Make app lifetime windows property an observable collection

[justinshannon]

What does the pull request do?

This PR updates the Windows collection in ClassicDesktopStyleApplicationLifetime from a read-only list to a read-only observable list. With this change, you can now subscribe to the CollectionChanged event to track when open windows are added or removed.

What is the current behavior?

At present, there's no way to track when windows are opened or closed.

What is the updated/expected behavior with this PR?

With this PR, the Windows collection in ClassicDesktopStyleApplicationLifetime becomes a read-only observable list, enabling you to monitor changes to the open windows. By subscribing to the CollectionChanged event, you can now detect when windows are added to or removed from the collection, allowing you to track when windows are opened or closed in real time.

How was the solution implemented (if it's not obvious)?

The solution was implemented by updating the type of the Windows collection in ClassicDesktopStyleApplicationLifetime from IReadOnlyList to IAvaloniaReadOnlyList. Since the private field declaration (_windows) is already an AvaloniaList, this change simply makes the public API an observable collection.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

N/A

Obsoletions / Deprecations

N/A

Fixed issues

N/A

[maxkatz6]

Unfortunately, this PR breaks binary compatibility, changing property type might break existing libraries/application that expected old type.

enabling you to monitor changes to the open windows

Note, you still can cast to INotifyCollectionChanged interface, and subscribe on the same event.
Or, probably event better, you can subscribe to Window.Opened.AddClassHandler event directly, without lifetimes in between.

[cla-avalonia]

@justinshannon,

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[thevortexcloud]

You can also create a Window manager class that keeps track of window states and raises events as needed without making any changes to Avalonia itself.