Detecting when a template was loaded in wpf

Question

I am working with an attached behavior for logging user actions on a ScrollBar.

my code:

class ScrollBarLogBehavior : Behavior<ScrollBar>
{
    protected override void OnAttached()
    {
        base.OnAttached();
        AssociatedObject.Loaded += new RoutedEventHandler(AssociatedObject_Loaded);
    }

    void AssociatedObject_Loaded(object sender, RoutedEventArgs e)
    {
        ...
        var track = (Track)AssociatedObject.Template.FindName("PART_Track", AssociatedObject);
        // ** HERE is the problem: track is null ! **
        ...
    }

How can I detect that the template has loaded and I can find the Track? (when I call AssociatedObject.Template.LoadContent() the result containt the requested Track, so it i a matter of timing and not a matter of wrong template or naming)

Answer 1

I did not find any good way to detect when the template was loaded. However, I did find a way to find the Track:

1. in OnAttached() - register to Scroll event fo the ScrollBar (this can only happen after the entire template is loaded, of course):

    protected override void OnAttached()
    {
        base.OnAttached();
        _scrollHandler = new ScrollEventHandler(AssociatedObject_Scroll);
        AssociatedObject.AddHandler(ScrollBar.ScrollEvent, _scrollHandler, true);
    }

2. When handling the Scroll event, remove registration and find the Thumb:

    void AssociatedObject_Scroll(object sender, ScrollEventArgs e)
    {
        var track = (Track)AssociatedObject.Template.FindName("PART_Track", Associated
        if (track == null)
            return;
        AssociatedObject.RemoveHandler(ScrollBar.ScrollEvent, _scrollHandler);
        // do my work with Track
        ...
    }

Answer 2

Override the method OnApplyTemplate

    public override void OnApplyTemplate()
    {
        base.OnApplyTemplate();
        var textBox = Template.FindName("PART_Textbox", this) as TextBox;
    }

Answer 3

If I understand correctly, you wish to create an attached behavior that will reference a template part after the ScrollBar has been loaded.

The following should work:

    internal static class ScrollBarLogBehavior
    {
        public static readonly DependencyProperty LogUserActionProperty = DependencyProperty.RegisterAttached(
            "LogUserAction",
            typeof(bool),
            typeof(ScrollBarLogBehavior),
            new UIPropertyMetadata(default(bool), LogUserActionChanged));

        public static bool GetLogUserAction(DependencyObject obj)
        {
            return (bool)obj.GetValue(LogUserActionProperty);
        }

        public static void SetLogUserAction(DependencyObject obj, bool value)
        {
            obj.SetValue(LogUserActionProperty, value);
        }

        public static void LogUserActionChanged(DependencyObject s, DependencyPropertyChangedEventArgs e)
        {
            if (s is ScrollBar scrollBar)
            {
                scrollBar.Loaded += OnScrollBarLoaded;
            }
        }

        private static void OnScrollBarLoaded(object sender, RoutedEventArgs e)
        {
            if (sender is ScrollBar scrollBar)
            {
                if (scrollBar.Template != null)
                {
                    // I'm not sure, but the `name` in the following method call might be case sensitive.
                    if (scrollBar.Template.FindName("PART_Track", scrollBar) is Track track)
                    {
                        // do work with `track` here
                    }
                }
            }
        }
    }

where you would "attach" the behavior in your XAML with:

<ScrollBar guiControls:ScrollBarLogBehavior.LogUserAction="True">
  <!-- more here -->
</ScrollBar>

BE ADVISED: this implementation completely ignores the bool value that is being set for LogUserAction