DataTrigger: Evaluate initial value on element load

[brianlagunas]

Description of Change

Online 43 of the DataTrigger.cs file, added an event handler for the Loaded event of the AssociatedObject. Within this handler, we call EvaluateBindingChange to force the evaluation of the binding value when the element first loads. After the element is loaded and the bindings have been evaluated, we immediately unhook the Loaded event handler.

Note: tests were not added because I was unable to simulate a loaded event on a UI component. This was tested with a running sample.

Bugs Fixed

• DataTrigger: Evaluate the binding's initial value like WPF #11

API Changes

None

Behavioral Changes

When an element first loads, the binding value is now evaluated by the DataTrigger.

PR Checklist

• Has tests (if omitted, state reason in description)
• Rebased on top of master at time of PR
• Changes adhere to coding standard

[DVaughan]

If AssociatedObject changes before the element is loaded, then the event unsubscription will be missed.
If EvaluateBindingChange throws an exception, the event should still be unsubscribed.
Here's one suggestion:

protected override void OnAttached()
{
    base.OnAttached();

    //fixes issue #11. We want to evaluate the binding's initial value when the element is first loaded
    if (AssociatedObject is FrameworkElement element)
    {
        void OnElementLoaded(object sender, RoutedEventArgs e)
        {
            try
            {
                this.EvaluateBindingChange(e);
            }
            finally
            {
                element.Loaded -= OnElementLoaded;
            }
        }

        element.Loaded += OnElementLoaded;
    }
}

[brianlagunas]

It is my understanding that the AssociatedObject can only be set when attaching. So if the AssociatedObject is to change before it is loaded, this implies that the previous AssociatedObject must also be detached first. Perhaps we should unsubscribe from the Loaded event in the OnDetaching override instead?

        protected override void OnAttached()
        {
            base.OnAttached();
            //fixes issue #11. We want to evaluate the binding's initial value when the element is first loaded
            if (AssociatedObject is FrameworkElement element)
            {
                element.Loaded += OnElementLoaded;
            }
        }

        protected override void OnDetaching()
        {
            base.OnDetaching();
            if (AssociatedObject is FrameworkElement element)
            {
                element.Loaded -= OnElementLoaded;
            }
        }

        private void OnElementLoaded(object sender, RoutedEventArgs e)
        {
            EvaluateBindingChange(e);
        }

Or we can assume the chances of the AssociatedObject changing before it's loaded is so rare, we don't need to worry about it, and go with what we already have.

[DVaughan]

Unsubscribing from the event in OnDetaching is a good idea.
The element.Loaded event may fire multiple times. Calling EvaluateBindingChange is probably not what we want. I'd say add the OnDetaching method to the last commit.

[brianlagunas]

Okay, I modified the code to not only unsubscribe from the LOaded event in the event handler, but also in the OnDetaching as a fail-safe.