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:
- 使用.Net Framework自带的Event Manager或者第三方库中的Event Manager(例如Prism)
- 使用泛型类WeakEventManager<EventSource, SomeEventEventArgs>
- 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();
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。
- 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();
- 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();
- 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:
和直接使用泛型类相比,自定义Event Manager性能要好一些。
其他资源
- https://docs.microsoft.com/en-us/dotnet/framework/wpf/advanced/weak-event-patterns
- https://www.codeproject.com/articles/738109/the-net-weak-event-pattern-in-csharp
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