How to create a event handler for event handler?

Question

I'm creating a Custom Control inherit TextBox , however, as PreviewTextInput and TextInput is not handled in TextBox by default, I wanted implement a PreviewTextInputLinked and TextInputLinked for my control, just for developer can add or remove these two events using += and -=

I have created a TextCompositionEventHandler called PreviewTextInputLinked TextInputLinked

public event TextCompositionEventHandler /*PreviewTextInputLinked*/ TextInputLinked = delegate { };
public MyTextBox()
{
   this.AddHandler(TextBox.PreviewTextInputEvent, /*PreviewTextInputLinked */TextInputLinked, true);
}

In the program XAML

/*<UControls:MyTextBox VerticalAlignment="Top" AcceptLetters="9" Mask=""
                     Width="122" 
                     PreviewTextInputLinked="MyTextBox_PreviewTextInputLinked"/>*/
<UControls:MyTextBox VerticalAlignment="Top" Width="122" 
                     TextInputLinked="MyTextBox_TextInputLinked"/>

Code behind

private void /*MyTextBox_PreviewTextInputLinked*/ MyTextBox_TextInputLinked(object sender,
                                              TextCompositionEventArgs e)
{
   MessageBox.Show("test");
}

But it is not doing anything, maybe I'm doing wrong, but I don't know what more can I do

Where is the problem?

Answer 1

PreviewTextInput will work since this is a tunnelling event whereas TextInput is a bubble event . In case it is handled while routing up, it won't bubble up to visual tree that's why hooking to PreviewTextInput works and TextInput doesn't because it might be handled somewhere en-route of bubbling.

AddHandler is a way to achieve that which you are using. Notice its third bool parameter which corresponds to handledEventsToo ie you still want handler to be called even it is handled below down Visual Tree by some element.

Now issue in your code is you pass an empty delegate to it which will get execute just like you want but you manually need to call other delegates (hooked via XAML to that event) which can be achieved like this -

public event TextCompositionEventHandler TextInputLinked = (sender, args) =>
{
   MyTextBox textBox = (MyTextBox)sender;
   if (textBox.TextInputLinked != null)
   {
       foreach (var handler in textBox.TextInputLinked.GetInvocationList())
       {
          if (handler.Target != null) <-- Check to avoid Stack overflow exception
          {
             handler.DynamicInvoke(sender, args);
          }
       }
   }
};

public MyTextBox()
{
    this.AddHandler(TextBox.TextInputEvent, TextInputLinked, true);
}

Now other handlers will be called as you desired.