在C#中使用Lambda的一次性事件

Question

我发现自己经常这样做: -

 EventHandler eh = null;  //can't assign lambda directly since it uses eh
 eh = (s, args) =>
 {
     //small snippet of code here

     ((SomeType)s).SomeEvent -= eh;
 }
 variableOfSomeType.SomeEvent += eh;

基本上我只想附加一个事件处理程序来监听事件中的一个镜头,之后我再也不想保持连接.通常,"代码片段"只是一行.

我的思绪有点麻木,我确信必须有一些我可以做的事情,所以我不需要重复所有这些开销.请记住,这EventHandler可能是EventHandler<T>.

任何想法如何我可以整理代码的重复部分,只是将片段留在Lambda中？

Answer 1

您可以将永久事件处理程序附加到事件中.然后,事件处理程序调用添加到内部队列的"一次性事件处理程序":

OneShotHandlerQueue<EventArgs> queue = new OneShotHandlerQueue<EventArgs>();

Test test = new Test();

// attach permanent event handler
test.Done += queue.Handle;

// add a "one shot" event handler
queue.Add((sender, e) => Console.WriteLine(e));
test.Start();

// add another "one shot" event handler
queue.Add((sender, e) => Console.WriteLine(e));
test.Start();

码:

class OneShotHandlerQueue<TEventArgs> where TEventArgs : EventArgs {
    private ConcurrentQueue<EventHandler<TEventArgs>> queue;
    public OneShotHandlerQueue() {
        this.queue = new ConcurrentQueue<EventHandler<TEventArgs>>();
    }
    public void Handle(object sender, TEventArgs e) {
        EventHandler<TEventArgs> handler;
        if (this.queue.TryDequeue(out handler) && (handler != null))
            handler(sender, e);
    }
    public void Add(EventHandler<TEventArgs> handler) {
        this.queue.Enqueue(handler);
    }
}

测试类:

class Test {
    public event EventHandler Done;
    public void Start() {
        this.OnDone(new EventArgs());
    }
    protected virtual void OnDone(EventArgs e) {
        EventHandler handler = this.Done;
        if (handler != null)
            handler(this, e);
    }
}

Answer 2

你可以使用反射:

public static class Listener {

  public static void ListenOnce(this object eventSource, string eventName, EventHandler handler) {
    var eventInfo = eventSource.GetType().GetEvent(eventName);
    EventHandler internalHandler = null;
    internalHandler = (src, args) => {
      eventInfo.RemoveEventHandler(eventSource, internalHandler);
      handler(src, args);
    };
    eventInfo.AddEventHandler(eventSource, internalHandler);
  }

  public static void ListenOnce<TEventArgs>(this object eventSource, string eventName, EventHandler<TEventArgs> handler) where TEventArgs : EventArgs {
    var eventInfo = eventSource.GetType().GetEvent(eventName);
    EventHandler<TEventArgs> internalHandler = null;
    internalHandler = (src, args) => {
      eventInfo.RemoveEventHandler(eventSource, internalHandler);
      handler(src, args);
    };
    eventInfo.AddEventHandler(eventSource, internalHandler);
  }

}

像这样使用它:

variableOfSomeType.ListenOnce("SomeEvent", 
  (s, args) => Console.WriteLine("I should print only once!"));

variableOfSomeType.ListenOnce<InterestingEventArgs>("SomeOtherEvent", 
  (s, args) => Console.WriteLine("I should print only once!"));

Answer 3

如果可以使用.NET的Reactive Extensions,则可以简化此操作.

您可以从事件中创建一个Observable,并且只使用第一个元素.Take(1)来执行您的小代码片段.这将整个过程变成几行代码.

编辑:为了演示,我已经制作了一个完整的示例程序(我将在下面粘贴).

我将可观察的创建和订阅移动到了方法(HandleOneShot)中.这使您可以通过单个方法调用执行您正在尝试的操作.为了演示,我创建了一个具有两个实现INotifyPropertyChanged的属性的类,并且正在侦听第一个属性更改事件,当它发生时写入控制台.

这将获取您的代码,并将其更改为:

HandleOneShot<SomeEventArgs>(variableOfSomeType, "SomeEvent",  e => { 
                    // Small snippet of code here
                }); 

请注意,所有订阅/取消订阅都会在幕后自动发生.没有必要手动处理订阅 - 只需订阅Observable,Rx就会为您处理这个问题.

运行时,此代码打印:

Setup...
Setting first property...
 **** Prop2 Changed! /new val
Setting second property...
Setting first property again.
Press ENTER to continue...

您只能获得一次单次触发事件.

namespace ConsoleApplication1
{
    using System;
    using System.ComponentModel;
    using System.Linq;

    class Test : INotifyPropertyChanged
    {
        private string prop2;
        private string prop;
        public string Prop
        {
            get {
                return prop;
            }
            set
            {
                if (prop != value)
                {
                    prop = value;
                    if (PropertyChanged!=null)
                        PropertyChanged(this, new PropertyChangedEventArgs("Prop"));
                }
            }
        }

        public string Prop2
        {
            get
            {
                return prop2;
            }
            set
            {
                if (prop2 != value)
                {
                    prop2 = value;
                    if (PropertyChanged != null)
                        PropertyChanged(this, new PropertyChangedEventArgs("Prop2"));
                }
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
    }


    class Program
    {
        static void HandleOneShot<TEventArgs>(object target, string eventName, Action<TEventArgs> action)  where TEventArgs : EventArgs
        {
            var obsEvent = Observable.FromEvent<TEventArgs>(target, eventName).Take(1);
            obsEvent.Subscribe(a => action(a.EventArgs));
        }

        static void Main(string[] args)
        {
            Test test = new Test();

            Console.WriteLine("Setup...");
            HandleOneShot<PropertyChangedEventArgs>(
                test, 
                "PropertyChanged", 
                e =>
                    {
                        Console.WriteLine(" **** {0} Changed! {1}/{2}!", e.PropertyName, test.Prop, test.Prop2);
                    });

            Console.WriteLine("Setting first property...");
            test.Prop2 = "new value";
            Console.WriteLine("Setting second property...");
            test.Prop = "second value";
            Console.WriteLine("Setting first property again...");
            test.Prop2 = "other value";

            Console.WriteLine("Press ENTER to continue...");
            Console.ReadLine();
        }
    }
}