/// <summary>
/// Creates a weak EventHandler wrapper
/// </summary>
/// <param name="eventHandler">EventHandler to create a weak wrapper for</param>
/// <param name="unregisterMethod">Lambda expression for unregistration (i.e. "h => obj.Event -= h")</param>
/// <exception cref="ArgumentNullException"><paramref name="eventHandler"/> is null.</exception>
/// <exception cref="NotSupportedException"><typeparamref name="TEventHandler"/> is not a delegate type.</exception>
/// <exception cref="ArgumentException"><paramref name="eventHandler"/> is a generated closure and may go out of scope prematurely. -or- <paramref name="unregisterMethod"/> references the <paramref name="eventHandler" /> target.</exception>
/// <remarks>
/// The <paramref name="unregisterMethod"/> is held on to with a strong reference, so anything referenced in
/// the lambda expression will not be garbage collected until the <paramref name="eventHandler"/> target goes
/// out of scope and the event is unregistered. It is recommended that you store the object containing the
/// event in a local variable and use the local variable for unsubcribing to minimize object retention
/// caused by property chains (i.e. "h => obj1.Property1.Property2.Event -= h" will hold on to obj1, which
/// will presumably hold on to whatever is in Property1 and Property2. Furthermore, if Property1 or
/// Property2 changes before the event is unsubscribed, the unsubscription will occur on the wrong
/// Property2 instance).
/// </remarks>
public WeakEventHandler(TEventHandler eventHandler, Action <TEventHandler> unregisterMethod)
{
if (eventHandler == null)
{
throw new ArgumentNullException("eventHandler");
}
// Make sure TEventHandler is a Delegate since we can't enforce it in a where clause
Delegate eventHandlerDelegate = eventHandler as Delegate;
if (eventHandlerDelegate == null)
{
throw new NotSupportedException(typeof(TEventHandler).Name + " is not a delegate type.");
}
// if there is no eventHandler target to create a weak reference to, the method will never go out of scope
if (eventHandlerDelegate.Method.IsStatic)
{
throw new ArgumentException("Cannot create weak event from static method, there is no object to reference that would go out of scope", "eventHandler");
}
// we create a weak reference to the eventHandler target, but hold a strong reference to the unregisterMethod
// if the unregisterMethod is on the eventHandler target, the target cannot be garbage collected and will be leaked.
if (unregisterMethod != null && eventHandlerDelegate.Target != null && eventHandlerDelegate.Target == unregisterMethod.Target)
{
throw new ArgumentException("Unregister method exists on same object as event handler, which will prevent the object from being garbage collected. This is typically the result of using a class variable instead of a local variable in the unregister method.", "unregisterMethod");
}
// the lifetime of a generated closue is only the lifetime of the delegate itself. Closures are not
// generated if the anonymous delegate only references variables passed in to it, or the "this" psuedo-variable.
if (OpenAction.HasClosureReference(eventHandlerDelegate))
{
throw new ArgumentException("Cannot create weak event from generated closure", "eventHandler");
}
if (_dispatchEventMethod == null)
{
// expression is slightly faster than reflection, and prevents FxCop from saying DispatchEvent is not referenced.
Expression <Action <object, TEventArgs> > expression = (o, e) => DispatchEvent(o, e);
var dispatchEvent = expression.Body as MethodCallExpression;
if (dispatchEvent == null)
{
throw new InvalidOperationException("Unable to locate DispatchEvent method");
}
_dispatchEventMethod = dispatchEvent.Method;
}
// create a TEventHandler delegate pointing at DispatchEvent
Handler = (TEventHandler)(object)Delegate.CreateDelegate(typeof(TEventHandler), this, _dispatchEventMethod);
// create weak reference to listener
_methodInfo = eventHandlerDelegate.Method;
_target = new WeakReference(eventHandlerDelegate.Target);
// create strong reference to unregister method - if it goes away, we can't unregister.
// this requires that the unregister method does not reference the listener, or the listener
// will never go out of scope!
_unregisterMethod = unregisterMethod;
}
/// <summary>
/// Constructs a new WeakAction from an existing Action
/// </summary>
/// <param name="action">Action to convert into a WeakAction.</param>
/// <exception cref="ArgumentNullException"><paramref name="action"/> is null.</exception>
/// <exception cref="ArgumentException"><paramref name="action"/> points at a generated closure.</exception>
public WeakAction(Action <T> action)
: base(action != null ? action.Target : null)
{
if (action == null)
{
throw new ArgumentNullException("action");
}
Method = action.Method;
if (Method.IsStatic)
{
Target = Method.DeclaringType;
_openAction = (target, param) => action(param);
}
else
{
// Closures are generated internal classes used to pass local variables to an anonymous delegate. The lifetime of
// the closure is determined by the lifetime of the delegate. Attempting to create a weak reference to the delegate
// will not keep the closure around without some external reference. Since we can't validate the presence of an
// external reference, we err on the side of caution and throw an exception. Closures are not generated if the
// anonymous delegate only references variables passed in to it, or the "this" psuedo-variable.
if (OpenAction.HasClosureReference(action))
{
throw new ArgumentException("Cannot create weak reference from generated closure");
}
}
}
