【发布时间】：2010-11-17 23:35:45
【问题描述】：
我最近开始深入研究 C#，但我一生无法弄清楚在该语言中实现观察者/可观察模式时委托是如何工作的。
谁能给我一个超级简单的例子来说明它是如何完成的？我用谷歌搜索了这个，但我发现的所有示例要么过于具体，要么过于“臃肿”。
【问题讨论】：
标签： c# events delegates observer-pattern
【发布时间】：2010-11-17 23:35:45
【问题描述】：
观察者模式通常用events实现。
这是一个例子：
using System;
class Observable
{
public event EventHandler SomethingHappened;
public void DoSomething() =>
SomethingHappened?.Invoke(this, EventArgs.Empty);
}
class Observer
{
public void HandleEvent(object sender, EventArgs args)
{
Console.WriteLine("Something happened to " + sender);
}
}
class Test
{
static void Main()
{
Observable observable = new Observable();
Observer observer = new Observer();
observable.SomethingHappened += observer.HandleEvent;
observable.DoSomething();
}
}
有关更多详细信息，请参阅链接文章。
请注意，上面的示例使用 C# 6 null-conditional 运算符安全地实现 DoSomething 以处理 SomethingHappened 尚未订阅并因此为 null 的情况。如果您使用的是旧版本的 C#，则需要如下代码：
public void DoSomething()
{
var handler = SomethingHappened;
if (handler != null)
{
handler(this, EventArgs.Empty);
}
}
【讨论】：
-
为了节省几行代码并避免空检查，请像这样初始化您的事件：stackoverflow.com/questions/340610/…
-
@Dinah：这并不能避免空值检查。您仍然可以稍后设置
SomethingHappened = null（如果是一种方便但不理想的取消订阅所有处理程序的方法），因此始终需要进行空检查。 -
@DanPuzey：你可以在课堂上，但同样你可以确保你不要这样做 - 而其他代码不能这样做，因为它只能订阅和取消订阅。如果你确保在你的类中从不故意将其设置为 null，那么可以避免 null 检查。
-
@JonSkeet：当然，我忘记了你不能在课堂外这样做。道歉！
-
我认为您可以将 DoSomething 中的所有内容替换为
SomethingHappened?.Invoke(this, EventArgs.Empty);
在此模型中，您的发布者会执行一些逻辑并发布“事件”。
然后，发布者将仅将其事件发送给已订阅接收特定事件的订阅者。
在 C# 中，任何对象都可以发布一组其他应用程序可以订阅的事件。
当发布类引发事件时，所有订阅的应用程序都会收到通知。
下图展示了这种机制。
关于 C# 中事件和委托的最简单示例：
代码是不言自明的，我还添加了 cmets 以清除代码。
using System;
public class Publisher //main publisher class which will invoke methods of all subscriber classes
{
public delegate void TickHandler(Publisher m, EventArgs e); //declaring a delegate
public TickHandler Tick; //creating an object of delegate
public EventArgs e = null; //set 2nd paramter empty
public void Start() //starting point of thread
{
while (true)
{
System.Threading.Thread.Sleep(300);
if (Tick != null) //check if delegate object points to any listener classes method
{
Tick(this, e); //if it points i.e. not null then invoke that method!
}
}
}
}
public class Subscriber1 //1st subscriber class
{
public void Subscribe(Publisher m) //get the object of pubisher class
{
m.Tick += HeardIt; //attach listener class method to publisher class delegate object
}
private void HeardIt(Publisher m, EventArgs e) //subscriber class method
{
System.Console.WriteLine("Heard It by Listener");
}
}
public class Subscriber2 //2nd subscriber class
{
public void Subscribe2(Publisher m) //get the object of pubisher class
{
m.Tick += HeardIt; //attach listener class method to publisher class delegate object
}
private void HeardIt(Publisher m, EventArgs e) //subscriber class method
{
System.Console.WriteLine("Heard It by Listener2");
}
}
class Test
{
static void Main()
{
Publisher m = new Publisher(); //create an object of publisher class which will later be passed on subscriber classes
Subscriber1 l = new Subscriber1(); //create object of 1st subscriber class
Subscriber2 l2 = new Subscriber2(); //create object of 2nd subscriber class
l.Subscribe(m); //we pass object of publisher class to access delegate of publisher class
l2.Subscribe2(m); //we pass object of publisher class to access delegate of publisher class
m.Start(); //starting point of publisher class
}
}
输出：
被听众听到
Listener2 听到的
被听众听到
Listener2 听到的
听者听过 . . . （无限次）
【讨论】：
这是一个简单的例子：
public class ObservableClass
{
private Int32 _Value;
public Int32 Value
{
get { return _Value; }
set
{
if (_Value != value)
{
_Value = value;
OnValueChanged();
}
}
}
public event EventHandler ValueChanged;
protected void OnValueChanged()
{
if (ValueChanged != null)
ValueChanged(this, EventArgs.Empty);
}
}
public class ObserverClass
{
public ObserverClass(ObservableClass observable)
{
observable.ValueChanged += TheValueChanged;
}
private void TheValueChanged(Object sender, EventArgs e)
{
Console.Out.WriteLine("Value changed to " +
((ObservableClass)sender).Value);
}
}
public class Program
{
public static void Main()
{
ObservableClass observable = new ObservableClass();
ObserverClass observer = new ObserverClass(observable);
observable.Value = 10;
}
}
注意：
- 这违反了一条规则，即我不会从 observable 中解开观察者，这对于这个简单的示例来说可能已经足够了，但请确保不要让观察者像那样挂在你的事件上。处理这种情况的一种方法是使 ObserverClass IDisposable，并让 .Dispose 方法执行与构造函数中的代码相反的操作
- 不执行错误检查，至少应在 ObserverClass 的构造函数中进行空检查
【讨论】：
我将上面的几个很好的例子（一如既往地感谢Mr. Skeet 和Mr. Karlsen）捆绑在一起，以包含几个不同的 Observable，并利用一个接口在 Observer 中跟踪它们并允许Observer 通过内部列表“观察”任意数量的 Observable：
namespace ObservablePattern
{
using System;
using System.Collections.Generic;
internal static class Program
{
private static void Main()
{
var observable = new Observable();
var anotherObservable = new AnotherObservable();
using (IObserver observer = new Observer(observable))
{
observable.DoSomething();
observer.Add(anotherObservable);
anotherObservable.DoSomething();
}
Console.ReadLine();
}
}
internal interface IObservable
{
event EventHandler SomethingHappened;
}
internal sealed class Observable : IObservable
{
public event EventHandler SomethingHappened;
public void DoSomething()
{
var handler = this.SomethingHappened;
Console.WriteLine("About to do something.");
if (handler != null)
{
handler(this, EventArgs.Empty);
}
}
}
internal sealed class AnotherObservable : IObservable
{
public event EventHandler SomethingHappened;
public void DoSomething()
{
var handler = this.SomethingHappened;
Console.WriteLine("About to do something different.");
if (handler != null)
{
handler(this, EventArgs.Empty);
}
}
}
internal interface IObserver : IDisposable
{
void Add(IObservable observable);
void Remove(IObservable observable);
}
internal sealed class Observer : IObserver
{
private readonly Lazy<IList<IObservable>> observables =
new Lazy<IList<IObservable>>(() => new List<IObservable>());
public Observer()
{
}
public Observer(IObservable observable) : this()
{
this.Add(observable);
}
public void Add(IObservable observable)
{
if (observable == null)
{
return;
}
lock (this.observables)
{
this.observables.Value.Add(observable);
observable.SomethingHappened += HandleEvent;
}
}
public void Remove(IObservable observable)
{
if (observable == null)
{
return;
}
lock (this.observables)
{
observable.SomethingHappened -= HandleEvent;
this.observables.Value.Remove(observable);
}
}
public void Dispose()
{
for (var i = this.observables.Value.Count - 1; i >= 0; i--)
{
this.Remove(this.observables.Value[i]);
}
}
private static void HandleEvent(object sender, EventArgs args)
{
Console.WriteLine("Something happened to " + sender);
}
}
}
【讨论】：
-
我知道这是旧的，但是......这看起来线程安全，但它不是。在 Observer.Add 和 Observer.Remove 中，空检查需要在锁内。 Dispose 还应该获取锁，并设置一个 isDispised 标志。否则是一个很好的完整示例。
根据MSDN，将观察者模式与c#中的委托和事件一起命名为“事件模式”这是一个细微的变化。
在本文中，您将找到结构良好的示例，说明如何在 c# 中以经典方式以及使用委托和事件来应用该模式。
Exploring the Observer Design Pattern
public class Stock
{
//declare a delegate for the event
public delegate void AskPriceChangedHandler(object sender,
AskPriceChangedEventArgs e);
//declare the event using the delegate
public event AskPriceChangedHandler AskPriceChanged;
//instance variable for ask price
object _askPrice;
//property for ask price
public object AskPrice
{
set
{
//set the instance variable
_askPrice = value;
//fire the event
OnAskPriceChanged();
}
}//AskPrice property
//method to fire event delegate with proper name
protected void OnAskPriceChanged()
{
AskPriceChanged(this, new AskPriceChangedEventArgs(_askPrice));
}//AskPriceChanged
}//Stock class
//specialized event class for the askpricechanged event
public class AskPriceChangedEventArgs : EventArgs
{
//instance variable to store the ask price
private object _askPrice;
//constructor that sets askprice
public AskPriceChangedEventArgs(object askPrice) { _askPrice = askPrice; }
//public property for the ask price
public object AskPrice { get { return _askPrice; } }
}//AskPriceChangedEventArgs
【讨论】：
/**********************Simple Example ***********************/
class Program
{
static void Main(string[] args)
{
Parent p = new Parent();
}
}
////////////////////////////////////////////
public delegate void DelegateName(string data);
class Child
{
public event DelegateName delegateName;
public void call()
{
delegateName("Narottam");
}
}
///////////////////////////////////////////
class Parent
{
public Parent()
{
Child c = new Child();
c.delegateName += new DelegateName(print);
//or like this
//c.delegateName += print;
c.call();
}
public void print(string name)
{
Console.WriteLine("yes we got the name : " + name);
}
}
【讨论】：
我不想更改我的源代码来添加额外的观察者，所以我写了以下简单的例子：
//EVENT DRIVEN OBSERVER PATTERN
public class Publisher
{
public Publisher()
{
var observable = new Observable();
observable.PublishData("Hello World!");
}
}
//Server will send data to this class's PublishData method
public class Observable
{
public event Receive OnReceive;
public void PublishData(string data)
{
//Add all the observer below
//1st observer
IObserver iObserver = new Observer1();
this.OnReceive += iObserver.ReceiveData;
//2nd observer
IObserver iObserver2 = new Observer2();
this.OnReceive += iObserver2.ReceiveData;
//publish data
var handler = OnReceive;
if (handler != null)
{
handler(data);
}
}
}
public interface IObserver
{
void ReceiveData(string data);
}
//Observer example
public class Observer1 : IObserver
{
public void ReceiveData(string data)
{
//sample observers does nothing with data :)
}
}
public class Observer2 : IObserver
{
public void ReceiveData(string data)
{
//sample observers does nothing with data :)
}
}
【讨论】：
类似这样的：
// interface implementation publisher
public delegate void eiSubjectEventHandler(eiSubject subject);
public interface eiSubject
{
event eiSubjectEventHandler OnUpdate;
void GenereteEventUpdate();
}
// class implementation publisher
class ecSubject : eiSubject
{
private event eiSubjectEventHandler _OnUpdate = null;
public event eiSubjectEventHandler OnUpdate
{
add
{
lock (this)
{
_OnUpdate -= value;
_OnUpdate += value;
}
}
remove { lock (this) { _OnUpdate -= value; } }
}
public void GenereteEventUpdate()
{
eiSubjectEventHandler handler = _OnUpdate;
if (handler != null)
{
handler(this);
}
}
}
// interface implementation subscriber
public interface eiObserver
{
void DoOnUpdate(eiSubject subject);
}
// class implementation subscriber
class ecObserver : eiObserver
{
public virtual void DoOnUpdate(eiSubject subject)
{
}
}
【讨论】：