C# 4 and .NET 4

events ❘ 203
Weak events
With events, the publisher and listener are directly connected. This can be a problem with garbage
collection. For example, if a listener is not directly referenced any more, there’s still a reference from the
publisher. The garbage collector cannot clean up memory from the listener, as the publisher still holds a
reference and fires events to the listener.
This strong connection can be resolved by using the weak event pattern and using the WeakEventManager
as an intermediate between the publisher and listeners.
The sample from before with the CarDealer as publisher and Consumer as listener is modified in the
following section to use the weak event pattern.
Weak event Manager
To use weak events you need to create a class that derives from WeakEventManager. WeakEventManager is
defined in the namespace System.Windows in the assembly WindowsBase.
The class WeakCarInfoEventManager is the weak event manager class that manages the connection
between the publisher and listener for the NewCarInfo event. This class implements a singleton pattern
so that only one instance is created. The static property CurrentManager creates an object of type
WeakCarInfoEventManager if it doesn’t exist, and returns a reference to it. WeakCarInfoEventManager.
CurrentManager is used to access the singleton object from the WeakCarInfoEventManager.
With the weak event pattern, the weak event manager class needs static methods AddListener() and
RemoveListener(). The listener is connected and disconnected to the events of the publisher with these
methods instead of using the events from the publisher directly. The listener also needs to implement the
interface IWeakEventListener that is shown shortly. With the AddListener() and RemoveListener()
methods, methods from the base class WeakEventManager are invoked to add and remove the listeners.
With the WeakCarInfoEventManager class you also need to override the StartListening() and
StopListening() methods from the base class. StartListening() is called when the first listener is
added, StopListening() when the last listener is removed. StartListening() and StopListening()
subscribes and unsubscribes a method from the weak event manager to listen for the event from the
publisher. In case the weak event manager class needs to connect to different publisher types, you can
check the type information from the source object before doing the cast. The event is then forwarded to
the listeners by calling the DeliverEvent() method from the base class, which in turn invokes the method
ReceiveWeakEvent() from the IWeakEventListener interface in the listeners:
using System.Windows;
namespace Wrox.ProCSharp.Delegates
{
public class WeakCarInfoEventManager: WeakEventManager
{
public static void AddListener(object source, IWeakEventListener listener)
{
CurrentManager.ProtectedAddListener(source, listener);
}
public static void RemoveListener(object source,
IWeakEventListener listener)
{
CurrentManager.ProtectedRemoveListener(source, listener);
}
public static WeakCarInfoEventManager CurrentManager
{
get
{
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