Weak Event Manager

Posted dereklovecc

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问题

通过传统的方式监听事件(即C#的+=语法),有可能会导致内存泄漏,原因是事件源会持有对事件Handler所在对象的强引用从而阻碍GC回收它,这样事件handler对象的生命周期受到了事件源对象的影响。

解决方案

此问题有两个解决办法:1) 确保通过-=语法反注册事件处理器 2)使用弱事件模式(Weak Event Pattern)。本文主要讲解Weak Event Pattern。

在使用Weak Event Pattern时,主要涉及到两个类:WeakEventManager和IWeakEventListener。WeakEventManager的StartListening和StopListening函数可以被子类覆盖来进行事件监听的注册和反注册。IWeakEventListener只有下面一个函数,当事件触发时,WeakEventManager会调用该函数从而执行Handler的代码。

bool ReceiveWeakEvent (Type managerType, object sender, EventArgs e);

根据构造WeakEventManager的方法不同,可分为如下三种方式来应用Weak Event Pattern:

  1. 使用.Net Framework自带的Event Manager或者第三方库中的Event Manager(例如Prism)
  2. 使用泛型类WeakEventManager<EventSource, SomeEventEventArgs>
  3. 自己编写Event Manager

首先我们通过如下代码来看看传统的事件注册方式有什么问题,代码通过+=语法把listener.HandleEvent方法注册到了eventSource.CustomEvent事件上,在设置listener为null后并触发GC,但listener对象并不会被GC收集且会继续处理新的消息,图一是其运行结果。

    public static class ProblemInNormalRegister
    
        public static void Test()
        
            var eventSource = new CustomEventSource();
            var listener = new CustomEventListener();

            //register event listener
            eventSource.CustomEvent += listener.HandleEvent;

            //trigger event and listener.HandleEvent will be executed
            eventSource.Raise("First Message from CustomEventSource");

            //set listener to null
            listener = null;

            //trigger gc but the listener object will not collected.
            GCUtils.TriggerGC();

            //trigger event and listener.HandleEvent stil will be executed
            eventSource.Raise("Second Message from CustomEventSource");

            Console.Read();
        
    

技术图片

接下来,让我们通过3种不同的方式来应用Weak Event Pattern来解决这个问题。

  1. 使用.Net Framework自带的Event Manager
    系统自带了一些像CollectionChangedEventManager,PropertyChangedEventManager的Weak Event Manager,每个事件管理器只处理一类事件,比如PropertyChangedEventManager是处理INotifyPropertyChanged.PropertyChanged事件的,完整列表请参考:https://docs.microsoft.com/en-us/dotnet/api/system.windows.weakeventmanager?view=netframework-4.8。 如果你要处理的事件恰好对应某个系统自带的Event Manager,那么你可以直接按照如下的方法拿来使用,其运行结果如图二所示,可以看到listener在被设置成null之后被回收了,从而也就不能再处理新的消息了
    public class UseExistingWeakEventManager_PropertyChangedEventManager
    
        public static void Test()
        
            var source = new PropertyChangedEventSource();
            var listener = new PropertyChangedEventEventListener();

            //Register listener to the source
            PropertyChangedEventManager.AddListener(source, listener, nameof(PropertyChangedEventSource.Name));

            //change property change and the ReceiveWeakEvent of PropertyChangedEventEventListener method will be called
            source.Name = "first name";

            //set listener to null so it will be ready for gc
            listener = null;

            //trigger gc and the listener will be collected.
            GCUtils.TriggerGC();

            //change property value but the ReceiveWeakEvent of PropertyChangedEventEventListener method will NOT be called
            source.Name = "second name"; 

            Console.Read();
        
    

技术图片

  1. 使用泛型类WeakEventManager<EventSource, SomeEventEventArgs>
    按照如下代码使用.net framework的泛型类WeakEventManager,非常方便而且listener在设置为null之后可以被回收,运行结果如图三所示。
    public class GenericManagerTest
    
        public static void Test()
        
            var eventSource = new CustomEventSource();
            var listener = new CustomEventListener();

            //add handler listener.HandleEvent to CustomEvent of CustomEventSource
            WeakEventManager<CustomEventSource, CustomEventArg>.AddHandler(eventSource, "CustomEvent", listener.HandleEvent);

            //trigger event and listener.HandleEvent will be executed
            eventSource.Raise("First message");

            //set listener to null
            listener = null;

            //trigger gc but the listener object will be collected.
            GCUtils.TriggerGC();

            //trigger event and listener.HandleEvent stil will NOT be executed
            eventSource.Raise("Second Message");

            Console.Read();
        
    

技术图片

  1. 自己编写Event Manager

首先,创建一个类如CustomizedWeakEventManager并继承自WeakEventManager,然后重写基类WeakEventManager中的StartListening和StopListening函数;此外CustomizedWeakEventManager也提供了AddListener和RemoveListener函数来方便添加和移除监听器,类的代码如下:

    public class CustomizedWeakEventManager : WeakEventManager
    
        private static CustomizedWeakEventManager CurrentManager
        
            get
            
                CustomizedWeakEventManager manager = (CustomizedWeakEventManager)GetCurrentManager(typeof(CustomizedWeakEventManager));

                if (manager == null)
                
                    manager = new CustomizedWeakEventManager();
                    SetCurrentManager(typeof(CustomizedWeakEventManager), manager);
                

                return manager;
            
        


        public static void AddListener(CustomEventSource source, IWeakEventListener listener)
        
            CurrentManager.ProtectedAddListener(source, listener);
        

        public static void RemoveListener(CustomEventSource source, IWeakEventListener listener)
        
            CurrentManager.ProtectedRemoveListener(source, listener);
        

        protected override void StartListening(object source)
        
            (source as CustomEventSource).CustomEvent += DeliverEvent;
        

        protected override void StopListening(object source)
        
            (source as CustomEventSource).CustomEvent -= DeliverEvent;
        
    

程序执行结果如下图:
技术图片和直接使用泛型类相比,自定义Event Manager性能要好一些。

完整的程序请参考:https://github.com/DerekLoveCC/Writings/blob/master/Blog/Code/AStockViewer/WeakEventPattern/WeakEventPattern.csproj

其他资源

  1. https://docs.microsoft.com/en-us/dotnet/framework/wpf/advanced/weak-event-patterns
  2. https://www.codeproject.com/articles/738109/the-net-weak-event-pattern-in-csharp

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