Recently we had a query from a developer on using WeakEventManager when subscribing to AutoCAD events. In this blog post we will look at three different versions all of which subscribe to the CommandEnded event of the Document class. The benefit of using the WeakEventManager is to prevent memory leaks when the events are left subscribed but no longer needed. The best practice is to unsubscribe the events when its no longer needed. Also, the use of WeakEventManager comes at a cost because the events are now delivered through the event manager. The additional cost of using the Weak event pattern is nicely explained in this blog post by Reed Copsey.
It is recommended to unsubscribe the AutoCAD events manually, but the following code snippets should demonstrate the use of WeakEventManager in case you need it.
public class Commands :
IExtensionApplication ,
System.Windows.IWeakEventListener
{
void IExtensionApplication .Initialize() { }
void IExtensionApplication .Terminate() { }
#region Version 1 (Without the use of WeakEventManager)
// Version : 1
bool bAdded1 = false ;
[CommandMethod ("StartMonitor1" )]
public void StartMonitorMethod()
{
Document doc
= Application .DocumentManager.MdiActiveDocument;
if (bAdded1 == false )
{
doc.CommandEnded
+= new CommandEventHandler (doc_CommandEnded);
bAdded1 = true ;
doc.Editor.WriteMessage(
String .Format(
"{0} Subscribed to CommandEnded event" ,
Environment .NewLine));
}
}
[CommandMethod ("EndMonitor1" )]
public void EndMonitorMethod()
{
Document doc
= Application .DocumentManager.MdiActiveDocument;
if (bAdded1 == true )
{
doc.CommandEnded
-= new CommandEventHandler (doc_CommandEnded);
bAdded1 = false ;
doc.Editor.WriteMessage(
String .Format(
"{0} Unsubscribed CommandEnded event" ,
Environment .NewLine));
}
}
void doc_CommandEnded(object sender, CommandEventArgs e)
{
Document doc
= Application .DocumentManager.MdiActiveDocument;
Editor ed = doc.Editor;
if (bAdded3)
ed.WriteMessage(
String .Format("{0} (Weak event) Command Ended : {1}" ,
Environment .NewLine, e.GlobalCommandName));
else
ed.WriteMessage(
String .Format("{0} Command Ended : {1}" ,
Environment .NewLine, e.GlobalCommandName));
}
#endregion
//(Using WeakEventManager (non-generic version)
#region Version 2
bool bAdded2 = false ;
[CommandMethod ("StartMonitor2" )]
public void StartMonitor2Method()
{
Document doc
= Application .DocumentManager.MdiActiveDocument;
if (bAdded2 == false )
{
CommandEndedEventManager .AddListener(doc, this );
bAdded2 = true ;
doc.Editor.WriteMessage(
String .Format(
"{0} Subscribed to CommandEnded weak event" ,
Environment .NewLine));
}
}
[CommandMethod ("EndMonitor2" )]
public void EndMonitor2Method()
{
Document doc
= Application .DocumentManager.MdiActiveDocument;
if (bAdded2 == true )
{
CommandEndedEventManager .RemoveListener
(doc, this );
bAdded2 = false ;
doc.Editor.WriteMessage(
String .Format(
"{0} Unsubscribed CommandEnded weak event" ,
Environment .NewLine));
}
}
bool System.Windows.IWeakEventListener .ReceiveWeakEvent
(Type managerType, object sender, EventArgs e)
{
if (managerType.Equals(
typeof (CommandEndedEventManager )))
{
Document doc = sender as Document ;
CommandEventArgs ce = e as CommandEventArgs ;
Editor ed = doc.Editor;
ed.WriteMessage(
String .Format("{0} (Weak event) Command Ended : {1}" ,
Environment .NewLine, ce.GlobalCommandName));
return true ;
}
return false ;
}
#endregion
//(Using generic WeakEventManager from .Net framework 4.5)
#region Version 3
bool bAdded3 = false ;
[CommandMethod ("StartMonitor3" )]
public void StartMonitor3Method()
{
Document doc
= Application .DocumentManager.MdiActiveDocument;
if (bAdded3 == false )
{
System.Windows.WeakEventManager
<Document , CommandEventArgs >
.AddHandler(
doc,
"CommandEnded" ,
doc_CommandEnded);
bAdded3 = true ;
doc.Editor.WriteMessage(
String .Format(
"{0} Subscribed to CommandEnded weak event" ,
Environment .NewLine));
}
}
[CommandMethod ("EndMonitor3" )]
public void EndMonitor3Method()
{
Document doc
= Application .DocumentManager.MdiActiveDocument;
if (bAdded3 == true )
{
System.Windows.WeakEventManager
<Document , CommandEventArgs >
.RemoveHandler(
doc,
"CommandEnded" ,
doc_CommandEnded);
bAdded3 = false ;
doc.Editor.WriteMessage(
String .Format(
"{0} Unsubscribed CommandEnded weak event" ,
Environment .NewLine));
}
}
#endregion
}
// For Version 2
// Using Non-generic version of WeakEventManager
public class CommandEndedEventManager
: System.Windows.WeakEventManager
{
public static CommandEndedEventManager CurrentManager
{
get
{
CommandEndedEventManager manager
= System.Windows.WeakEventManager .GetCurrentManager(
typeof (CommandEndedEventManager ))
as CommandEndedEventManager ;
if (manager == null )
{
manager = new CommandEndedEventManager ();
System.Windows.WeakEventManager .SetCurrentManager
(typeof (CommandEndedEventManager ), manager);
}
return manager;
}
}
public static void AddListener(
Document source,
System.Windows.IWeakEventListener listener)
{
CurrentManager.ProtectedAddListener
(source, listener);
}
public static void RemoveListener(
Document source,
System.Windows.IWeakEventListener listener)
{
CurrentManager.ProtectedRemoveListener
(source, listener);
}
protected override void StartListening(object source)
{
try
{
((Document )source).CommandEnded
+= deliver_CommandEnded;
}
catch (System.Exception ex)
{
Document doc
= Application .DocumentManager.MdiActiveDocument;
Editor ed = doc.Editor;
ed.WriteMessage(ex.Message);
}
}
void deliver_CommandEnded(
object sender, CommandEventArgs e)
{
this .DeliverEvent(sender, e);
}
protected override void StopListening(object source)
{
try
{
((Document )source).CommandEnded
-= deliver_CommandEnded;
}
catch (System.Exception ex)
{
System.Windows.MessageBox .Show(ex.Message);
}
}
}
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