Add Loaded/Unloaded Events from WinUI/UWP

Is your feature request related to a problem? Please describe.

UWP has the Loaded/Unloaded events which are useful for control authors to connect/disconnect things.

• Loaded : “Occurs when a FrameworkElement has been constructed and added to the object tree, and is ready for interaction.”
• Unloaded : “Occurs when this object is no longer connected to the main object tree.”

OnAttachedToVisualTree and OnDetachedFromVisualTree are similar but the sequence these methods/events are called is not the same. OnAttachedToVisualTree is called BEFORE OnApplyTemplate which is a problem if you are connecting events and the like. Therefore, Loaded would be invoked just before (or during) the control is first shown to the user – after both OnApplyTemplate and OnAttachedToVisualTree.

Describe the solution you’d like

Add closely equivalent Loaded/Unloaded events and new OnLoaded/OnUnloaded methods to Control. This will help control authors and apps coming over from WinUI and UWP. It might also simplify some Avalonia controls.

Describe alternatives you’ve considered

Additional context Add any other context or screenshots about the feature request here.

[robloo]

Yes, forgot to add it, but that’s what the _IsLoaded field is for. May be a DirectProperty or a CLR property, haven’t decided yet and will check WPF.

[amwx]

@amwx This is near the top of my list for next issues to close. I imagine I’ll get to it next weekend. Would appreciate a review at that time or even some ideas on good places to raise these events. I’m not very familiar with the control lifecycle in Avalonia at the moment.

I had a peek at the WPF source and checked a call stack for how this is implemented. Doesn’t make a whole lot of sense as its scattered in 10 different places with weirdly named helper classes in typical MS fashion, but it looks like they fire it after the entire layout pass is completed and just before the render pass. The exact details of this are a bit more obscure. In Avalonia, it probably could be done in LayoutManager.ExecuteLayoutPass as the last step, by keeping track of which elements haven’t loaded yet. But, I’m not that familiar with the layout => rendering pipeline other than just generally how it works so one of the core team will probably have better advice.

Unloaded is much less of an issue, and I think DetachedFromVisualTree does the same thing, and Unloaded may not even be needed. Though for parity, it makes sense to include it and can probably be raised at the same time.