Chain of Responsibility - 职责链模式

Class EventHandler {
public:
    EventHandler(EventHandler* eh) : m_handler(eh) { }
    virtual void handle(Event* e) { if (m_handler) m_handler->handle(e); }
private:
    EventHandler* m_handler;
}
class Application : EventHandler {
public:
    Applicatoin();
    void exec() { ... }
}
class Widget : EventHandler {
public:
    Widget(Widget* parent) : EventHandler(parent) { }
    void draw() { ... }
}
class Window : public Widget {
public:
    virtual void handle(Event* e) {
        if(e->type() == WheelEvent)
            ...;
        else
            Widget::Handle(e);
    }
}
class Button : public Widget{
public:
    virutal void handle(Event* e) {
        if(e->type == MousePressEvent)
            ...
        else
            Widget::handle(e);
    }
}
int main()
{
    Application app;
    Widget widget;
    Button button(widget);
    return app.exe();
}