Init the flex item before adding it to the layout

[mattleibow]

Description of Change

In some cases, adding an item before may trigger an update before the add operation is complete. For example, adding a label with HTML text may cause the layout to run while the label is being constructed.

To prevent that layout pass from crashing because the flex item is not yet set, we make sure to first set the flex item, and then add it to the layout. Then, when the layout pass runs, the item is ready to be used.

This error might not typically appear in normal XAML layouts because the items are first set, and then the entire page is inflated. However, if the layout is part of a CollectionView or items are added dynamically after the UI is loaded, this layout during add may occur.

Issues Fixed

Fixes #21711

[MartyIX]

(Not want to hijack this PR but I wrote this comment #9075 (comment) and perhaps "it's obviously wrong there"¹ and thus an opportunity to kill two bugs with one PR.)

Footnotes

1. Perhaps not, I can't really tell. ↩

[mattleibow]

@MartyIX Oh wow, that is interesting... Layout being called in a measure... which may call measure...

But I think this is unrelated since this is iOS and that was Windows. I also found something that is probably the reason for this issue on iOS (not saying there is not another issue with measure/layout, just there are probably more than one):

https://developer.apple.com/documentation/foundation/nsattributedstring/1524613-initwithdata#discussion

The HTML importer should not be called from a background thread (that is, the options dictionary includes NSDocumentTypeDocumentAttribute with a value of NSHTMLTextDocumentType). It will try to synchronize with the main thread, fail, and time out. Calling it from the main thread works (but can still time out if the HTML contains references to external resources, which should be avoided at all costs). The HTML import mechanism is meant for implementing something like markdown (that is, text styles, colors, and so on), not for general HTML import.

Seems the HTML type for NSAttributedString is somehow causing the main thread to run one last loop - and thus a measure. The constructor is doing bad things for sure.

maui/src/Core/src/Platform/iOS/LabelExtensions.cs

Lines 76 to 90 in 8410edb

	internal static void UpdateTextHtml(this UILabel platformLabel, ILabel label)
	{
	string text = label.Text ?? string.Empty;
	var attr = new NSAttributedStringDocumentAttributes
	{
	DocumentType = NSDocumentType.HTML,
	StringEncoding = NSStringEncoding.UTF8
	};
	NSError nsError = new();
	#pragma warning disable CS8601
	platformLabel.AttributedText = new NSAttributedString(text, attr, ref nsError);
	#pragma warning restore CS8601
	}

[mattleibow]

IOS UI tests are all green. Android and Windows are not adding items (or the test is broken) so there is some issue with adding items at runtime.

[mattleibow]

Hmm, I blame Appium. Manually running the tests and clicking on buttons work. Appium times out. Time to investigate!

[mattleibow]

Looks like Android and Windows are now passing after I added the AutomationId.

[PureWeen]

Failing test not related