What's the difference between the updateProperties method introduced and the viewWillLayoutSubviews introduced for Observation tracking in UIKit? Which should be used in what context?

The updateProperties method and viewWillLayoutSubviews serve different purposes in UIKit, particularly with the introduction of automatic observation tracking.

updateProperties Method

• Purpose: It is a new method introduced to support automatic trait and observation tracking. It allows you to update properties without forcing a layout, providing finer-grained updates.
• Usage: Use updateProperties to populate content, apply styling, or configure behaviors. It automatically tracks any observable you read, and you can manually trigger it by calling setNeedsUpdateProperties.
• Timing: It runs just before layoutSubviews but is independent of it, allowing you to invalidate properties without triggering a layout pass.
• Example: If you have an observable model object, changes to this model will trigger updateProperties, allowing you to update the view accordingly without unnecessary layout passes.

viewWillLayoutSubviews

• Purpose: This method is part of the traditional layout cycle in UIKit, used to make changes just before the layout of subviews.
• Usage: Use viewWillLayoutSubviews when you need to adjust the layout of subviews in response to changes in the view's bounds or other layout-related changes.
• Timing: It is called during the layout pass, after updateProperties has been executed.

When to Use Each

• Use updateProperties when you need to update properties based on observable changes without affecting the layout. It is ideal for updating content or styles that do not require a layout pass.
• Use viewWillLayoutSubviews when you need to adjust the layout of subviews in response to changes in the view's size or other layout-related events.

For more details, you can refer to the session What’s new in UIKit (12:30) where these methods are discussed.