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[ASP.net教程]C#事件(Event)学习日记


event 关键字的来由,为了简化自定义方法的构建来为委托调用列表增加和删除方法。

在编译器处理 event 关键字的时候,它会自动提供注册和注销方法以及任何必要的委托类型成员变量。

这些委托成员变量总是声明为私有的,因此不能直接从触发事件对象访问它们。

 

温馨提示:如果您对于委托不是很了解,您可以先看 C#委托(Delegate) ,这对您理解本章会有所帮助。

定义一个事件的步骤:

  1. 需要定义一个委托,它包含事件触发时将要调用方法
  2. 通过 event 关键字用相关委托声明这个事件

话不多说,我们来看一个示例:

 

1. 定义Car类:

public class Car{  // 这个委托用来与Car事件协作  public delegate void CarEngineHandler(string msg);

  // 这种汽车可以发送这些事件 public event CarEngineHandler Exploded; public event CarEngineHandler AboutToBlow; public int CurrentSpeed { get; set; } public int MaxSpeed { get; set; } public string PetName { get; set; } private bool CarIsDead; public Car() { MaxSpeed = 100; } public Car(string name, int maxSp, int currSp) { CurrentSpeed = currSp; MaxSpeed = maxSp; PetName = name; } public void Accelerate(int delta) { // 如果Car无法使用了,触发Exploded事件 if (CarIsDead) { if (Exploded != null) { Exploded("sorry,this car is dead"); } } else { CurrentSpeed += delta; // 确认已无法使用,触发AboutToBlow事件 if ((MaxSpeed - CurrentSpeed) == 10 && AboutToBlow != null) { AboutToBlow("careful buddy ! gonna blow !"); } if (CurrentSpeed >= MaxSpeed) { CarIsDead = true; } else { Console.WriteLine($"CurrentSpeed={CurrentSpeed}"); } } }}

 以上我们已经设定了Car对象发送两个自定义事件,这不再需要自定义注册函数,也不需要声明委托成员变量。稍后我们将说到如何使用这个汽车,在此之前,让我们了解一下事件的架构,揭开事件的神秘面纱。

 

2. 事件神秘面纱

C#事件事实上会扩展两个隐藏的公共方法,一个 add_事件名称,一个 remove_事件名称。

 

add_Exploded() CIL指令

 

remove_Exploded() CIL指令

 

代表事件本身的CIL代码使用 .addon 和 .removeon 指令调用对应的 add_xxx() 和 remove_xxx()方法

 

3. 使用Car类

了解了这些之后,我们来使用之前定义的Car类:

public class MyEvent{  public static void Show()  {    WriteLine("fun with events");    Car c1 = new Car("bwm", 100, 10);    // 注册事件处理程序    c1.AboutToBlow += new Car.CarEngineHandler(CarIsAlomostDoomed);    c1.AboutToBlow += new Car.CarEngineHandler(CarAboutToBlow);    Car.CarEngineHandler d = new Car.CarEngineHandler(CarExploded);    c1.Exploded += d;    WriteLine("******Speeding up******");    for (int i = 0; i < 6; i++)    {      c1.Accelerate(20);    }    // 注销,从调用列表中移除CarExploded()方法    c1.Exploded -= d;    WriteLine("******Speeding up******");    for (int i = 0; i < 6; i++)    {      c1.Accelerate(20);    }  }  private static void CarExploded(string msg) => WriteLine($"CarExploded-> {msg}");  private static void CarAboutToBlow(string msg) => WriteLine($"CarAboutToBlow=>{msg}");  private static void CarIsAlomostDoomed(string msg) => WriteLine($"CarIsAlomostDoomed-> {msg}");}

 

运行效果图:

 

为了进一步简化事件注册,我们可以用到委托章节学习到的方法组转换语法(解释:我可以在调用以委托作为参数的方法时,直接提供方法的名称,而不是委托对象)

下面请看使用方法组转换,注册和注销事件,粗体部分:

public static void Show(){  WriteLine("fun with events");  Car c1 = new Car("bwm", 100, 10);  // 注册事件处理程序   c1.AboutToBlow += CarIsAlomostDoomed;  c1.AboutToBlow += CarAboutToBlow;  c1.Exploded += CarExploded;  WriteLine("******Speeding up******");  for (int i = 0; i < 6; i++)  {    c1.Accelerate(20);  }
// 注销,从调用列表中移除CarExploded()方法 c1.Exploded -= CarExploded; WriteLine("******Speeding up******"); for (int i = 0; i < 6; i++) { c1.Accelerate(20); }}

 

4. 创建自定义事件参数

微软的事件模式:(System.Object sender,System.EventArgs args)这一两个参数的模型。

第一个参数 sender :表示一个对发送事件的对象(Car)的引用,

第二个参数 args :与该事件相关的信息

System.EventArgs 基类源代码:

public class EventArgs {  public static readonly EventArgs Empty = new EventArgs();    public EventArgs()   {  }}

那么对于简单的事件类型来说,我们可以直接传递一个EventArgs的实例,但是如果我们期望传递自定义的数据,就应该从System.EventArgs派生出一个子类。
我们接下来就为我们的 Car 自定义一个符合这种事件模式的事件参数,新建一个 CarEventArgs 类,包含一个字符串,表示要发送给接收者的信息:

public class CarEventArgs : EventArgs{  public readonly string msg;  public CarEventArgs(string message)  {    msg = message;  }}

 

我们修改一下Car类,新添加一个 CarCustomEngineHandler 委托,并且更改相应的事件代码:

public class Car{  public delegate void CarCustomEngineHandler(object sender, CarEventArgs e);  // 模仿微软正规(object sender, EventArgs e)写法  public event CarCustomEngineHandler CustomExploded;  public event CarCustomEngineHandler CustomAboutToBlow;  public void AccelerateCustom(int delta)  {    if (CarIsDead)    {      if (CustomExploded != null)      {        CustomExploded(this, new CarEventArgs("sorry,this car is dead"));      }    }    else    {      CurrentSpeed += delta;      if ((MaxSpeed - CurrentSpeed) == 10 && CustomAboutToBlow != null)      {        CustomAboutToBlow(this, new CarEventArgs("careful buddy ! gonna blow !"));      }      if (CurrentSpeed >= MaxSpeed)      {        CarIsDead = true;      }      else      {        Console.WriteLine($"CurrentSpeed={CurrentSpeed}");      }    }  }}

 

看一下调用粗体部分(是如何使用传递的参数sender,e的):

public class MyCustomEvents{  public static void Show()  {    WriteLine("fun with events");    Car c1 = new Car("bwm", 100, 10);    c1.CustomAboutToBlow += CarIsAlomostDoomed;    c1.CustomAboutToBlow += CarAboutToBlow;    Car.CarCustomEngineHandler d = CarExploded;    c1.CustomExploded += d;    WriteLine("******Speeding up******");    for (int i = 0; i < 6; i++)    {      c1.AccelerateCustom(20);    }    c1.CustomExploded -= d;    WriteLine("******Speeding up******");    for (int i = 0; i < 6; i++)    {      c1.AccelerateCustom(20);    }  }  private static void CarExploded(object sender, CarEventArgs e) => WriteLine($"CarExploded->{((Car)sender)?.PetName} {e.msg}");  private static void CarAboutToBlow(object sender, CarEventArgs e) => WriteLine($"CarAboutToBlow=>{((Car)sender)?.PetName} {e.msg}");  private static void CarIsAlomostDoomed(object sender, CarEventArgs e) => WriteLine($"CarIsAlomostDoomed->{((Car)sender)?.PetName} {e.msg}");}


5. 泛型 EventHandler<T> 委托

public delegate void EventHandler<TEventArgs>(object sender, TEventArgs e);

由于很多自定义委托接受(object,EventArgs)这样的参数结构,那么我们可以使用框架内置的 EventHandler<> 来简化我们的事件 委托。

 

首先修改一下Car类:

public class Car{  public event EventHandler<CarEventArgs> StandardExploded;  public event EventHandler<CarEventArgs> StandardAboutToBlow;  public void AccelerateStandard(int delta)  {    if (CarIsDead)    {      if (StandardExploded != null)      {        StandardExploded(this, new CarEventArgs("sorry,this car is dead"));      }    }    else    {      CurrentSpeed += delta;      if ((MaxSpeed - CurrentSpeed) == 10 && StandardAboutToBlow != null)      {        StandardAboutToBlow(this, new CarEventArgs("careful buddy ! gonna blow !"));      }      if (CurrentSpeed >= MaxSpeed)      {        CarIsDead = true;      }      else      {        Console.WriteLine($"CurrentSpeed={CurrentSpeed}");      }    }  }}


调用代码其实和上一段并没有太大差异,这里还是贴出来:

public class MyStandardEvent{  public static void Show()  {    WriteLine("fun with events");    Car c1 = new Car("bwm", 100, 10);    c1.StandardAboutToBlow += CarIsAlomostDoomed;    c1.StandardAboutToBlow += CarAboutToBlow;    EventHandler<CarEventArgs> d = CarExploded;    c1.StandardExploded += d;    WriteLine("******Speeding up******");    for (int i = 0; i < 6; i++)    {      c1.AccelerateStandard(20);    }    c1.StandardExploded -= d;    WriteLine("******Speeding up******");    for (int i = 0; i < 6; i++)    {      c1.AccelerateStandard(20);    }  }  private static void CarExploded(object sender, CarEventArgs e) => WriteLine($"CarExploded->{((Car)sender)?.PetName} {e.msg}");  private static void CarAboutToBlow(object sender, CarEventArgs e) => WriteLine($"CarAboutToBlow=>{((Car)sender)?.PetName} {e.msg}");  private static void CarIsAlomostDoomed(object sender, CarEventArgs e) => WriteLine($"CarIsAlomostDoomed->{((Car)sender)?.PetName} {e.msg}");}

 

 

6.匿名方法

这么简单的处理操作, CarExploded() ,CarAboutToBlow()这一的方法很少会被调用委托之外的任何程序所调用。从生成效率来说,手工定义一个由委托对象调用的方法有点麻烦耶。

为了解决这种情况,现在事件注册时,可以直接将一个委托与一段代码关联 -- 匿名方法

我们修改一下调用Car类的地方(注意粗体部分、最后一个大括号 ";" 结束):

public class MyAnonymousMtehoden{  public static void Show()  {    int aboutToBlowCounter = 0;    WriteLine("fun with events");    Car c1 = new Car("bwm", 100, 10);    c1.StandardAboutToBlow += delegate    {      WriteLine("Eek,going to fast");    };    c1.StandardAboutToBlow += delegate (object sender, CarEventArgs e)    {      aboutToBlowCounter++;      WriteLine($"CarAboutToBlow=>{((Car)sender)?.PetName} {e.msg}");    };    c1.StandardExploded += delegate (object sender, CarEventArgs e)    {      aboutToBlowCounter++;      WriteLine($"Exploded=>{((Car)sender)?.PetName} {e.msg}");    };    for (int i = 0; i < 6; i++)    {      c1.AccelerateStandard(20);    }    WriteLine($"aboutToBlowCounter={aboutToBlowCounter}");  }}

 

本文参考《精通C#》

 

学无止境,望各位看官多多指教。