How to subscribe to IsEffectivelyVisible changed

[danipen]

I'm writing a rotator component that updates some properties every frame. Obviously, I don't want to update properties when the component is not visible to avoid wasting CPU.

Issue

I'm trying to figure out how to manage if a Control changed its visibility, and it's being a little bit tricky because this component is not being hidden itself directly (it's part of the ControlTemplate). So the component is indirectly hidden when some of its parents is hidden.

Workaroud

I noticed that there is an IsEffectivelyVisible property that is useful to determine the visibility in this scenario but was unable to be notified when the EffectivelyVisibility was changed.

I made a sample application to test indirect visibility, and I noticed that when a control is indirectly set to visible/hidden, the property TransformedBounds is changed, so I wrote this ugly workaround:

protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
{
    base.OnPropertyChanged(change);

    if (change.Property == TransformedBoundsProperty || change.Property == IsVisibleProperty)
    {
        if (IsEffectivelyVisible)
            Start(this);
       else
            Stop(this);
    }
}

Questions

So I'm missing an IsEffectivelyVisibleProperty that would really be useful for this scenario.

• How difficult would be to implement IsEffectivelyVisibleProperty?
• Is any other (better) way to do this?
• Any ideas / comments?

[danipen]

There is an interesting discussion about something similar for WPF, here: microsoft/microsoft-ui-xaml#674

[robloo]

That discussion was actually for UWP/WinUI instead of WPF but it's the practically the same problem encountered in all XAML frameworks to date.

[danipen]

Ohh sorry... yes you're right I misread WPF...

So... No easy solution?

[robloo]

As far as I know, you are correct, there is no easy solution. Either you redesign your app so the situation never happens or you have some specialized code like you have.

I think adding a new IsUserVisible property would be great to handle all this. IsEffectivelyVisible won't tell you if an item is scrolled off screen for example.