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C#语言进阶(二)—事件全解

news 来源：原创 2024/9/29 7:19:38

总目录
C# 语法总目录

事件全解目录

事件
- 1.基本用法发布订阅模式
- 2. .net标准事件模型
- 3. 事件访问器

事件

事件修饰符: virtual , override , abstract , sealed

解释：

广播者是声明委托并使用的类，它控制什么时候使用委托。

订阅者就是给广播类声明的委托，使用 += 或 -= 改变委托数量的这一操作过程者。

用法：声明事件就是在声明委托时前面加上 event 关键字。

作用：事件的作用是让委托的广播者和订阅者的结构更清晰。

过程：编译器看到这个委托添加了事件关键字，会自动让委托变成私有的，然后给它添加对应的访问器类似类属性的{set;get;}，这样订阅者就只能通过访问器来修改自己的委托。

所以，简单来说，事件就是把委托默认设置为私有，然后创建一个默认的访问器让外部只能通过访问器+=或-=修改委托的成员，且不能将这个委托置null。

1.基本用法发布订阅模式

#region 普通的  发布订阅模式
public delegate void ScoreChangeHandle(decimal oldScore, decimal newScore);
//广播者
internal class BroadCaster
{private string? name;private decimal score;public event ScoreChangeHandle? ScoreChangedEvent;public BroadCaster(string name){this.name = name;}public decimal Score{get { return score;}set{if (score == value) return;decimal oldScore = score;score = value;ScoreChangedEvent?.Invoke(oldScore, score);}}
}
//订阅者
internal class Subscriber
{private readonly string _id;public Subscriber(string id, BroadCaster broad){_id = id;broad.ScoreChangedEvent += ScoreChange;}void ScoreChange(decimal oldScore, decimal newScore){Console.WriteLine("this id is: "+_id+",  oldscore is " + oldScore + "  ,new Score is: " + newScore+"  ,time is: "+ DateTime.Now);}
}
#endregionstatic void Main(string[] args)
{BroadCaster bc = new BroadCaster("bc1");Subscriber sub1 = new Subscriber("01", bc);Subscriber sub2 = new Subscriber("02", bc);bc.Score = 10;
}//输出
this id is: 01,  oldscore is 0  ,new Score is: 10  ,time is: 2024/4/18 16:19:33
this id is: 02,  oldscore is 0  ,new Score is: 10  ,time is: 2024/4/18 16:19:33

2. .net标准事件模型

标准事件模型是 .net framwork 定义的一个标准。可用可不用，只是一个标准而已。

官方为这个标准定义了一个事件参数类，用于给事件传递参数。这就是上面说的，这个模型可用可不用，不用官方的，自己也能做一个类似的，做个开发，没必要搞得这么复杂。

以下是上面案例根据标准事件模型的修改版本。

这里使用 .net framwork的标准事件模型参数类： System.EventArgs 类，来模拟标准事件模型

#region .net Framework 标准事件模型
//继承标准事件模型参数类型
//这个父类啥都没有，只有一个静态参数，一个构造方法，可以点进去看
public class ScoreChangedEventArgs : EventArgs
{public static readonly new ScoreChangedEventArgs? Empty;//通常标准事件模型传递的参数设置为只读类型public readonly decimal oldScore;public readonly decimal newScore;public ScoreChangedEventArgs(decimal oldScore,decimal newScore){this.oldScore = oldScore;this.newScore = newScore;}
}
//发布者
public class BroadCasterStandar
{private string? name;private decimal score;//事件标准委托public event EventHandler<ScoreChangedEventArgs>? ScoreChanged;protected virtual void OnScoreChanged(ScoreChangedEventArgs? e){ScoreChanged?.Invoke(this, e);}public BroadCasterStandar(string name){this.name = name;}public decimal Score{get { return score; }set{if (score == value) return;decimal oldScore = score;score = value;OnScoreChanged(new ScoreChangedEventArgs(oldScore, score));//如果不需要传值，那么可以用下面代替//OnScoreChanged(ScoreChangedEventArgs.Empty);}}
}
//订阅者
internal class SubscriberStandar
{private readonly string _id;public SubscriberStandar(string id, BroadCasterStandar broad){_id = id;//订阅信息broad.ScoreChanged += ScoreChanged;}//处理广播信息void ScoreChanged(object? obj, ScoreChangedEventArgs e){if (e == ScoreChangedEventArgs.Empty){return;}Console.WriteLine("this id is: " + _id + ",  oldscore is " + e.oldScore + "  ,new Score is: " + e.newScore + "  ,time is: " + DateTime.Now);}
}
#endregionstatic void Main(string[] args)
{BroadCasterStandar bcs = new BroadCasterStandar("bcs");SubscriberStandar sbs1 = new SubscriberStandar("01", bcs);SubscriberStandar sbs2 = new SubscriberStandar("02", bcs);//广播信息bcs.Score = 15;
}//输出
this id is: 01,  oldscore is 0  ,new Score is: 15  ,time is: 2024/4/18 16:43:12
this id is: 02,  oldscore is 0  ,new Score is: 15  ,time is: 2024/4/18 16:43:12

3. 事件访问器

默认情况下，编译器会默认实现事件的访问器，如果显示的去实现，那么编译器就不会自动取生成默认的访问器。

从功能上，自己手动写访问器和编译器默认生成是一样的，但是编译器默认生成的使用了无锁的比较并交换算法，保证在更新委托时的线程安全性，在多线程情况下更安全些。

一般情况不需要自己去显示写事件访问器，如果需要更多高级操作，那么就不得不用。

以下是一些可能需要用的情况 (当然也可以不用)：

当事件很多，订阅者却不多，这种情况下需要自定义访问器的add方法，在方法里使用字典来存储委托引用，这样比原来的开销小
当广播类继承了多个事件接口，并且有多个接口的事件名称是相同的，那么需要把这些接口的事件显示的去创建访问器，用来区分它们用哪个访问器访问

#region 继承多个接口 显示创建访问器
public interface IMathScore
{event EventHandler<ScoreChangedCusEventArgs> ScoreChanged;
}
public interface IEnglishScore
{event EventHandler<ScoreChangedCusEventArgs> ScoreChanged;
}public class ScoreChangedCusEventArgs : EventArgs
{public static readonly new ScoreChangedCusEventArgs? Empty;//通常标准事件模型传递的参数设置为只读类型public readonly decimal oldScore;public readonly decimal newScore;public ScoreChangedCusEventArgs(decimal oldScore, decimal newScore){this.oldScore = oldScore;this.newScore = newScore;}
}
//发布者
public class BroadCasterCusStandar:IMathScore,IEnglishScore
{private string? name;private decimal englishScore;private decimal mathScore;event EventHandler<ScoreChangedCusEventArgs> MathEvent;event EventHandler<ScoreChangedCusEventArgs> EnglishEvent;object objectLock = new Object();event EventHandler<ScoreChangedCusEventArgs>? IMathScore.ScoreChanged{add {lock (objectLock){MathEvent += value;}}remove {lock (objectLock){MathEvent -= value;}}}event EventHandler<ScoreChangedCusEventArgs>? IEnglishScore.ScoreChanged{add{lock (objectLock){EnglishEvent += value;}}remove{lock (objectLock){EnglishEvent -= value;}}}protected virtual void OnMathScoreChanged(ScoreChangedCusEventArgs? e){MathEvent?.Invoke(this, e);}protected virtual void OnEnglishScoreChanged(ScoreChangedCusEventArgs? e){EnglishEvent?.Invoke(this, e);}public BroadCasterCusStandar(string name){this.name = name;}public decimal MathScore{get { return mathScore; }set{if (mathScore == value) return;decimal oldMathScore = mathScore;mathScore = value;OnMathScoreChanged(new ScoreChangedCusEventArgs(oldMathScore, mathScore));//如果不需要传值，那么可以用下面代替//OnMathScoreChanged(ScoreChangedCusEventArgs.Empty);}}public decimal EnglishScore{get { return englishScore; }set{if (englishScore == value) return;decimal oldEnglishScore = englishScore;englishScore = value;OnEnglishScoreChanged(new ScoreChangedCusEventArgs(oldEnglishScore, englishScore));//如果不需要传值，那么可以用下面代替//OnEnglishScoreChanged(ScoreChangedCusEventArgs.Empty);}}
}
//订阅者
internal class SubscriberCus1Standar
{private readonly string _id;public SubscriberCus1Standar(string id, BroadCasterCusStandar broad){_id = id;IEnglishScore englishBroad = (IEnglishScore)broad;//订阅信息englishBroad.ScoreChanged += ScoreChanged;}//处理广播信息void ScoreChanged(object? obj, ScoreChangedCusEventArgs e){if (e == ScoreChangedCusEventArgs.Empty){return;}Console.WriteLine("this id is: " + _id + ",  oldscore is " + e.oldScore + "  ,new Score is: " + e.newScore + "  ,time is: " + DateTime.Now);}
}
internal class SubscriberCus2Standar
{private readonly string _id;public SubscriberCus2Standar(string id, BroadCasterCusStandar broad){_id = id;IMathScore englishBroad = (IMathScore)broad;//订阅信息englishBroad.ScoreChanged += ScoreChanged;}//处理广播信息void ScoreChanged(object? obj, ScoreChangedCusEventArgs e){if (e == ScoreChangedCusEventArgs.Empty){return;}Console.WriteLine("this id is: " + _id + ",  oldscore is " + e.oldScore + "  ,new Score is: " + e.newScore + "  ,time is: " + DateTime.Now);}
}
#endregion//输出
this id is: 02,  oldscore is 0  ,new Score is: 15  ,time is: 2024/4/18 17:34:35
this id is: 01,  oldscore is 0  ,new Score is: 20  ,time is: 2024/4/18 17:34:35