---
title: "Mediator"
type: "design-pattern"
slug: "mediator"
url: "http://localhost:3000/en/design-patterns/mediator.md"
category: "Behavioral Patterns"
description: "Mediator is a behavioral design pattern that lets you reduce chaotic dependencies between objects. The pattern restricts direct communications between the objects and forces them to collaborate only via a mediator object."
languages: ["java", "csharp", "cpp", "go", "php", "python", "ruby", "rust", "swift", "typescript"]
---
# Mediator

> Mediator is a behavioral design pattern that lets you reduce chaotic dependencies between objects. The pattern restricts direct communications between the objects and forces them to collaborate only via a mediator object.

## Intent

**Mediator** is a behavioral design pattern that lets you reduce chaotic dependencies between objects. The pattern restricts direct communications between the objects and forces them to collaborate only via a mediator object.

## Problem

Say you have a dialog for creating and editing customer profiles. It consists of various form controls such as text fields, checkboxes, buttons, etc.

Relations between elements of the user interface can become chaotic as the application evolves.

Some of the form elements may interact with others. For instance, selecting the “I have a dog” checkbox may reveal a hidden text field for entering the dog’s name. Another example is the submit button that has to validate values of all fields before saving the data.

Elements can have lots of relations with other elements. Hence, changes to some elements may affect the others.

By having this logic implemented directly inside the code of the form elements you make these elements’ classes much harder to reuse in other forms of the app. For example, you won’t be able to use that checkbox class inside another form, because it’s coupled to the dog’s text field. You can use either all the classes involved in rendering the profile form, or none at all.

## Solution

The Mediator pattern suggests that you should cease all direct communication between the components which you want to make independent of each other. Instead, these components must collaborate indirectly, by calling a special mediator object that redirects the calls to appropriate components. As a result, the components depend only on a single mediator class instead of being coupled to dozens of their colleagues.

In our example with the profile editing form, the dialog class itself may act as the mediator. Most likely, the dialog class is already aware of all of its sub-elements, so you won’t even need to introduce new dependencies into this class.

UI elements should communicate indirectly, via the mediator object.

The most significant change happens to the actual form elements. Let’s consider the submit button. Previously, each time a user clicked the button, it had to validate the values of all individual form elements. Now its single job is to notify the dialog about the click. Upon receiving this notification, the dialog itself performs the validations or passes the task to the individual elements. Thus, instead of being tied to a dozen form elements, the button is only dependent on the dialog class.

You can go further and make the dependency even looser by extracting the common interface for all types of dialogs. The interface would declare the notification method which all form elements can use to notify the dialog about events happening to those elements. Thus, our submit button should now be able to work with any dialog that implements that interface.

This way, the Mediator pattern lets you encapsulate a complex web of relations between various objects inside a single mediator object. The fewer dependencies a class has, the easier it becomes to modify, extend or reuse that class.

## Structure

1. **Components** are various classes that contain some business logic. Each component has a reference to a mediator, declared with the type of the mediator interface. The component isn’t aware of the actual class of the mediator, so you can reuse the component in other programs by linking it to a different mediator.
2. The **Mediator** interface declares methods of communication with components, which usually include just a single notification method. Components may pass any context as arguments of this method, including their own objects, but only in such a way that no coupling occurs between a receiving component and the sender’s class.
3. **Concrete Mediators** encapsulate relations between various components. Concrete mediators often keep references to all components they manage and sometimes even manage their lifecycle.
4. Components must not be aware of other components. If something important happens within or to a component, it must only notify the mediator. When the mediator receives the notification, it can easily identify the sender, which might be just enough to decide what component should be triggered in return.
From a component’s perspective, it all looks like a total black box. The sender doesn’t know who’ll end up handling its request, and the receiver doesn’t know who sent the request in the first place.

## Pseudocode

In this example, the **Mediator** pattern helps you eliminate mutual dependencies between various UI classes: buttons, checkboxes and text labels.

Structure of the UI dialog classes.

An element, triggered by a user, doesn’t communicate with other elements directly, even if it looks like it’s supposed to. Instead, the element only needs to let its mediator know about the event, passing any contextual info along with that notification.

In this example, the whole authentication dialog acts as the mediator. It knows how concrete elements are supposed to collaborate and facilitates their indirect communication. Upon receiving a notification about an event, the dialog decides what element should address the event and redirects the call accordingly.

// The mediator interface declares a method used by components
// to notify the mediator about various events. The mediator may
// react to these events and pass the execution to other
// components.
interface Mediator is
    method notify(sender: Component, event: string)

// The concrete mediator class. The intertwined web of
// connections between individual components has been untangled
// and moved into the mediator.
class AuthenticationDialog implements Mediator is
    private field title: string
    private field loginOrRegisterChkBx: Checkbox
    private field loginUsername, loginPassword: Textbox
    private field registrationUsername, registrationPassword,
                  registrationEmail: Textbox
    private field okBtn, cancelBtn: Button

    constructor AuthenticationDialog() is
        // Create all component objects by passing the current
        // mediator into their constructors to establish links.

    // When something happens with a component, it notifies the
    // mediator. Upon receiving a notification, the mediator may
    // do something on its own or pass the request to another
    // component.
    method notify(sender, event) is
        if (sender == loginOrRegisterChkBx and event == "check")
            if (loginOrRegisterChkBx.checked)
                title = "Log in"
                // 1. Show login form components.
                // 2. Hide registration form components.
            else
                title = "Register"
                // 1. Show registration form components.
                // 2. Hide login form components

        if (sender == okBtn && event == "click")
            if (loginOrRegister.checked)
                // Try to find a user using login credentials.
                if (!found)
                    // Show an error message above the login
                    // field.
            else
                // 1. Create a user account using data from the
                // registration fields.
                // 2. Log that user in.
                // ...

// Components communicate with a mediator using the mediator
// interface. Thanks to that, you can use the same components in
// other contexts by linking them with different mediator
// objects.
class Component is
    field dialog: Mediator

    constructor Component(dialog) is
        this.dialog = dialog

    method click() is
        dialog.notify(this, "click")

    method keypress() is
        dialog.notify(this, "keypress")

// Concrete components don't talk to each other. They have only
// one communication channel, which is sending notifications to
// the mediator.
class Button extends Component is
    // ...

class Textbox extends Component is
    // ...

class Checkbox extends Component is
    method check() is
        dialog.notify(this, "check")
    // ...

## Applicability

Use the Mediator pattern when it’s hard to change some of the classes because they are tightly coupled to a bunch of other classes.

 The pattern lets you extract all the relationships between classes into a separate class, isolating any changes to a specific component from the rest of the components.

 Use the pattern when you can’t reuse a component in a different program because it’s too dependent on other components.

 After you apply the Mediator, individual components become unaware of the other components. They could still communicate with each other, albeit indirectly, through a mediator object. To reuse a component in a different app, you need to provide it with a new mediator class.

 Use the Mediator when you find yourself creating tons of component subclasses just to reuse some basic behavior in various contexts.

 Since all relations between components are contained within the mediator, it’s easy to define entirely new ways for these components to collaborate by introducing new mediator classes, without having to change the components themselves.

## How to Implement

1. Identify a group of tightly coupled classes which would benefit from being more independent (e.g., for easier maintenance or simpler reuse of these classes).
2. Declare the mediator interface and describe the desired communication protocol between mediators and various components. In most cases, a single method for receiving notifications from components is sufficient.
This interface is crucial when you want to reuse component classes in different contexts. As long as the component works with its mediator via the generic interface, you can link the component with a different implementation of the mediator.
3. Implement the concrete mediator class. Consider storing references to all components inside the mediator. This way, you could call any component from the mediator’s methods.
4. You can go even further and make the mediator responsible for the creation and destruction of component objects. After this, the mediator may resemble a [factory](/design-patterns/abstract-factory) or a [facade](/design-patterns/facade).
5. Components should store a reference to the mediator object. The connection is usually established in the component’s constructor, where a mediator object is passed as an argument.
6. Change the components’ code so that they call the mediator’s notification method instead of methods on other components. Extract the code that involves calling other components into the mediator class. Execute this code whenever the mediator receives notifications from that component.

## Pros

* _Single Responsibility Principle_. You can extract the communications between various components into a single place, making it easier to comprehend and maintain.
* _Open/Closed Principle_. You can introduce new mediators without having to change the actual components.
* You can reduce coupling between various components of a program.
* You can reuse individual components more easily.

## Cons

* Over time a mediator can evolve into a [God Object](/antipatterns/god-object).

## Relations with Other Patterns

* [Chain of Responsibility](/design-patterns/chain-of-responsibility), [Command](/design-patterns/command), [Mediator](/design-patterns/mediator) and [Observer](/design-patterns/observer) address various ways of connecting senders and receivers of requests:

  * _Chain of Responsibility_ passes a request sequentially along a dynamic chain of potential receivers until one of them handles it.
  * _Command_ establishes unidirectional connections between senders and receivers.
  * _Mediator_ eliminates direct connections between senders and receivers, forcing them to communicate indirectly via a mediator object.
  * _Observer_ lets receivers dynamically subscribe to and unsubscribe from receiving requests.
* [Facade](/design-patterns/facade) and [Mediator](/design-patterns/mediator) have similar jobs: they try to organize collaboration between lots of tightly coupled classes.

  * _Facade_ defines a simplified interface to a subsystem of objects, but it doesn’t introduce any new functionality. The subsystem itself is unaware of the facade. Objects within the subsystem can communicate directly.
  * _Mediator_ centralizes communication between components of the system. The components only know about the mediator object and don’t communicate directly.
* The difference between [Mediator](/design-patterns/mediator) and [Observer](/design-patterns/observer) is often elusive. In most cases, you can implement either of these patterns; but sometimes you can apply both simultaneously. Let’s see how we can do that.
The primary goal of _Mediator_ is to eliminate mutual dependencies among a set of system components. Instead, these components become dependent on a single mediator object. The goal of _Observer_ is to establish dynamic one-way connections between objects, where some objects act as subordinates of others.
There’s a popular implementation of the _Mediator_ pattern that relies on _Observer_. The mediator object plays the role of publisher, and the components act as subscribers which subscribe to and unsubscribe from the mediator’s events. When _Mediator_ is implemented this way, it may look very similar to _Observer_.
When you’re confused, remember that you can implement the Mediator pattern in other ways. For example, you can permanently link all the components to the same mediator object. This implementation won’t resemble _Observer_ but will still be an instance of the Mediator pattern.
Now imagine a program where all components have become publishers, allowing dynamic connections between each other. There won’t be a centralized mediator object, only a distributed set of observers.
## Relations

**Related patterns**

- [Chain of Responsibility](/en/design-patterns/chain-of-responsibility.md)
- [Command](/en/design-patterns/command.md)
- [Observer](/en/design-patterns/observer.md)
- [Facade](/en/design-patterns/facade.md)

## Code Examples

### java

```java
package refactoring_guru.mediator.example.components;

import refactoring_guru.mediator.example.mediator.Mediator;

/**
 * Common component interface.
 */
public interface Component {
    void setMediator(Mediator mediator);
    String getName();
}

package refactoring_guru.mediator.example.components;

import refactoring_guru.mediator.example.mediator.Mediator;
import refactoring_guru.mediator.example.mediator.Note;

import javax.swing.*;
import java.awt.event.ActionEvent;

/**
 * Concrete components don't talk with each other. They have only one
 * communication channel–sending requests to the mediator.
 */
public class AddButton extends JButton implements Component {
    private Mediator mediator;

    public AddButton() {
        super("Add");
    }

    @Override
    public void setMediator(Mediator mediator) {
        this.mediator = mediator;
    }

    @Override
    protected void fireActionPerformed(ActionEvent actionEvent) {
        mediator.addNewNote(new Note());
    }

    @Override
    public String getName() {
        return "AddButton";
    }
}

package refactoring_guru.mediator.example.components;

import refactoring_guru.mediator.example.mediator.Mediator;

import javax.swing.*;
import java.awt.event.ActionEvent;

/**
 * Concrete components don't talk with each other. They have only one
 * communication channel–sending requests to the mediator.
 */
public class DeleteButton extends JButton  implements Component {
    private Mediator mediator;

    public DeleteButton() {
        super("Del");
    }

    @Override
    public void setMediator(Mediator mediator) {
        this.mediator = mediator;
    }

    @Override
    protected void fireActionPerformed(ActionEvent actionEvent) {
        mediator.deleteNote();
    }

    @Override
    public String getName() {
        return "DelButton";
    }
}

package refactoring_guru.mediator.example.components;

import refactoring_guru.mediator.example.mediator.Mediator;
import refactoring_guru.mediator.example.mediator.Note;

import javax.swing.*;
import java.awt.event.KeyEvent;
import java.util.ArrayList;

/**
 * Concrete components don't talk with each other. They have only one
 * communication channel–sending requests to the mediator.
 */
public class Filter extends JTextField implements Component {
    private Mediator mediator;
    private ListModel listModel;

    public Filter() {}

    @Override
    public void setMediator(Mediator mediator) {
        this.mediator = mediator;
    }

    @Override
    protected void processComponentKeyEvent(KeyEvent keyEvent) {
        String start = getText();
        searchElements(start);
    }

    public void setList(ListModel listModel) {
        this.listModel = listModel;
    }

    private void searchElements(String s) {
        if (listModel == null) {
            return;
        }

        if (s.equals("")) {
            mediator.setElementsList(listModel);
            return;
        }

        ArrayList<Note> notes = new ArrayList<>();
        for (int i = 0; i < listModel.getSize(); i++) {
            notes.add((Note) listModel.getElementAt(i));
        }
        DefaultListModel<Note> listModel = new DefaultListModel<>();
        for (Note note : notes) {
            if (note.getName().contains(s)) {
                listModel.addElement(note);
            }
        }
        mediator.setElementsList(listModel);
    }

    @Override
    public String getName() {
        return "Filter";
    }
}

package refactoring_guru.mediator.example.components;

import refactoring_guru.mediator.example.mediator.Mediator;
import refactoring_guru.mediator.example.mediator.Note;

import javax.swing.*;

/**
 * Concrete components don't talk with each other. They have only one
 * communication channel–sending requests to the mediator.
 */
@SuppressWarnings("unchecked")
public class List extends JList implements Component {
    private Mediator mediator;
    private final DefaultListModel LIST_MODEL;

    public List(DefaultListModel listModel) {
        super(listModel);
        this.LIST_MODEL = listModel;
        setModel(listModel);
        this.setLayoutOrientation(JList.VERTICAL);
        Thread thread = new Thread(new Hide(this));
        thread.start();
    }

    @Override
    public void setMediator(Mediator mediator) {
        this.mediator = mediator;
    }

    public void addElement(Note note) {
        LIST_MODEL.addElement(note);
        int index = LIST_MODEL.size() - 1;
        setSelectedIndex(index);
        ensureIndexIsVisible(index);
        mediator.sendToFilter(LIST_MODEL);
    }

    public void deleteElement() {
        int index = this.getSelectedIndex();
        try {
            LIST_MODEL.remove(index);
            mediator.sendToFilter(LIST_MODEL);
        } catch (ArrayIndexOutOfBoundsException ignored) {}
    }

    public Note getCurrentElement() {
        return (Note)getSelectedValue();
    }

    @Override
    public String getName() {
        return "List";
    }

    private class Hide implements Runnable {
        private List list;

        Hide(List list) {
            this.list = list;
        }

        @Override
        public void run() {
            while (true) {
                try {
                    Thread.sleep(300);
                } catch (InterruptedException ex) {
                    ex.printStackTrace();
                }
                if (list.isSelectionEmpty()) {
                    mediator.hideElements(true);
                } else {
                    mediator.hideElements(false);
                }
            }
        }
    }
}

package refactoring_guru.mediator.example.components;

import refactoring_guru.mediator.example.mediator.Mediator;

import javax.swing.*;
import java.awt.event.ActionEvent;

/**
 * Concrete components don't talk with each other. They have only one
 * communication channel–sending requests to the mediator.
 */
public class SaveButton extends JButton implements Component {
    private Mediator mediator;

    public SaveButton() {
        super("Save");
    }

    @Override
    public void setMediator(Mediator mediator) {
        this.mediator = mediator;
    }

    @Override
    protected void fireActionPerformed(ActionEvent actionEvent) {
        mediator.saveChanges();
    }

    @Override
    public String getName() {
        return "SaveButton";
    }
}

package refactoring_guru.mediator.example.components;

import refactoring_guru.mediator.example.mediator.Mediator;

import javax.swing.*;
import java.awt.event.KeyEvent;

/**
 * Concrete components don't talk with each other. They have only one
 * communication channel–sending requests to the mediator.
 */
public class TextBox extends JTextArea implements Component {
    private Mediator mediator;

    @Override
    public void setMediator(Mediator mediator) {
        this.mediator = mediator;
    }

    @Override
    protected void processComponentKeyEvent(KeyEvent keyEvent) {
        mediator.markNote();
    }

    @Override
    public String getName() {
        return "TextBox";
    }
}

package refactoring_guru.mediator.example.components;

import refactoring_guru.mediator.example.mediator.Mediator;

import javax.swing.*;
import java.awt.event.KeyEvent;

/**
 * Concrete components don't talk with each other. They have only one
 * communication channel–sending requests to the mediator.
 */
public class Title extends JTextField implements Component {
    private Mediator mediator;

    @Override
    public void setMediator(Mediator mediator) {
        this.mediator = mediator;
    }

    @Override
    protected void processComponentKeyEvent(KeyEvent keyEvent) {
        mediator.markNote();
    }

    @Override
    public String getName() {
        return "Title";
    }
}

package refactoring_guru.mediator.example.mediator;

import refactoring_guru.mediator.example.components.Component;

import javax.swing.*;

/**
 * Common mediator interface.
 */
public interface Mediator {
    void addNewNote(Note note);
    void deleteNote();
    void getInfoFromList(Note note);
    void saveChanges();
    void markNote();
    void clear();
    void sendToFilter(ListModel listModel);
    void setElementsList(ListModel list);
    void registerComponent(Component component);
    void hideElements(boolean flag);
    void createGUI();
}

package refactoring_guru.mediator.example.mediator;

import refactoring_guru.mediator.example.components.*;
import refactoring_guru.mediator.example.components.Component;
import refactoring_guru.mediator.example.components.List;

import javax.swing.*;
import javax.swing.border.LineBorder;
import java.awt.*;

/**
 * Concrete mediator. All chaotic communications between concrete components
 * have been extracted to the mediator. Now components only talk with the
 * mediator, which knows who has to handle a request.
 */
public class Editor implements Mediator {
    private Title title;
    private TextBox textBox;
    private AddButton add;
    private DeleteButton del;
    private SaveButton save;
    private List list;
    private Filter filter;

    private JLabel titleLabel = new JLabel("Title:");
    private JLabel textLabel = new JLabel("Text:");
    private JLabel label = new JLabel("Add or select existing note to proceed...");
  
    /**
     * Here the registration of components by the mediator.
     */
    @Override
    public void registerComponent(Component component) {
        component.setMediator(this);
        switch (component.getName()) {
            case "AddButton":
                add = (AddButton)component;
                break;
            case "DelButton":
                del = (DeleteButton)component;
                break;
            case "Filter":
                filter = (Filter)component;
                break;
            case "List":
                list = (List)component;
                this.list.addListSelectionListener(listSelectionEvent -> {
                    Note note = (Note)list.getSelectedValue();
                    if (note != null) {
                        getInfoFromList(note);
                    } else {
                        clear();
                    }
                });
                break;
            case "SaveButton":
                save = (SaveButton)component;
                break;
            case "TextBox":
                textBox = (TextBox)component;
                break;
            case "Title":
                title = (Title)component;
                break;
        }
    }

    /**
     * Various methods to handle requests from particular components.
     */
    @Override
    public void addNewNote(Note note) {
        title.setText("");
        textBox.setText("");
        list.addElement(note);
    }

    @Override
    public void deleteNote() {
        list.deleteElement();
    }

    @Override
    public void getInfoFromList(Note note) {
        title.setText(note.getName().replace('*', ' '));
        textBox.setText(note.getText());
    }

    @Override
    public void saveChanges() {
        try {
            Note note = (Note) list.getSelectedValue();
            note.setName(title.getText());
            note.setText(textBox.getText());
            list.repaint();
        } catch (NullPointerException ignored) {}
    }

    @Override
    public void markNote() {
        try {
            Note note = list.getCurrentElement();
            String name = note.getName();
            if (!name.endsWith("*")) {
                note.setName(note.getName() + "*");
            }
            list.repaint();
        } catch (NullPointerException ignored) {}
    }

    @Override
    public void clear() {
        title.setText("");
        textBox.setText("");
    }

    @Override
    public void sendToFilter(ListModel listModel) {
        filter.setList(listModel);
    }

    @SuppressWarnings("unchecked")
    @Override
    public void setElementsList(ListModel list) {
        this.list.setModel(list);
        this.list.repaint();
    }

    @Override
    public void hideElements(boolean flag) {
        titleLabel.setVisible(!flag);
        textLabel.setVisible(!flag);
        title.setVisible(!flag);
        textBox.setVisible(!flag);
        save.setVisible(!flag);
        label.setVisible(flag);
    }

    @Override
    public void createGUI() {
        JFrame notes = new JFrame("Notes");
        notes.setSize(960, 600);
        notes.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        JPanel left = new JPanel();
        left.setBorder(new LineBorder(Color.BLACK));
        left.setSize(320, 600);
        left.setLayout(new BoxLayout(left, BoxLayout.Y_AXIS));
        JPanel filterPanel = new JPanel();
        filterPanel.add(new JLabel("Filter:"));
        filter.setColumns(20);
        filterPanel.add(filter);
        filterPanel.setPreferredSize(new Dimension(280, 40));
        JPanel listPanel = new JPanel();
        list.setFixedCellWidth(260);
        listPanel.setSize(320, 470);
        JScrollPane scrollPane = new JScrollPane(list);
        scrollPane.setPreferredSize(new Dimension(275, 410));
        listPanel.add(scrollPane);
        JPanel buttonPanel = new JPanel();
        add.setPreferredSize(new Dimension(85, 25));
        buttonPanel.add(add);
        del.setPreferredSize(new Dimension(85, 25));
        buttonPanel.add(del);
        buttonPanel.setLayout(new FlowLayout());
        left.add(filterPanel);
        left.add(listPanel);
        left.add(buttonPanel);
        JPanel right = new JPanel();
        right.setLayout(null);
        right.setSize(640, 600);
        right.setLocation(320, 0);
        right.setBorder(new LineBorder(Color.BLACK));
        titleLabel.setBounds(20, 4, 50, 20);
        title.setBounds(60, 5, 555, 20);
        textLabel.setBounds(20, 4, 50, 130);
        textBox.setBorder(new LineBorder(Color.DARK_GRAY));
        textBox.setBounds(20, 80, 595, 410);
        save.setBounds(270, 535, 80, 25);
        label.setFont(new Font("Verdana", Font.PLAIN, 22));
        label.setBounds(100, 240, 500, 100);
        right.add(label);
        right.add(titleLabel);
        right.add(title);
        right.add(textLabel);
        right.add(textBox);
        right.add(save);
        notes.setLayout(null);
        notes.getContentPane().add(left);
        notes.getContentPane().add(right);
        notes.setResizable(false);
        notes.setLocationRelativeTo(null);
        notes.setVisible(true);
    }
}

package refactoring_guru.mediator.example.mediator;

/**
 * Note class.
 */
public class Note {
    private String name;
    private String text;

    public Note() {
        name = "New note";
    }

    public void setName(String name) {
        this.name = name;
    }

    public void setText(String text) {
        this.text = text;
    }

    public String getName() {
        return name;
    }

    public String getText() {
        return text;
    }

    @Override
    public String toString() {
        return name;
    }
}

package refactoring_guru.mediator.example;

import refactoring_guru.mediator.example.components.*;
import refactoring_guru.mediator.example.mediator.Editor;
import refactoring_guru.mediator.example.mediator.Mediator;

import javax.swing.*;

/**
 * Demo class. Everything comes together here.
 */
public class Demo {
    public static void main(String[] args) {
        Mediator mediator = new Editor();

        mediator.registerComponent(new Title());
        mediator.registerComponent(new TextBox());
        mediator.registerComponent(new AddButton());
        mediator.registerComponent(new DeleteButton());
        mediator.registerComponent(new SaveButton());
        mediator.registerComponent(new List(new DefaultListModel()));
        mediator.registerComponent(new Filter());

        mediator.createGUI();
    }
}
```

### csharp

```csharp
using System;

namespace RefactoringGuru.DesignPatterns.Mediator.Conceptual
{
    // The Mediator interface declares a method used by components to notify the
    // mediator about various events. The Mediator may react to these events and
    // pass the execution to other components.
    public interface IMediator
    {
        void Notify(object sender, string ev);
    }

    // Concrete Mediators implement cooperative behavior by coordinating several
    // components.
    class ConcreteMediator : IMediator
    {
        private Component1 _component1;

        private Component2 _component2;

        public ConcreteMediator(Component1 component1, Component2 component2)
        {
            this._component1 = component1;
            this._component1.SetMediator(this);
            this._component2 = component2;
            this._component2.SetMediator(this);
        } 

        public void Notify(object sender, string ev)
        {
            if (ev == "A")
            {
                Console.WriteLine("Mediator reacts on A and triggers following operations:");
                this._component2.DoC();
            }
            if (ev == "D")
            {
                Console.WriteLine("Mediator reacts on D and triggers following operations:");
                this._component1.DoB();
                this._component2.DoC();
            }
        }
    }

    // The Base Component provides the basic functionality of storing a
    // mediator's instance inside component objects.
    class BaseComponent
    {
        protected IMediator _mediator;

        public BaseComponent(IMediator mediator = null)
        {
            this._mediator = mediator;
        }

        public void SetMediator(IMediator mediator)
        {
            this._mediator = mediator;
        }
    }

    // Concrete Components implement various functionality. They don't depend on
    // other components. They also don't depend on any concrete mediator
    // classes.
    class Component1 : BaseComponent
    {
        public void DoA()
        {
            Console.WriteLine("Component 1 does A.");

            this._mediator.Notify(this, "A");
        }

        public void DoB()
        {
            Console.WriteLine("Component 1 does B.");

            this._mediator.Notify(this, "B");
        }
    }

    class Component2 : BaseComponent
    {
        public void DoC()
        {
            Console.WriteLine("Component 2 does C.");

            this._mediator.Notify(this, "C");
        }

        public void DoD()
        {
            Console.WriteLine("Component 2 does D.");

            this._mediator.Notify(this, "D");
        }
    }
    
    class Program
    {
        static void Main(string[] args)
        {
            // The client code.
            Component1 component1 = new Component1();
            Component2 component2 = new Component2();
            new ConcreteMediator(component1, component2);

            Console.WriteLine("Client triggers operation A.");
            component1.DoA();

            Console.WriteLine();

            Console.WriteLine("Client triggers operation D.");
            component2.DoD();
        }
    }
}

Client triggers operation A.
Component 1 does A.
Mediator reacts on A and triggers following operations:
Component 2 does C.

Client triggers operation D.
Component 2 does D.
Mediator reacts on D and triggers following operations:
Component 1 does B.
Component 2 does C.
```

### cpp

```cpp
#include <iostream>
#include <string>
/**
 * The Mediator interface declares a method used by components to notify the
 * mediator about various events. The Mediator may react to these events and
 * pass the execution to other components.
 */
class BaseComponent;
class Mediator {
 public:
  virtual void Notify(BaseComponent *sender, std::string event) const = 0;
};

/**
 * The Base Component provides the basic functionality of storing a mediator's
 * instance inside component objects.
 */
class BaseComponent {
 protected:
  Mediator *mediator_;

 public:
  BaseComponent(Mediator *mediator = nullptr) : mediator_(mediator) {
  }
  void set_mediator(Mediator *mediator) {
    this->mediator_ = mediator;
  }
};

/**
 * Concrete Components implement various functionality. They don't depend on
 * other components. They also don't depend on any concrete mediator classes.
 */
class Component1 : public BaseComponent {
 public:
  void DoA() {
    std::cout << "Component 1 does A.\n";
    this->mediator_->Notify(this, "A");
  }
  void DoB() {
    std::cout << "Component 1 does B.\n";
    this->mediator_->Notify(this, "B");
  }
};

class Component2 : public BaseComponent {
 public:
  void DoC() {
    std::cout << "Component 2 does C.\n";
    this->mediator_->Notify(this, "C");
  }
  void DoD() {
    std::cout << "Component 2 does D.\n";
    this->mediator_->Notify(this, "D");
  }
};

/**
 * Concrete Mediators implement cooperative behavior by coordinating several
 * components.
 */
class ConcreteMediator : public Mediator {
 private:
  Component1 *component1_;
  Component2 *component2_;

 public:
  ConcreteMediator(Component1 *c1, Component2 *c2) : component1_(c1), component2_(c2) {
    this->component1_->set_mediator(this);
    this->component2_->set_mediator(this);
  }
  void Notify(BaseComponent *sender, std::string event) const override {
    if (event == "A") {
      std::cout << "Mediator reacts on A and triggers following operations:\n";
      this->component2_->DoC();
    }
    if (event == "D") {
      std::cout << "Mediator reacts on D and triggers following operations:\n";
      this->component1_->DoB();
      this->component2_->DoC();
    }
  }
};

/**
 * The client code.
 */

void ClientCode() {
  Component1 *c1 = new Component1;
  Component2 *c2 = new Component2;
  ConcreteMediator *mediator = new ConcreteMediator(c1, c2);
  std::cout << "Client triggers operation A.\n";
  c1->DoA();
  std::cout << "\n";
  std::cout << "Client triggers operation D.\n";
  c2->DoD();

  delete c1;
  delete c2;
  delete mediator;
}

int main() {
  ClientCode();
  return 0;
}

Client triggers operation A.
Component 1 does A.
Mediator reacts on A and triggers following operations:
Component 2 does C.

Client triggers operation D.
Component 2 does D.
Mediator reacts on D and triggers following operations:
Component 1 does B.
Component 2 does C.
```

### go

```go
package main

type Train interface {
	arrive()
	depart()
	permitArrival()
}

package main

import "fmt"

type PassengerTrain struct {
	mediator Mediator
}

func (g *PassengerTrain) arrive() {
	if !g.mediator.canArrive(g) {
		fmt.Println("PassengerTrain: Arrival blocked, waiting")
		return
	}
	fmt.Println("PassengerTrain: Arrived")
}

func (g *PassengerTrain) depart() {
	fmt.Println("PassengerTrain: Leaving")
	g.mediator.notifyAboutDeparture()
}

func (g *PassengerTrain) permitArrival() {
	fmt.Println("PassengerTrain: Arrival permitted, arriving")
	g.arrive()
}

package main

import "fmt"

type FreightTrain struct {
	mediator Mediator
}

func (g *FreightTrain) arrive() {
	if !g.mediator.canArrive(g) {
		fmt.Println("FreightTrain: Arrival blocked, waiting")
		return
	}
	fmt.Println("FreightTrain: Arrived")
}

func (g *FreightTrain) depart() {
	fmt.Println("FreightTrain: Leaving")
	g.mediator.notifyAboutDeparture()
}

func (g *FreightTrain) permitArrival() {
	fmt.Println("FreightTrain: Arrival permitted")
	g.arrive()
}

package main

type Mediator interface {
	canArrive(Train) bool
	notifyAboutDeparture()
}

package main

type StationManager struct {
	isPlatformFree bool
	trainQueue     []Train
}

func newStationManger() *StationManager {
	return &StationManager{
		isPlatformFree: true,
	}
}

func (s *StationManager) canArrive(t Train) bool {
	if s.isPlatformFree {
		s.isPlatformFree = false
		return true
	}
	s.trainQueue = append(s.trainQueue, t)
	return false
}

func (s *StationManager) notifyAboutDeparture() {
	if !s.isPlatformFree {
		s.isPlatformFree = true
	}
	if len(s.trainQueue) > 0 {
		firstTrainInQueue := s.trainQueue[0]
		s.trainQueue = s.trainQueue[1:]
		firstTrainInQueue.permitArrival()
	}
}

package main

func main() {
	stationManager := newStationManger()

	passengerTrain := &PassengerTrain{
		mediator: stationManager,
	}
	freightTrain := &FreightTrain{
		mediator: stationManager,
	}

	passengerTrain.arrive()
	freightTrain.arrive()
	passengerTrain.depart()
}

PassengerTrain: Arrived
FreightTrain: Arrival blocked, waiting
PassengerTrain: Leaving
FreightTrain: Arrival permitted
FreightTrain: Arrived
```

### php

```php
<?php

namespace RefactoringGuru\Mediator\Conceptual;

/**
 * The Mediator interface declares a method used by components to notify the
 * mediator about various events. The Mediator may react to these events and
 * pass the execution to other components.
 */
interface Mediator
{
    public function notify(object $sender, string $event): void;
}

/**
 * Concrete Mediators implement cooperative behavior by coordinating several
 * components.
 */
class ConcreteMediator implements Mediator
{
    private $component1;

    private $component2;

    public function __construct(Component1 $c1, Component2 $c2)
    {
        $this->component1 = $c1;
        $this->component1->setMediator($this);
        $this->component2 = $c2;
        $this->component2->setMediator($this);
    }

    public function notify(object $sender, string $event): void
    {
        if ($event == "A") {
            echo "Mediator reacts on A and triggers following operations:\n";
            $this->component2->doC();
        }

        if ($event == "D") {
            echo "Mediator reacts on D and triggers following operations:\n";
            $this->component1->doB();
            $this->component2->doC();
        }
    }
}

/**
 * The Base Component provides the basic functionality of storing a mediator's
 * instance inside component objects.
 */
class BaseComponent
{
    protected $mediator;

    public function __construct(Mediator $mediator = null)
    {
        $this->mediator = $mediator;
    }

    public function setMediator(Mediator $mediator): void
    {
        $this->mediator = $mediator;
    }
}

/**
 * Concrete Components implement various functionality. They don't depend on
 * other components. They also don't depend on any concrete mediator classes.
 */
class Component1 extends BaseComponent
{
    public function doA(): void
    {
        echo "Component 1 does A.\n";
        $this->mediator->notify($this, "A");
    }

    public function doB(): void
    {
        echo "Component 1 does B.\n";
        $this->mediator->notify($this, "B");
    }
}

class Component2 extends BaseComponent
{
    public function doC(): void
    {
        echo "Component 2 does C.\n";
        $this->mediator->notify($this, "C");
    }

    public function doD(): void
    {
        echo "Component 2 does D.\n";
        $this->mediator->notify($this, "D");
    }
}

/**
 * The client code.
 */
$c1 = new Component1();
$c2 = new Component2();
$mediator = new ConcreteMediator($c1, $c2);

echo "Client triggers operation A.\n";
$c1->doA();

echo "\n";
echo "Client triggers operation D.\n";
$c2->doD();

Client triggers operation A.
Component 1 does A.
Mediator reacts on A and triggers following operations:
Component 2 does C.

Client triggers operation D.
Component 2 does D.
Mediator reacts on D and triggers following operations:
Component 1 does B.
Component 2 does C.

<?php

namespace RefactoringGuru\Mediator\RealWorld;

/**
 * The Event Dispatcher class acts as a Mediator and contains the subscription
 * and notification logic. While a classic Mediator often depends on concrete
 * component classes, this one is only tied to their abstract interfaces.
 *
 * We are able to achieve this level of indirection thanks to the way the
 * connections between components are established. The components themselves may
 * subscribe to specific events that they are interested in via the Mediator's
 * subscription interface.
 *
 * Note, we can't use the PHP's built-in Subject/Observer interfaces here
 * because we'll be stretching them too far from what they were designed for.
 */
class EventDispatcher
{
    /**
     * @var array
     */
    private $observers = [];

    public function __construct()
    {
        // The special event group for observers that want to listen to all
        // events.
        $this->observers["*"] = [];
    }

    private function initEventGroup(string &$event = "*"): void
    {
        if (!isset($this->observers[$event])) {
            $this->observers[$event] = [];
        }
    }

    private function getEventObservers(string $event = "*"): array
    {
        $this->initEventGroup($event);
        $group = $this->observers[$event];
        $all = $this->observers["*"];

        return array_merge($group, $all);
    }

    public function attach(Observer $observer, string $event = "*"): void
    {
        $this->initEventGroup($event);

        $this->observers[$event][] = $observer;
    }

    public function detach(Observer $observer, string $event = "*"): void
    {
        foreach ($this->getEventObservers($event) as $key => $s) {
            if ($s === $observer) {
                unset($this->observers[$event][$key]);
            }
        }
    }

    public function trigger(string $event, object $emitter, $data = null): void
    {
        echo "EventDispatcher: Broadcasting the '$event' event.\n";
        foreach ($this->getEventObservers($event) as $observer) {
            $observer->update($event, $emitter, $data);
        }
    }
}

/**
 * A simple helper function to provide global access to the event dispatcher.
 */
function events(): EventDispatcher
{
    static $eventDispatcher;
    if (!$eventDispatcher) {
        $eventDispatcher = new EventDispatcher();
    }

    return $eventDispatcher;
}

/**
 * The Observer interface defines how components receive the event
 * notifications.
 */
interface Observer
{
    public function update(string $event, object $emitter, $data = null);
}

/**
 * Unlike our Observer pattern example, this example makes the UserRepository
 * act as a regular component that doesn't have any special event-related
 * methods. Like any other component, this class relies on the EventDispatcher
 * to broadcast its events and listen for the other ones.
 *
 * @see \RefactoringGuru\Observer\RealWorld\UserRepository
 */
class UserRepository implements Observer
{
    /**
     * @var array List of application's users.
     */
    private $users = [];

    /**
     * Components can subscribe to events by themselves or by client code.
     */
    public function __construct()
    {
        events()->attach($this, "users:deleted");
    }

    /**
     * Components can decide whether they'd like to process an event using its
     * name, emitter or any contextual data passed along with the event.
     */
    public function update(string $event, object $emitter, $data = null): void
    {
        switch ($event) {
            case "users:deleted":
                if ($emitter === $this) {
                    return;
                }
                $this->deleteUser($data, true);
                break;
        }
    }

    // These methods represent the business logic of the class.

    public function initialize(string $filename): void
    {
        echo "UserRepository: Loading user records from a file.\n";
        // ...
        events()->trigger("users:init", $this, $filename);
    }

    public function createUser(array $data, bool $silent = false): User
    {
        echo "UserRepository: Creating a user.\n";

        $user = new User();
        $user->update($data);

        $id = bin2hex(openssl_random_pseudo_bytes(16));
        $user->update(["id" => $id]);
        $this->users[$id] = $user;

        if (!$silent) {
            events()->trigger("users:created", $this, $user);
        }

        return $user;
    }

    public function updateUser(User $user, array $data, bool $silent = false): ?User
    {
        echo "UserRepository: Updating a user.\n";

        $id = $user->attributes["id"];
        if (!isset($this->users[$id])) {
            return null;
        }

        $user = $this->users[$id];
        $user->update($data);

        if (!$silent) {
            events()->trigger("users:updated", $this, $user);
        }

        return $user;
    }

    public function deleteUser(User $user, bool $silent = false): void
    {
        echo "UserRepository: Deleting a user.\n";

        $id = $user->attributes["id"];
        if (!isset($this->users[$id])) {
            return;
        }

        unset($this->users[$id]);

        if (!$silent) {
            events()->trigger("users:deleted", $this, $user);
        }
    }
}

/**
 * Let's keep the User class trivial since it's not the focus of our example.
 */
class User
{
    public $attributes = [];

    public function update($data): void
    {
        $this->attributes = array_merge($this->attributes, $data);
    }

    /**
     * All objects can trigger events.
     */
    public function delete(): void
    {
        echo "User: I can now delete myself without worrying about the repository.\n";
        events()->trigger("users:deleted", $this, $this);
    }
}

/**
 * This Concrete Component logs any events it's subscribed to.
 */
class Logger implements Observer
{
    private $filename;

    public function __construct($filename)
    {
        $this->filename = $filename;
        if (file_exists($this->filename)) {
            unlink($this->filename);
        }
    }

    public function update(string $event, object $emitter, $data = null)
    {
        $entry = date("Y-m-d H:i:s") . ": '$event' with data '" . json_encode($data) . "'\n";
        file_put_contents($this->filename, $entry, FILE_APPEND);

        echo "Logger: I've written '$event' entry to the log.\n";
    }
}

/**
 * This Concrete Component sends initial instructions to new users. The client
 * is responsible for attaching this component to a proper user creation event.
 */
class OnboardingNotification implements Observer
{
    private $adminEmail;

    public function __construct(string $adminEmail)
    {
        $this->adminEmail = $adminEmail;
    }

    public function update(string $event, object $emitter, $data = null): void
    {
        // mail($this->adminEmail,
        //     "Onboarding required",
        //     "We have a new user. Here's his info: " .json_encode($data));

        echo "OnboardingNotification: The notification has been emailed!\n";
    }
}

/**
 * The client code.
 */

$repository = new UserRepository();
events()->attach($repository, "facebook:update");

$logger = new Logger(__DIR__ . "/log.txt");
events()->attach($logger, "*");

$onboarding = new OnboardingNotification("1@example.com");
events()->attach($onboarding, "users:created");

// ...

$repository->initialize(__DIR__ . "users.csv");

// ...

$user = $repository->createUser([
    "name" => "John Smith",
    "email" => "john99@example.com",
]);

// ...

$user->delete();

UserRepository: Loading user records from a file.
EventDispatcher: Broadcasting the 'users:init' event.
Logger: I've written 'users:init' entry to the log.
UserRepository: Creating a user.
EventDispatcher: Broadcasting the 'users:created' event.
OnboardingNotification: The notification has been emailed!
Logger: I've written 'users:created' entry to the log.
User: I can now delete myself without worrying about the repository.
EventDispatcher: Broadcasting the 'users:deleted' event.
UserRepository: Deleting a user.
Logger: I've written 'users:deleted' entry to the log.
```

### python

```python
from __future__ import annotations
from abc import ABC


class Mediator(ABC):
    """
    The Mediator interface declares a method used by components to notify the
    mediator about various events. The Mediator may react to these events and
    pass the execution to other components.
    """

    def notify(self, sender: object, event: str) -> None:
        pass


class ConcreteMediator(Mediator):
    def __init__(self, component1: Component1, component2: Component2) -> None:
        self._component1 = component1
        self._component1.mediator = self
        self._component2 = component2
        self._component2.mediator = self

    def notify(self, sender: object, event: str) -> None:
        if event == "A":
            print("Mediator reacts on A and triggers following operations:")
            self._component2.do_c()
        elif event == "D":
            print("Mediator reacts on D and triggers following operations:")
            self._component1.do_b()
            self._component2.do_c()


class BaseComponent:
    """
    The Base Component provides the basic functionality of storing a mediator's
    instance inside component objects.
    """

    def __init__(self, mediator: Mediator = None) -> None:
        self._mediator = mediator

    @property
    def mediator(self) -> Mediator:
        return self._mediator

    @mediator.setter
    def mediator(self, mediator: Mediator) -> None:
        self._mediator = mediator


"""
Concrete Components implement various functionality. They don't depend on other
components. They also don't depend on any concrete mediator classes.
"""


class Component1(BaseComponent):
    def do_a(self) -> None:
        print("Component 1 does A.")
        self.mediator.notify(self, "A")

    def do_b(self) -> None:
        print("Component 1 does B.")
        self.mediator.notify(self, "B")


class Component2(BaseComponent):
    def do_c(self) -> None:
        print("Component 2 does C.")
        self.mediator.notify(self, "C")

    def do_d(self) -> None:
        print("Component 2 does D.")
        self.mediator.notify(self, "D")


if __name__ == "__main__":
    # The client code.
    c1 = Component1()
    c2 = Component2()
    mediator = ConcreteMediator(c1, c2)

    print("Client triggers operation A.")
    c1.do_a()

    print("\n", end="")

    print("Client triggers operation D.")
    c2.do_d()

Client triggers operation A.
Component 1 does A.
Mediator reacts on A and triggers following operations:
Component 2 does C.


Client triggers operation D.
Component 2 does D.
Mediator reacts on D and triggers following operations:
Component 1 does B.
Component 2 does C.
```

### ruby

```ruby
# The Mediator interface declares a method used by components to notify the
# mediator about various events. The Mediator may react to these events and pass
# the execution to other components.
class Mediator
  # @abstract
  #
  # @param [Object] sender
  # @param [String] event
  def notify(_sender, _event)
    raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
  end
end

class ConcreteMediator < Mediator
  # @param [Component1] component1
  # @param [Component2] component2
  def initialize(component1, component2)
    @component1 = component1
    @component1.mediator = self
    @component2 = component2
    @component2.mediator = self
  end

  # @param [Object] sender
  # @param [String] event
  def notify(_sender, event)
    if event == 'A'
      puts 'Mediator reacts on A and triggers following operations:'
      @component2.do_c
    elsif event == 'D'
      puts 'Mediator reacts on D and triggers following operations:'
      @component1.do_b
      @component2.do_c
    end
  end
end

# The Base Component provides the basic functionality of storing a mediator's
# instance inside component objects.
class BaseComponent
  # @return [Mediator]
  attr_accessor :mediator

  # @param [Mediator] mediator
  def initialize(mediator = nil)
    @mediator = mediator
  end
end

# Concrete Components implement various functionality. They don't depend on
# other components. They also don't depend on any concrete mediator classes.
class Component1 < BaseComponent
  def do_a
    puts 'Component 1 does A.'
    @mediator.notify(self, 'A')
  end

  def do_b
    puts 'Component 1 does B.'
    @mediator.notify(self, 'B')
  end
end

class Component2 < BaseComponent
  def do_c
    puts 'Component 2 does C.'
    @mediator.notify(self, 'C')
  end

  def do_d
    puts 'Component 2 does D.'
    @mediator.notify(self, 'D')
  end
end

# The client code.
c1 = Component1.new
c2 = Component2.new
ConcreteMediator.new(c1, c2)

puts 'Client triggers operation A.'
c1.do_a

puts "\n"

puts 'Client triggers operation D.'
c2.do_d

Client triggers operation A.
Component 1 does A.
Mediator reacts on A and triggers following operations:
Component 2 does C.

Client triggers operation D.
Component 2 does D.
Mediator reacts on D and triggers following operations:
Component 1 does B.
Component 2 does C.
```

### rust

```rust
// A train gets a mediator object by reference.
pub trait Train {
    fn name(&self) -> &String;
    fn arrive(&mut self, mediator: &mut dyn Mediator);
    fn depart(&mut self, mediator: &mut dyn Mediator);
}

// Mediator has notification methods.
pub trait Mediator {
    fn notify_about_arrival(&mut self, train_name: &str) -> bool;
    fn notify_about_departure(&mut self, train_name: &str);
}

let train1 = PassengerTrain::new("Train 1");
let train2 = FreightTrain::new("Train 2");

// Station has `accept` and `depart` methods,
// but it also implements `Mediator`.
let mut station = TrainStation::default();

// Station is taking ownership of the trains.
station.accept(train1);
station.accept(train2);

// `train1` and `train2` have been moved inside,
// but we can use train names to depart them.
station.depart("Train 1");
station.depart("Train 2");
station.depart("Train 3");

use std::collections::{HashMap, VecDeque};

use crate::trains::Train;

// Mediator has notification methods.
pub trait Mediator {
    fn notify_about_arrival(&mut self, train_name: &str) -> bool;
    fn notify_about_departure(&mut self, train_name: &str);
}

#[derive(Default)]
pub struct TrainStation {
    trains: HashMap<String, Box<dyn Train>>,
    train_queue: VecDeque<String>,
    train_on_platform: Option<String>,
}

impl Mediator for TrainStation {
    fn notify_about_arrival(&mut self, train_name: &str) -> bool {
        if self.train_on_platform.is_some() {
            self.train_queue.push_back(train_name.into());
            false
        } else {
            self.train_on_platform.replace(train_name.into());
            true
        }
    }

    fn notify_about_departure(&mut self, train_name: &str) {
        if Some(train_name.into()) == self.train_on_platform {
            self.train_on_platform = None;

            if let Some(next_train_name) = self.train_queue.pop_front() {
                let mut next_train = self.trains.remove(&next_train_name).unwrap();
                next_train.arrive(self);
                self.trains.insert(next_train_name.clone(), next_train);

                self.train_on_platform = Some(next_train_name);
            }
        }
    }
}

impl TrainStation {
    pub fn accept(&mut self, mut train: impl Train + 'static) {
        if self.trains.contains_key(train.name()) {
            println!("{} has already arrived", train.name());
            return;
        }

        train.arrive(self);
        self.trains.insert(train.name().clone(), Box::new(train));
    }

    pub fn depart(&mut self, name: &'static str) {
        let train = self.trains.remove(name);
        if let Some(mut train) = train {
            train.depart(self);
        } else {
            println!("'{}' is not on the station!", name);
        }
    }
}

mod freight_train;
mod passenger_train;

pub use freight_train::FreightTrain;
pub use passenger_train::PassengerTrain;

use crate::train_station::Mediator;

// A train gets a mediator object by reference.
pub trait Train {
    fn name(&self) -> &String;
    fn arrive(&mut self, mediator: &mut dyn Mediator);
    fn depart(&mut self, mediator: &mut dyn Mediator);
}

use super::Train;
use crate::train_station::Mediator;

pub struct FreightTrain {
    name: String,
}

impl FreightTrain {
    pub fn new(name: &'static str) -> Self {
        Self { name: name.into() }
    }
}

impl Train for FreightTrain {
    fn name(&self) -> &String {
        &self.name
    }

    fn arrive(&mut self, mediator: &mut dyn Mediator) {
        if !mediator.notify_about_arrival(&self.name) {
            println!("Freight train {}: Arrival blocked, waiting", self.name);
            return;
        }

        println!("Freight train {}: Arrived", self.name);
    }

    fn depart(&mut self, mediator: &mut dyn Mediator) {
        println!("Freight train {}: Leaving", self.name);
        mediator.notify_about_departure(&self.name);
    }
}

use super::Train;
use crate::train_station::Mediator;

pub struct PassengerTrain {
    name: String,
}

impl PassengerTrain {
    pub fn new(name: &'static str) -> Self {
        Self { name: name.into() }
    }
}

impl Train for PassengerTrain {
    fn name(&self) -> &String {
        &self.name
    }

    fn arrive(&mut self, mediator: &mut dyn Mediator) {
        if !mediator.notify_about_arrival(&self.name) {
            println!("Passenger train {}: Arrival blocked, waiting", self.name);
            return;
        }

        println!("Passenger train {}: Arrived", self.name);
    }

    fn depart(&mut self, mediator: &mut dyn Mediator) {
        println!("Passenger train {}: Leaving", self.name);
        mediator.notify_about_departure(&self.name);
    }
}

mod train_station;
mod trains;

use train_station::TrainStation;
use trains::{FreightTrain, PassengerTrain};

fn main() {
    let train1 = PassengerTrain::new("Train 1");
    let train2 = FreightTrain::new("Train 2");

    // Station has `accept` and `depart` methods,
    // but it also implements `Mediator`.
    let mut station = TrainStation::default();

    // Station is taking ownership of the trains.
    station.accept(train1);
    station.accept(train2);

    // `train1` and `train2` have been moved inside,
    // but we can use train names to depart them.
    station.depart("Train 1");
    station.depart("Train 2");
    station.depart("Train 3");
}

Passenger train Train 1: Arrived
Freight train Train 2: Arrival blocked, waiting
Passenger train Train 1: Leaving
Freight train Train 2: Arrived
Freight train Train 2: Leaving
'Train 3' is not on the station!
```

### swift

```swift
import XCTest

/// The Mediator interface declares a method used by components to notify the
/// mediator about various events. The Mediator may react to these events and
/// pass the execution to other components.
protocol Mediator: AnyObject {

    func notify(sender: BaseComponent, event: String)
}

/// Concrete Mediators implement cooperative behavior by coordinating several
/// components.
class ConcreteMediator: Mediator {

    private var component1: Component1
    private var component2: Component2

    init(_ component1: Component1, _ component2: Component2) {
        self.component1 = component1
        self.component2 = component2

        component1.update(mediator: self)
        component2.update(mediator: self)
    }

    func notify(sender: BaseComponent, event: String) {
        if event == "A" {
            print("Mediator reacts on A and triggers following operations:")
            self.component2.doC()
        }
        else if (event == "D") {
            print("Mediator reacts on D and triggers following operations:")
            self.component1.doB()
            self.component2.doC()
        }
    }
}

/// The Base Component provides the basic functionality of storing a mediator's
/// instance inside component objects.
class BaseComponent {

    fileprivate weak var mediator: Mediator?

    init(mediator: Mediator? = nil) {
        self.mediator = mediator
    }

    func update(mediator: Mediator) {
        self.mediator = mediator
    }
}

/// Concrete Components implement various functionality. They don't depend on
/// other components. They also don't depend on any concrete mediator classes.
class Component1: BaseComponent {

    func doA() {
        print("Component 1 does A.")
        mediator?.notify(sender: self, event: "A")
    }

    func doB() {
        print("Component 1 does B.\n")
        mediator?.notify(sender: self, event: "B")
    }
}

class Component2: BaseComponent {

    func doC() {
        print("Component 2 does C.")
        mediator?.notify(sender: self, event: "C")
    }

    func doD() {
        print("Component 2 does D.")
        mediator?.notify(sender: self, event: "D")
    }
}

/// Let's see how it all works together.
class MediatorConceptual: XCTestCase {

    func testMediatorConceptual() {

        let component1 = Component1()
        let component2 = Component2()

        let mediator = ConcreteMediator(component1, component2)
        print("Client triggers operation A.")
        component1.doA()

        print("\nClient triggers operation D.")
        component2.doD()

        print(mediator)
    }
}

Client triggers operation A.
Component 1 does A.
Mediator reacts on A and triggers following operations:
Component 2 does C.

Client triggers operation D.
Component 2 does D.
Mediator reacts on D and triggers following operations:
Component 1 does B.

Component 2 does C.

import XCTest

class MediatorRealWorld: XCTestCase {

    func test() {

        let newsArray = [News(id: 1, title: "News1", likesCount: 1),
                         News(id: 2, title: "News2", likesCount: 2)]

        let numberOfGivenLikes = newsArray.reduce(0, { $0 + $1.likesCount })

        let mediator = ScreenMediator()

        let feedVC = NewsFeedViewController(mediator, newsArray)
        let newsDetailVC = NewsDetailViewController(mediator, newsArray.first!)
        let profileVC = ProfileViewController(mediator, numberOfGivenLikes)

        mediator.update([feedVC, newsDetailVC, profileVC])

        feedVC.userLikedAllNews()
        feedVC.userDislikedAllNews()
    }
}

class NewsFeedViewController: ScreenUpdatable {

    private var newsArray: [News]
    private weak var mediator: ScreenUpdatable?

    init(_ mediator: ScreenUpdatable?, _ newsArray: [News]) {
        self.newsArray = newsArray
        self.mediator = mediator
    }

    func likeAdded(to news: News) {

        print("News Feed: Received a liked news model with id \(news.id)")

        for var item in newsArray {
            if item == news {
                item.likesCount += 1
            }
        }
    }

    func likeRemoved(from news: News) {

        print("News Feed: Received a disliked news model with id \(news.id)")

        for var item in newsArray {
            if item == news {
                item.likesCount -= 1
            }
        }
    }

    func userLikedAllNews() {
        print("\n\nNews Feed: User LIKED all news models")
        print("News Feed: I am telling to mediator about it...\n")
        newsArray.forEach({ mediator?.likeAdded(to: $0) })
    }

    func userDislikedAllNews() {
        print("\n\nNews Feed: User DISLIKED all news models")
        print("News Feed: I am telling to mediator about it...\n")
        newsArray.forEach({ mediator?.likeRemoved(from: $0) })
    }
}

class NewsDetailViewController: ScreenUpdatable {

    private var news: News
    private weak var mediator: ScreenUpdatable?

    init(_ mediator: ScreenUpdatable?, _ news: News) {
        self.news = news
        self.mediator = mediator
    }

    func likeAdded(to news: News) {
        print("News Detail: Received a liked news model with id \(news.id)")
        if self.news == news {
            self.news.likesCount += 1
        }
    }

    func likeRemoved(from news: News) {
        print("News Detail: Received a disliked news model with id \(news.id)")
        if self.news == news {
            self.news.likesCount -= 1
        }
    }
}

class ProfileViewController: ScreenUpdatable {

    private var numberOfGivenLikes: Int
    private weak var mediator: ScreenUpdatable?

    init(_ mediator: ScreenUpdatable?, _ numberOfGivenLikes: Int) {
        self.numberOfGivenLikes = numberOfGivenLikes
        self.mediator = mediator
    }

    func likeAdded(to news: News) {
        print("Profile: Received a liked news model with id \(news.id)")
        numberOfGivenLikes += 1
    }

    func likeRemoved(from news: News) {
        print("Profile: Received a disliked news model with id \(news.id)")
        numberOfGivenLikes -= 1
    }
}

protocol ScreenUpdatable: class {

    func likeAdded(to news: News)

    func likeRemoved(from news: News)
}

class ScreenMediator: ScreenUpdatable {

    private var screens: [ScreenUpdatable]?

    func update(_ screens: [ScreenUpdatable]) {
        self.screens = screens
    }

    func likeAdded(to news: News) {
        print("Screen Mediator: Received a liked news model with id \(news.id)")
        screens?.forEach({ $0.likeAdded(to: news) })
    }

    func likeRemoved(from news: News) {
        print("ScreenMediator: Received a disliked news model with id \(news.id)")
        screens?.forEach({ $0.likeRemoved(from: news) })
    }
}

struct News: Equatable {

    let id: Int

    let title: String

    var likesCount: Int

    /// Other properties

    static func == (left: News, right: News) -> Bool {
        return left.id == right.id
    }
}

News Feed: User LIKED all news models
News Feed: I am telling to mediator about it...

Screen Mediator: Received a liked news model with id 1
News Feed: Received a liked news model with id 1
News Detail: Received a liked news model with id 1
Profile: Received a liked news model with id 1
Screen Mediator: Received a liked news model with id 2
News Feed: Received a liked news model with id 2
News Detail: Received a liked news model with id 2
Profile: Received a liked news model with id 2


News Feed: User DISLIKED all news models
News Feed: I am telling to mediator about it...

ScreenMediator: Received a disliked news model with id 1
News Feed: Received a disliked news model with id 1
News Detail: Received a disliked news model with id 1
Profile: Received a disliked news model with id 1
ScreenMediator: Received a disliked news model with id 2
News Feed: Received a disliked news model with id 2
News Detail: Received a disliked news model with id 2
Profile: Received a disliked news model with id 2
```

### typescript

```typescript
/**
 * The Mediator interface declares a method used by components to notify the
 * mediator about various events. The Mediator may react to these events and
 * pass the execution to other components.
 */
interface Mediator {
    notify(sender: object, event: string): void;
}

/**
 * Concrete Mediators implement cooperative behavior by coordinating several
 * components.
 */
class ConcreteMediator implements Mediator {
    private component1: Component1;

    private component2: Component2;

    constructor(c1: Component1, c2: Component2) {
        this.component1 = c1;
        this.component1.setMediator(this);
        this.component2 = c2;
        this.component2.setMediator(this);
    }

    public notify(sender: object, event: string): void {
        if (event === 'A') {
            console.log('Mediator reacts on A and triggers following operations:');
            this.component2.doC();
        }

        if (event === 'D') {
            console.log('Mediator reacts on D and triggers following operations:');
            this.component1.doB();
            this.component2.doC();
        }
    }
}

/**
 * The Base Component provides the basic functionality of storing a mediator's
 * instance inside component objects.
 */
class BaseComponent {
    protected mediator: Mediator;

    constructor(mediator?: Mediator) {
        this.mediator = mediator!;
    }

    public setMediator(mediator: Mediator): void {
        this.mediator = mediator;
    }
}

/**
 * Concrete Components implement various functionality. They don't depend on
 * other components. They also don't depend on any concrete mediator classes.
 */
class Component1 extends BaseComponent {
    public doA(): void {
        console.log('Component 1 does A.');
        this.mediator.notify(this, 'A');
    }

    public doB(): void {
        console.log('Component 1 does B.');
        this.mediator.notify(this, 'B');
    }
}

class Component2 extends BaseComponent {
    public doC(): void {
        console.log('Component 2 does C.');
        this.mediator.notify(this, 'C');
    }

    public doD(): void {
        console.log('Component 2 does D.');
        this.mediator.notify(this, 'D');
    }
}

/**
 * The client code.
 */
const c1 = new Component1();
const c2 = new Component2();
const mediator = new ConcreteMediator(c1, c2);

console.log('Client triggers operation A.');
c1.doA();

console.log('');
console.log('Client triggers operation D.');
c2.doD();

Client triggers operation A.
Component 1 does A.
Mediator reacts on A and triggers following operations:
Component 2 does C.

Client triggers operation D.
Component 2 does D.
Mediator reacts on D and triggers following operations:
Component 1 does B.
Component 2 does C.
```