Programatically attach an event handler to a custom child element routed event

Question

I have a Window that has a Button and a Popup. When the button is clicked, the event handler in code behind opens the Popup. On the Popup I have Button when clicked, the event handler in code behind closes the Popup. Simple. Crude.

I also have a UserControl that has a custom routed event and a Button that raises that event. That UserControl has been placed on the Popup.

I have added an event handler in XAML on the Popup element for the UserControl custom event. In code behind I show a message box.

This all works fine and dandy.

This is an extremely boiled down example. My ultimate question is, how do I programmatically attach an event handler to the custom routed event at the Popup level?

The UserControl XAML:

  <Grid>
    <Button
      Width="100"
      Height="100"
      Click="Button_Click"
      Content="Event!" />
  </Grid>

The UserControl code behind:

    public static readonly RoutedEvent CustomEventEvent = EventManager.RegisterRoutedEvent(
      nameof(CustomEvent), RoutingStrategy.Bubble, typeof(RoutedEventHandler), typeof(UserControl1));

    public event RoutedEventHandler CustomEvent
    {
      add => this.AddHandler(CustomEventEvent, value);
      remove => this.RemoveHandler(CustomEventEvent, value);
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
      RaiseEvent(new RoutedEventArgs(CustomEventEvent));
    }

Main window XAML:

  <Grid>

    <Button
      Width="200"
      Height="100"
      Click="Button_Click"
      Content="Popup" />

    <Popup
      x:Name="MyPopup"
      local:UserControl1.CustomEvent="MyPopup_CustomEvent"
      AllowsTransparency="True"
      Loaded="MyPopup_Loaded"
      Placement="Right">
      <Border
        Background="Azure"
        BorderBrush="Gray"
        BorderThickness="2"
        CornerRadius="3">
        <Grid Width="500" Height="300">
          <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition />
          </Grid.RowDefinitions>

          <Button
            Grid.Row="0"
            Width="100"
            Height="100"
            Click="Button_Click_1"
            Content="Close" />

          <local:UserControl1
            Grid.Row="1"
            Width="100"
            Height="100" />
        </Grid>
      </Border>
    </Popup>

  </Grid>

The main window code behind (minus the boring popup open/close button clicks):

    private void MyPopup_CustomEvent(object sender, RoutedEventArgs e)
    {
      MessageBox.Show("YAY");
    }

    private void MyPopup_Loaded(object sender, RoutedEventArgs e)
    {
      if (sender is Popup popup)
      {
        // Here is where the wheels fall off.
        // How do I find/attach to the routed event after it has bubbled up to the popup?
        // popup.CustomEvent += LocalMyPopup_CustomEvent;
      }
    }

    private void LocalMyPopup_CustomEvent(object sender, RoutedEventArgs e)
    {
    }

Ultimately, there will be several of the UserControls in many popups and I'd rather not have to attach event handlers to each and every one of them. Especially considering I can get them all at the Popup level using MyPopup_CustomEvent. I just want to replicate that behavior.

Answer 1

Turns out the solution is as trivial as it is (in hindsight) obvious. The AddHandler has to be called directly on the Popup and passing in the static Routed Event reference.

private void MyPopup_Loaded(object sender, RoutedEventArgs e)
{
  if (sender is Popup popup)
  {
    popup.AddHandler(UserControl1.CustomEventEvent, new RoutedEventHandler(LocalMyPopup_CustomEvent), true);
    // popup.CustomEvent += LocalMyPopup_CustomEvent;
  }
}