/// <summary>
/// Initializes a new instance of the <see cref="WeakEventSubscription{TDelegateInstanceObject, TEventArgs}"/> class.
/// </summary>
/// <param name="eventHandler">The event handler.</param>
/// <param name="unsubscribeFromEventAction">The delegate method that unsubscribes from the event.</param>
public WeakEventSubscription(EventHandler <TEventArgs> eventHandler, UnsubscribeFromEventAction <TEventArgs> unsubscribeFromEventAction)
{
if (eventHandler == null)
{
throw new ArgumentNullException(nameof(eventHandler));
}
// Hold a weak reference to the target of the event handler - not the event handler itself, which would cause a strong reference.
this.weakReferenceToEventHandlerTarget = new WeakReference(eventHandler.Target);
// Create an unbound delegate to the MethodInfo for the event handler - not the event handler itself, which would cause a strong reference.
this.unboundDelegateEventHandler = (UnboundDelegateEventHandler)Delegate.CreateDelegate(typeof(UnboundDelegateEventHandler), null, eventHandler.Method);
// The actual event handler is a method in this class - not the event handler from the other object.
this.eventHandler = this.InvokeEventHandler;
this.unsubscribeFromEventAction = unsubscribeFromEventAction;
}
private void InvokeEventHandler(object sender, TEventArgs e)
{
var instanceObject = this.weakReferenceToEventHandlerTarget.Target as TDelegateInstanceObject;
if (instanceObject != null)
{
this.unboundDelegateEventHandler.Invoke(instanceObject, sender, e);
}
else if (this.unsubscribeFromEventAction != null)
{
// If instance object from the weak reference has been garbage collected, then unsubscribe from the event.
try
{
this.unsubscribeFromEventAction.Invoke(this.eventHandler);
}
finally
{
this.unsubscribeFromEventAction = null;
}
}
}
private static IWeakEventSubscription <TEventArgs> CreateWeakEventSubscription <TEventArgs>(EventHandler <TEventArgs> eventHandler, UnsubscribeFromEventAction <TEventArgs> unsubscribeFromEventDelegate) where TEventArgs : EventArgs
{
Type type = typeof(WeakEventSubscription <,>).MakeGenericType(eventHandler.Method.DeclaringType, typeof(TEventArgs));
ConstructorInfo constructor = type.GetConstructor(new Type[] { typeof(EventHandler <TEventArgs>), typeof(UnsubscribeFromEventAction <TEventArgs>) });
return((IWeakEventSubscription <TEventArgs>)constructor.Invoke(new object[] { eventHandler, unsubscribeFromEventDelegate }));
}
/// <summary>
/// Creates a weak event subscription/handler, specifying a delegate method that can unsubscribe the event handler from the event.
/// </summary>
/// <typeparam name="TEventArgs">The type of the event arguments.</typeparam>
/// <param name="eventHandler">The event handler.</param>
/// <param name="unsubscribeFromEventDelegate">The unsubscribe from event delegate.</param>
/// <returns>
/// A weak event handler.
/// </returns>
/// <exception cref="System.ArgumentNullException">If eventHandler is null.</exception>
/// <exception cref="System.ArgumentException">A WeakEventSubscription can only be created on instance methods.;eventHandler.</exception>
internal static IWeakEventSubscription <TEventArgs> Create <TEventArgs>(EventHandler <TEventArgs> eventHandler, UnsubscribeFromEventAction <TEventArgs> unsubscribeFromEventDelegate) where TEventArgs : EventArgs
{
if (eventHandler == null)
{
throw new ArgumentNullException(nameof(eventHandler));
}
if (eventHandler.Method.IsStatic || eventHandler.Target == null)
{
throw new ArgumentException("A WeakEventSubscription can only be created on instance methods.", nameof(eventHandler));
}
IWeakEventSubscription <TEventArgs> weakEventSubscription = CreateWeakEventSubscription <TEventArgs>(eventHandler, unsubscribeFromEventDelegate);
return(weakEventSubscription);
}
