5. 第二章：C#委托和事件之事件

第二章：C#委托和事件

 

第二节：事件

事件其实没什么不好理解的，声明一个事件不过类似于声明一个委托类型的变量而已。

    public class MyTestClass
    {
        //定义委托
        public delegate void MyDelegateHandler(string param);

        //构造函数
        public MyTestClass()
        {
            this.MyDelegateHappened += this.MyInsideDelegateMethod;
        }

        //内部方法
        private void MyInsideDelegateMethod(string param)
        {
            Console.WriteLine("I am running inside the class "MyClass" -> void MyInsideDelegateMethod(string param)");
            Console.WriteLine(param);
        }

        //执行
        public void Execute(string param)
        {
            this.OnMyDeletateHappened(param);
        }

        //事件
        public event MyDelegateHandler MyDelegateHappened;
        protected virtual void OnMyDeletateHappened(string param)
        {
            MyDelegateHandler handler = this.MyDelegateHappened;
            if (handler != null)
            {
                handler(param);
            }
        }
    }

调用：

        static void Main(string[] args)
        {
            Console.WriteLine("Only the inside method will be executed!");
            //实例化
            MyTestClass myTestClass = new MyTestClass();
            myTestClass.Execute("Hello, world!");
            Console.WriteLine();

            Console.WriteLine("The outside method will be added to the delegate logic!");
            //新增外部委托的方法
            myTestClass.MyDelegateHappened += MyOutsideDelegateMethod;
            myTestClass.Execute("Hello, world!");
            Console.WriteLine();

            Console.WriteLine("The outside method will be removed from the delegate logic!");
            //去除外部委托的方法
            myTestClass.MyDelegateHappened -= MyOutsideDelegateMethod;
            myTestClass.Execute("Hello, world!");
            Console.ReadLine();
        }

        private static void MyOutsideDelegateMethod(string param)
        {
            Console.WriteLine("I am running ouside the class "Program" -> void MyOutsideDelegateMethod(string param)");
            Console.WriteLine(param);
        }

运行结果：

Only the inside method will be executed!

I am running inside the class "MyClass" -> void MyInsideDelegateMethod(string param)

Hello, world!

 

The outside method will be added to the delegate logic!

I am running inside the class "MyClass" -> void MyInsideDelegateMethod(string param)

Hello, world!

I am running ouside the class "Program" -> void MyOutsideDelegateMethod(string param)

Hello, world!

 

The outside method will be removed from the delegate logic!

I am running inside the class "MyClass" -> void MyInsideDelegateMethod(string param)

Hello, world!

 

相信有部分读者看到这里，会想起C#里面常见的EventHandler

a. 不带参数的

        public event EventHandler MyEventHappened;
        protected virtual void OnMyEventHanppened()
        {
            EventHandler handler = this.MyEventHappened;
            if (handler != null)
            {
                handler(this, EventArgs.Empty);
            }
        }

其实，请看EventHandler实际的本质是什么：（F12 Go To Definition）

#region Assembly mscorlib.dll, v4.0.0.0
// C:Program Files (x86)Reference AssembliesMicrosoftFramework.NETFrameworkv4.5mscorlib.dll
#endregion

using System.Runtime.InteropServices;

namespace System
{
    // Summary:
    //     Represents the method that will handle an event that has no event data.
    //
    // Parameters:
    //   sender:
    //     The source of the event.
    //
    //   e:
    //     An System.EventArgs that contains no event data.
    [Serializable]
    [ComVisible(true)]
    public delegate void EventHandler(object sender, EventArgs e);
}

b. 带参数的

        public event EventHandler<EventArgs> MyEventHappened;
        protected virtual void OnMyEventHanppened(EventArgs args)
        {
            EventHandler<EventArgs> handler = this.MyEventHappened;
            if (handler != null)
            {
                handler(this, args);
            }
        }

#region Assembly mscorlib.dll, v4.0.0.0
// C:Program Files (x86)Reference AssembliesMicrosoftFramework.NETFrameworkv4.5mscorlib.dll
#endregion

namespace System
{
    // Summary:
    //     Represents the method that will handle an event.
    //
    // Parameters:
    //   sender:
    //     The source of the event.
    //
    //   e:
    //     An System.EventArgs that contains the event data.
    //
    // Type parameters:
    //   TEventArgs:
    //     The type of the event data generated by the event.
    [Serializable]
    public delegate void EventHandler<TEventArgs>(object sender, TEventArgs e);
}

带参数的EventArgs，可以我们自己定义继承类去传递我们需要的数据：

    public class MyEventArgs : EventArgs
    {
        private string _param;

        public MyEventArgs(string param)
        {
            this._param = param;
        }

        public string Param
        {
            get { return this._param; }
        }
    }

定义event的时候：

        public event EventHandler<MyEventArgs> MyEventHappened2;
        protected virtual void OnMyEventHappened2(MyEventArgs args)
        {
            EventHandler<MyEventArgs> handler = this.MyEventHappened2;
            if (handler != null)
            {
                handler(this, args);
            }
        }

上述两个示例说明：EventHandler本质，依然是一个delegate！只不过它是由系统定义好了而已，现在编程者常用这种方式，便于使用。

代码：

    public class MyTestClass2 {
        public void Execute(string param)
        {
            this.OnMyEventHappened(new MyEventArgs(param));
        }

        public event EventHandler<MyEventArgs> MyEventHappened;
        protected virtual void OnMyEventHappened(MyEventArgs args)
        {
            EventHandler<MyEventArgs> handler = this.MyEventHappened;
            if (handler != null)
            {
                handler(this, args);
            }
        }
    }

    public class MyEventArgs : EventArgs
    {
        private string _param;

        public MyEventArgs(string param)
        {
            this._param = param;
        }

        public string Param
        {
            get { return this._param; }
        }
    }

调用：

        static void Main(string[] args)
        {
            MyTestClass2 myTestClass = new MyTestClass2();
            myTestClass.MyEventHappened += MyTestClass_MyEventHappened;
            myTestClass.Execute("Hi~");

            Console.ReadLine();
        }

        private static void MyTestClass_MyEventHappened(object sender, MyEventArgs e)
        {
            Console.WriteLine(e.Param);
        }

运行结果：
Hi~

此程序中：myTestClass.Execute("Hi~");其实也可以由类的内部调用——这样做，就可以做到类的实例内部触发事件，然后做到“通知”外部注册时逻辑被执行的效果！

综上所述，大家是不是对委托和事件有更清晰的认识了呢？