Make Window.DialogResult a public property.

[rabbitism]

What does the pull request do?

Make Window.DialogResult a public property

What is the current behavior?

The only way to return a result when closing a dialog is to call Close(object?), which means we don't have any way to return a value when closing from system title bar (or press ALt+F4).

What is the updated/expected behavior with this PR?

User can set this property in advance if they want to return some value as default DialogResult. If user choose to close with Close(object?) method, then this paramter has a higher priority, which is the original behavior.

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

Similar behaviors from other frameworks:

1. Winform: there is a public property, but it is an enum.
2. WPF: there is a public property, but a nullable boolean.
3. WinUI: no such a behavior. But when close from close button(or press ESC), a ContentDilaogResult.None is returned

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

Obsoletions / Deprecations

Fixed issues

Closes #17691

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0055026-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

Public API for review:

namespace Avalonia.Controls
{
    public class Window : WindowBase, IFocusScope, ILayoutRoot
    {
+        public object? DialogResult { get; set; }
    }
}

[MrJul]

During the API review meeting, it was noticed that in WPF, DialogResult immediately closes the window, which isn't the case at all in this PR. We aren't really comfortable adding the exact same API member as WPF with a totally different behavior.

We were against mimicking the WPF behavior as it would effectively be exactly the same as Close(DialogResult?).
We discussed calling the property something like DefaultDialogResult.

At the end of the day, we aren't sure this API is needed at all.
The caller of ShowDialog would need to handle a default/null result anyway and be prepared for that.
As mentioned above, if the WPF behavior is needed, Close(DialogResult?) works today.

Therefore, the API is rejected.