事件(二)

如何：实现接口事件

接口可声明事件。下面的示例演示如何在类中实现接口事件。接口事件的实现规则与任何接口方法或属性的实现规则基本相同。

在类中实现接口事件

在类中声明事件，然后在适当的位置调用该事件。

public interface IDrawingObject
...{
    event EventHandler ShapeChanged;
}
public class MyEventArgs : EventArgs ...{…}
public class Shape : IDrawingObject
...{
    event EventHandler ShapeChanged;
    void ChangeShape()
    ...{
        // Do something before the event…
        OnShapeChanged(new MyEventsArgs(…));
        // or do something after the event. 
    }
    protected virtual void OnShapeChanged(MyEventArgs e)
    ...{
        if(ShapeChanged != null)
        ...{
           ShapeChanged(this, e);
        }
    }
}

 

示例

下面的示例演示如何处理以下的不常见情况：您的类是从两个以上的接口继承的，每个接口都含有同名事件。在这种情况下，您至少要为其中一个事件提供显式接口实现。为事件编写显式接口实现时，必须编写add和remove事件访问器。这两个事件访问器通常由编译器提供，但在这种情况下编译器不能提供。

您可以提供自己的访问器，以便指定这两个事件是由您的类中的同一事件表示，还是由不同事件表示。例如，根据接口规范，如果事件应在不同时间引发，则可以将每个事件与类中的一个单独实现关联。在下面的示例中，订户将形状引用强制转换为IShape或IDrawingObject，从而确定自己将会接受哪个OnDraw事件。

namespace WrapTwoInterfaceEvents...{
        using System;
        
        public interface IDrawingObject...{
                //Raise this event before drawing
                //the object
                event EventHandler OnDraw;
            }
        
        public interface IShape...{
                //Raise this event after drawing
                //the shape.
                event EventHandler OnDraw;
            }
        
        //Base class event publisher inherits two
        //interfaces, each with an OnDraw event
        public class Shape : IDrawingObject, IShape...{
                //Create an event for each interface event
                event EventHandler PreDrawEvent;
                event EventHandler PostDrawEvent;
                
                //Explicit interface implementation required.
                //Associate IDrawingObject's event with
                //PreDrawEvent
                event EventHandler IDrawingObject.OnDraw...{
                        add...{
                                PreDrawEvent += value;
                            }
                        remove...{
                                PreDrawEvent -= value;
                            }
                    }
                
                //Explicit interface implementation required.
                //Associate IShape's event with
                //PostDrawEvent
                event EventHandler IShape.OnDraw...{
                        add...{
                                PostDrawEvent += value;
                            }
                        remove...{
                                PostDrawEvent -= value;
                            }
                    }
                
                //For the sake of simplicity this one method
                //implements both interfaces.
                public void Draw()...{
                        //Raise IDrawingObject's event before the object is drawn.
                        EventHandler handler = PreDrawEvent;
                        if (handler != null)...{
                                handler(this,new EventArgs());
                            }
                        Console.WriteLine("Drawing a shape.");
                        
                        //RaiseIShape's event after the object is drawn.
                        handler = PostDrawEvent;
                        if (handler != null)...{
                                handler(this,new EventArgs());
                            }
                    }
            }
            
        public class Subscriber1...{
                    //References the shape object as an IDrawingObject
                    public Subscriber1(Shape shape)...{
                            IDrawingObject d = (IDrawingObject)shape;
                            d.OnDraw += new EventHandler(d_OnDraw);
                        }
                    
                    void d_OnDraw(object sender,EventArgs e)...{
                            Console.WriteLine("Sub1 receives the IDrawingObject event.");
                        }
                }
                
        //References the shape object as an IShape
        public class Subscriber2...{
                public Subscriber2(Shape shape)...{
                        IShape d = (IShape)shape;
                        d.OnDraw += new EventHandler(d_OnDraw);
                    }
                        
                void d_OnDraw(object sender,EventArgs e)...{
                        Console.WriteLine("Sub2 receives the IShape event.");
                    }
            }
                    
        public class Program2...{
                static void Main(string[] args)...{
                        Shape shape = new Shape();
                        Subscriber1 sub = new Subscriber1(shape);
                        Subscriber2 sub2 = new Subscriber2(shape);
                        shape.Draw();
                        
                        Console.WriteLine("Press Enter to close this window.");
                        Console.ReadLine();
                    }
            }
    }

如何：使用字典存储事件实例

accessor-declarations的一种用法是公开大量的事件但不为每个事件分配字段，而是使用字典来存储这些事件实例。这只有在具有非常多的事件、但您预计大部分事件都不会实现时才有用。

示例

public delegate void EventHandler1(int i);
public delegate void EventHandler2(string s);

public class PropertyEventsSample...{
        private System.Collections.Generic.Dictionary<string,System.Delegate> eventTable;
        
        public PropertyEventsSample()...{
                eventTable = new System.Collections.Generic.Dictionary<string,System.Delegate>();
                eventTable.Add("Event1",null);
                eventTable.Add("Event2",null);
            }
        
        public event EventHandler1 Event1...{
                add...{
                        eventTable["Event1"] = (EventHandler1)eventTable["Event1"] + value;
                    }
                remove...{
                        eventTable["Event1"] = (EventHandler1)eventTable["Event1"] - value;
                    }
            }
        
        public event EventHandler2 Event2...{
                add...{
                        eventTable["Event2"] = (EventHandler2)eventTable["Event2"] + value;
                    }
                remove...{
                        eventTable["Event2"] = (EventHandler2)eventTable["Event2"] - value;
                    }
            }
        
        internal void RaiseEvent1(int i)...{
                EventHandler1 handler1;
                if (null != (handler1 = (EventHandler1)eventTable["Event1"]))...{
                        handler1(i);
                    }
            }
        
        internal void RaiseEvent2(string s)...{
                EventHandler2 handler2;
                if (null != (handler2 = (EventHandler2)eventTable["Event2"]))...{
                        handler2(s);
                    }
            }
    }


public class TestClass2...{
        public static void Delegate1Method(int i)...{
                System.Console.WriteLine(i);
            }
        
        public static void Delegate2Method(string s)...{
                System.Console.WriteLine(s);
            }
        
        static void Main()...{
                PropertyEventsSample p = new PropertyEventsSample();
                
                p.Event1 += new EventHandler1(TestClass.Delegate1Method);
                p.Event1 += new EventHandler1(TestClass.Delegate1Method);
                p.Event1 -= new EventHandler1(TestClass.Delegate1Method);
                p.RaiseEvent1(2);
                
                p.Event2 += new EventHandler2(TestClass.Delegate2Method);
                p.Event2 += new EventHandler2(TestClass.Delegate2Method);
                p.Event2 -= new EventHandler2(TestClass.Delegate2Method);
                p.RaiseEvent2("TestString");
            }
    }