Property to Track OnScreen Visibility of a Control

[robloo]

This topic comes up from time-to-time. There isn't really a good way to tell if a control is ACTUALLY visible to the user on the screen. By "actually visible" I mean drawn on the screen where the user can see it -- not clipped, hidden, etc. -- actually there on the screen.

Here is the topic from the WinUI repo which has further discussion about this: microsoft/microsoft-ui-xaml#674

Now in that discussion I don't recall the topic of automation API's coming up. However, the automation APIs of course need this information themselves to figure out what to announce to users. Therefore, we have the IsOffscreen property and certain modes get us most of the way there.

However, what is everyone's opinion about:

1. Making a new property for this information and exposing it to Visuals/Controls?
2. Then adding an event for when it changes?
3. Bonus would be adding information on when Render() is actually called for a control as well so we know when the first render is complete (in 3rd party controls).

This is all helpful in some use cases where you only want to do certain expensive operations or perhaps UI animations/updates when a control is actually shown to the user.

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Note that this functionality can be achieved by watching the Bounds property right now. That method is used in some third party controls as well. If Bounds.Size is (0,0) the control isn't shown yet. When the Bounds has an actual value you can assume it is now visible. Composition/Rendering is efficient and already has the needed information.

[maxkatz6]

Just adding some options here

Then adding an event for when it changes?

Avalonia and UWP have EffectiveViewportChanged event, which is typically enough to get complete information about control visibility.
And at least in Avalonia it should include occlusions (it seems not to be the case in UWP I guess? from the linked discussion).
This event is expensive though and only enables viewport calculation when there is any subscriber.

This event provides more complete result than Bounds, as goes through every ancestor boundaries/clip.

Yet, it's not complete enough, as it only can read/respect UI thread (i.e. - layout) information.
Anything that happens with composition visual on render thread is there. Like a composition animation with offset. A rare use-case, probably?

Bonus would be adding information on when Render() is actually called for a control as well so we know when the first render is complete (in 3rd party controls).

What's wrong with Loaded event? Although, neither (loaded/render method) indicate moment of first frame drawn with this element.

[robloo]

Avalonia and UWP have EffectiveViewportChanged event... This event is expensive though and only enables viewport calculation when there is any subscriber.

Yes, I forgot about that. However, it is a bit different at least in the nuances. It would be very nice for applications if all available information was pre-summarized in an IsOnscreen property (with perhaps IsCompletelyOnscreen or something too).

That said, for now I should probably be using this instead of the Bounds.Size method as you said.

Anything that happens with composition visual on render thread is there. Like a composition animation with offset. A rare use-case, probably?

But this is part of my point. I believe the composition/rendering system already has the necessary information to calculate IsOnscreen quickly without extra overhead -- including edge cases. We just have to get that information back out. It could be used not only for new properties as proposed but also for improved automation IsOffscreen calculation.

What's wrong with Loaded event? Although, neither (loaded/render method) indicate moment of first frame drawn with this element.

Exactly, this came up with controls analogous to a map. The tiles in the background take a while to load/render. It would be nice to have a standard way of knowing when the control actually finished rendering so animations can then be run (fly to location for example).

When the control finishes rendering is different than when it is considered Loaded by Avalonia.