笔记：Gof设计模式--Mediator

1、意图

Define an object that encapsulates how a set of objects interact.Mediator promotes loose coupling by keeping objects from referring toeach other explicitly, and it lets you vary their interactionindependently.

2、适应性

Use the Mediator pattern when

• a set of objects communicate in well-defined but complex ways. Theresulting interdependencies are unstructured and difficult tounderstand.

• reusing an object is difficult because it refers to and communicateswith many other objects.

• a behavior that's distributed between several classes should becustomizable without a lot of subclassing.

3、结构





4、示例代码

We'll use a DialogDirector to implement the font dialog box shown inthe Motivation. The abstract class DialogDirector definesthe interface for directors.

class DialogDirector {
public:
virtual ~DialogDirector();
virtual void ShowDialog();
virtual void WidgetChanged(Widget*) = 0;
protected:
DialogDirector();
virtual void CreateWidgets() = 0;
};

Widget is the abstract base class for widgets. Awidget knows its director.

class Widget {
public:
Widget(DialogDirector*);
virtual void Changed();
virtual void HandleMouse(MouseEv
// ...
private:
DialogDirector* _director;
};

//  Widgets call WidgetChanged on their director toinform it of a significant event.
void Widget::Changed ()
{
_director->WidgetChanged(this);
}

The ListBox, EntryField, and Button aresubclasses of Widget for specialized user interfaceelements. ListBox provides a GetSelectionoperation to get the current selection, and EntryField'sSetText operation puts new text into
the field.

class ListBox : public Widget {
public:
ListBox(DialogDirector*);
virtual const char* GetSelection();
virtual void SetList(List<char*>* listItems);
virtual void HandleMouse(MouseEvent& event);
// ...
};

class EntryField : public Widget {
public:
EntryField(DialogDirector*);
virtual void SetText(const char* text);
virtual const char* GetText();
virtual void HandleMouse(MouseEvent& event);
// ...
};

class Button : public Widget {
public:
Button(DialogDirector*);
virtual void SetText(const char* text);
virtual void HandleMouse(MouseEvent& event);
// ...
};

void Button::HandleMouse (MouseEvent& event) {
// ...
Changed();
}

The FontDialogDirector class mediates between widgets in thedialog box. FontDialogDirector is a subclass ofDialogDirector:

class FontDialogDirector : public DialogDirector {
public:
FontDialogDirector();
virtual ~FontDialogDirector();
virtual void WidgetChanged(Widget*);
protected:
virtual void CreateWidgets();
private:
Button* _ok;
Button* _cancel;
ListBox* _fontList;
EntryField* _fontName;
};

// FontDialogDirector keeps track of the widgets it displays.
// It redefines CreateWidgets to create the widgets and initialize itsreferences to
// them:
void FontDialogDirector::CreateWidgets () {
_ok = new Button(this);
_cancel = new Button(this);
_fontList = new ListBox(this);
_fontName = new EntryField(this);
// fill the listBox with the available font names
// assemble the widgets in the dialog
}

// WidgetChanged ensures that the widgets work together properly:
void FontDialogDirector::WidgetChanged ( Widget* theChangedWidget ) {
if (theChangedWidget == _fontList) {
_fontName->SetText(_fontList->GetSelection());
} else if (theChangedWidget == _ok) {
// apply font change and dismiss dialog
// ...
} else if (theChangedWidget == _cancel) {
// dismiss dialog
}
}