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);
}
}
#endregion
static 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);
}
}
#endregion
static 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
