我最近开始研究C#，但是当在这门语言中实现观察者/可观察者模式时，我无论如何也无法理解委托的工作原理。
有没有人可以给我一个超级简单的例子来说明它是如何实现的？我已经通过Google搜索了，但我找到的所有示例要么过于特定于某个问题，要么过于“臃肿”。
8个回答
227
观察者模式通常使用事件来实现。
以下是一个示例：
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();
}
}
请注意，上面的示例使用了C# 6的 null-conditional 运算符来实现
DoSomething 的安全处理，以处理未订阅 SomethingHappened 且为null的情况。如果您正在使用旧版本的C＃，则需要像这样的代码：public void DoSomething()
{
var handler = SomethingHappened;
if (handler != null)
{
handler(this, EventArgs.Empty);
}
}
- Jon Skeet
15
18
在这个模型中，有发布者会执行一些逻辑并发布一个"事件"。
然后，发布者只会将它们的事件发送给已经订阅特定事件的订阅者。
然后，发布者只会将它们的事件发送给已经订阅特定事件的订阅者。
在C#中，任何对象都可以发布一组事件供其他应用程序订阅。
当发布类触发一个事件时，所有已订阅的应用程序都会被通知。
下图显示了这个机制。
C#中最简单的事件和委托示例：
代码是自解释的，另外我已经添加了注释来清楚地解释代码。
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
被 Listener2 听到
被听众听到
被 Listener2 听到
被听众听到
......（无限次）
- GorvGoyl
15
以下是一个简单的示例：
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：
- 这违反了一个规则，即我没有从可观察者上取消观察者的挂钩，在这个简单的例子中可能足够好，但确保你不要像这样让观察者一直挂在你的事件上。处理这种情况的一种方法是使ObserverClass成为IDisposable，并让.Dispose方法执行与构造函数中代码相反的操作。
- 未执行任何错误检查，至少应该在ObserverClass的构造函数中执行空值检查。
- Lasse V. Karlsen
5
应用委托和事件在C#中实现观察者模式被称为“事件模式”，这是微软MSDN上的一个小变化。本文将提供结构良好的示例，介绍如何在C#中应用该模式，包括传统方式和使用委托和事件的方式。请参考以下链接了解更多关于观察者设计模式的内容：探索观察者设计模式。
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
- Anestis Kivranoglou
5
我已经将上面两个优秀的例子（感谢Skeet先生和Karlsen先生）综合起来，包括了几个不同的可观察对象，并利用接口在观察者中跟踪这些对象，并允许观察者通过内部列表观察任意数量的可观察对象:
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);
}
}
}
- Jesse C. Slicer
1
我知道这很老，但是... 这看起来是线程安全的，但实际上并不是。在Observer.Add和Observer.Remove中，空值检查需要在锁内部进行。Dispose也应该获取锁，并设置一个isDisposed标志。否则，这是一个很好、完整的例子。 - user5151179
1
/**********************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);
}
}
- Narottam Goyal
0
我不想改变我的源代码来添加额外的观察者，因此我编写了以下简单示例：
//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 :)
}
}
- Imran Rizvi
0
类似这样：
// 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)
{
}
}
- Elena K
网页内容由stack overflow 提供, 点击上面的可以查看英文原文，
原文链接
原文链接
SomethingHappened = null（一种方便但懒惰和不理想的取消所有处理程序的方法），因此空值检查始终是必要的。 - Dan PuzeySomethingHappened?.Invoke(this, EventArgs.Empty);替换 DoSomething 中的所有内容。 - Junior Mayhé