[Epic] Controls initialization sequence

[carldebilly]

Epic issue - Controls initialization sequence

This issue will try to regroup the following ones:

• Loading, Loaded & Unloaded events are called too early #2895 Loading, Loaded & Unloaded events are called too early
• [Android] LayoutUpdated may be invoked twice #2769 [Android] LayoutUpdated may be invoked twice
• [Wasm] Loaded even is raised too early #1763 [Wasm] Loaded even is raised too early
• [iOS, Android] Page.NavigatedTo is invoked after Page.Loaded #1478 Page.NavigatedTo is invoked after Page.Loaded
• Frame.Navigated and page's OnNavigatedTo fire in a different order on android than on uwp. #1863 Frame.Navigated and page's OnNavigatedTo fire in a different order on android than on uwp.
  • There's an unmerged PR for this one: Moved Navigated before OnNavigatedTo and OnNavigatedFrom #2008
• [Android, iOS] Control templates are applied too early #190 Control templates are applied too early
• Dialog (ShowAsync()) in MainPage.Loaded event #4550 ContentDialog not appearing
• ScrollViewer performance ScrollViewer performance #4331 (collapsed items are being measured)
• ContentControl Content DataContext is incorrect on macOS MacOS binding problem caused by ContentControl #4626
• x;Bind not working on navigation events x:Bind not working with Frame navigation #2666
• NavigationView items are not rendered properly ItemsRepeater OnElementPrepared is called after NavigationViewItem.OnApplyTemplate #4689
• FrameworkElements in Resources are not loaded [iOS] FrameworkElement in Resources is not loaded #5208
• Popups layout events Unloaded event in ContentDialog is fired when the ContentDialog is opened in Uno.WinUI on MacOS #5404
• TabView items Loaded event is raised too early [WASM] Pivot items are not lazy loaded #5838
• LayoutSlot is zero on first load #7001
• Page OnNavigatedTo fires at the wrong time #7531
• OnApplyTemplate is called no matter its initial visibility #13019

Sequence of events on UWP

Lets suppose we have 2 controls in a direct parent/child relationship (on the logical tree) called outer and inner. Those 2 controls are created at the same time from the same XAML file like this:

<Grid DataContext="myContext">
    <OuterControl x:Name="outer" Style="{StaticResource OuterControlStyle}">
        <OuterControl.Content>
            <InnerControl x:name="inner" Style="{StaticResource InnerControlStyle}" />
        </OuterControl.Content>
    </OuterControl>
</Grid>

Here's the sequence of the initialization of those controls on UWP:

1. outer receive DataContextChanged event
2. outer gets assignments for all setters from OuterControlStyle -- note: templates are not materialized yet
3. outer receive Loading event
4. outer control template gets materialized
5. outer gets called for ApplyTemplate()
6. outer gets called for MeasureOverride(). Calling base.MeasureOverride() will call:
   a. inner receive DataContextChanged event
   b. inner gets assignments for all setters from InnerControlStyle
   c. inner receive Loading event
   d. inner control template gets materialized
   e. inner gets called for ApplyTemplate()
   f. inner gets called for MeasureOverride()
7. outer gets called for ArrangeOverride(). Calling base.ArrangeOverride() will call:
   a. inner gets called for ArrangeOverride()
8. outer receive the SizeChanged event
9. inner receive the SizeChanged event
10. outer receive the Loaded event
11. inner received the Loaded event

Affected platform(s):

• iOS
• Android
• WebAssembly
• macOS [not tested yet]

Problems

1. On UWP, OnApplyTemplate() is called during the first measure phase (step 6). Currently on Uno (all platforms) it's called synchronously with the assignation to the Template (ControlTemplate) property (step 4)
2. On Wasm, inner receives twice t he Loading event
3. On Uno (all platforms), the DataContextChanged is raised synchronously with the assignation. Should be deferred to the first measure phase (6.a.)
4. On Uno (all platforms), the control template gets materialized synchrnously during the Template assignation and ApplyTemplate is also called immediatly. They should be be deferred to the first measure phase. Or, at least, the call to ApplyTemplate should be deferred.
5. On iOS, the inner gets unloaded + reloaded after its template has been materialized
6. Loading should be raised on outer before inner (incorrect on iOS)
7. SizeChanged should be raised on outer before inner (correct only on iOS)
8. SizeChanged should be raised BEFORE the Loaded event for both inner & outer (all platforms)
9. Loaded should be raison on outer before inner (correct only on Android)