Mediator

• Defines an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.

Classical Structure

Scenario

• Build a dialog box/form for editing articles using GUI classes from some framework TextBox, ListBox, Button.
• When you select article in listbox, textbox should be populated with the title of article and save button should be enabled.
• When textbox is cleared the button should be disabled
• When button is clicked, the text should be fetched

Problem

• UI controls need to collaborate
• You can't change the source code of UI controls based on form/dialog

Solution

• ConcreteColleagues doesn't know about each other. They talk to each other via mediator
• Coupling between ConcreteMediator and ConcreteColleague

internal abstract class DialogBox
{
   public abstract void Changed(UIControl control);
}
internal class UIControl
{
    protected readonly DialogBox owner;

    public UIControl(DialogBox owner)
    {
        this.owner = owner;
    }
}
internal class TextBox : UIControl
{
    private string content;

    public TextBox(DialogBox owner) : base(owner)
    {
    }

    public string Content
    {
        get => content;
        set
        {
            content = value;
            owner.Changed(this);
        }
    }
}
internal class ListBox : UIControl
{
    private string selection;

    public ListBox(DialogBox owner) : base(owner)
    {
    }

    public string Selection
    {
        get => selection;
        set
        {
            selection = value;
            owner.Changed(this);
        }
    }
}
internal class Button : UIControl
{
    bool isEnabled;

    public Button(DialogBox owner) : base(owner)
    {
    }

    public bool IsEnabled { 
        get => isEnabled;
        set { 
            isEnabled = value;
            owner.Changed(this);
        }
    }
}
internal class ArticleDialogBox : DialogBox
{
    private ListBox articlesListBox;
    private TextBox titleTextBox;
    private Button saveButton;

    public ArticleDialogBox()
    {
        articlesListBox =  new ListBox(this);
        titleTextBox = new TextBox(this);
        saveButton = new Button(this);
    }

    public void SimulateUserInteraction()
    {
        articlesListBox.Selection = "Article1";
        //titleTextBox.Content = "";
        Console.WriteLine(titleTextBox.Content);
        Console.WriteLine(saveButton.IsEnabled);

    }

    public override void Changed(UIControl control)
    {
        if (control == articlesListBox)
            articleSelected();
        else if(control == titleTextBox)
            titleChanged();
    }

    private void articleSelected()
    {
        titleTextBox.Content = articlesListBox.Selection;
        saveButton.IsEnabled = true;
    }

    private void titleChanged()
    {
        var content = titleTextBox.Content;
        saveButton.IsEnabled = !String.IsNullOrEmpty(content);
    }
}
public class Program
{
    static void Main(string[] args)
    {
	    var articleDialogBox = new ArticleDialogBox();
		articleDialogBox.SimulateUserInteraction();
	}
}

Example Structure

Example Code

Mediator Using Observer

Example Code

internal abstract class UIControl
{
    List<Action> eventHandlers = new List<Action>();

    public void AddEventHandler(Action eventHandler)
    {
        eventHandlers.Add(eventHandler);
    }

    protected void NotifyEventHandlers()
    {
        foreach (Action handler in eventHandlers)
        {
            handler.Invoke();
        }
    }
}
internal class TextBox : UIControl
{
    private string content;

    public string Content
    {
        get => content;
        set
        {
            content = value;
            NotifyEventHandlers();
        }
    }
}
internal class ListBox : UIControl
{
    private string selection;

    public string Selection
    {
        get => selection;
        set
        {
            selection = value;
            NotifyEventHandlers();
        }
    }
}
internal class Button : UIControl
{
    bool isEnabled;

    public bool IsEnabled
    {
        get => isEnabled;
        set
        {
            isEnabled = value;
            NotifyEventHandlers();
        }
    }
}
internal class ArticleDialogBox
{
    private ListBox articlesListBox;
    private TextBox titleTextBox;
    private Button saveButton;

    public ArticleDialogBox()
    {
        articlesListBox = new ListBox();
        articlesListBox.AddEventHandler(ArticleSelected);

        titleTextBox = new TextBox();
        titleTextBox.AddEventHandler(TitleChanged);

        saveButton = new Button();
    }

    public void SimulateUserInteraction()
    {
        articlesListBox.Selection = "Article1";
        titleTextBox.Content = "";
        Console.WriteLine(titleTextBox.Content);
        Console.WriteLine(saveButton.IsEnabled);

    }

    private void ArticleSelected()
    {
        titleTextBox.Content = articlesListBox.Selection;
        saveButton.IsEnabled = true;
    }

    private void TitleChanged()
    {
        var content = titleTextBox.Content;
        saveButton.IsEnabled = !String.IsNullOrEmpty(content);
    }
}
public class Program
{
    static void Main(string[] args)
    {
	    var articleDialogBox = new ArticleDialogBox();
		articleDialogBox.SimulateUserInteraction();
	}
}