New api spec : Window.AppWindow api

specs/appwindow-spec.md

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+Window.AppWindow api
+===
+New api to simplify accessing [appwindow](https://docs.microsoft.com/en-us/windows/apps/windows-app-sdk/windowing/windowing-overview)
+functionality through WinUI code
+
+# Background
+Xaml has a [Window](https://docs.microsoft.com/uwp/api/Windows.UI.Xaml.Window) API that
+internally wraps an hwnd. Windows has an [AppWindow](https://docs.microsoft.com/uwp/api/Windows.UI.WindowManagement.AppWindow) class
+that similarly wraps an hwnd in UWP. WinAppSDK has a new [AppWindow](https://docs.microsoft.com/windows/windows-app-sdk/api/winrt/Microsoft.UI.Windowing.AppWindow) which wraps an hwnd and works on Desktop.
+
+You can get an AppWindow from a Xaml Window by calling a COM API to get Xaml and then
+a DLL export API to convert the hwnd to an AppWindow. This spec adds a simple `Window.AppWindow`
+property to make this much easier and more discoverable.
+
+[Xaml Window](https://docs.microsoft.com/en-us/windows/windows-app-sdk/api/winrt/microsoft.ui.xaml.window) will
+expose AppWindow object directly to app developer through an api. Instead of writing a lot of boiler plate
+code everywhere, app developer can use this api, reducing code bloat, and making appwindow apis easily
+accessible from winui code.
+
+These *before* and *after* C# code examples illustrate how this api simplifies integrating appwindow apis in winui codebase.
+
+### Before
+```c#
+// This is needed to get any Window from inside a Xaml control 
+var xamlWindow = WindowHelper.GetWindowForElement(this); // api to get window object from UIElement (not a real api)  
+
+// unnecessary boiler plate code 
+var windowId = Win32Interop.GetWindowIdFromWindow(WindowNative.GetWindowHandle(xamlWindow)); 
+var appWindow = AppWindow.GetFromWindowId(windowId); 
+
+//calling function foo  (not a real appwindow function)
+appWindow.foo();
+```
+
+### After
+```c#
+// This is needed to get any Window from inside a Xaml control 
+var xamlWindow = WindowHelper.GetWindowForElement(this); // api to get window object from UIElement (not a real api)  
+
+
+//calling function foo directly
+xamlWindow.AppWindow.foo();  
+```
+Notice how `xamlWindow.AppWindow.foo()` doesn't require additional steps to call the function `foo`.
+
+
+# API Pages
+
+## Window.AppWindow property
+
+Gets the `AppWindow` associated with this `Window`.
+```c#
+public Microsoft.UI.Windowing.AppWindow Window.AppWindow { get; }
+```
+_Spec note: Mapping an hwnd to an AppWindow causes the hwnd to be subclassed, meaning that the timing of
+when [AppWindow.GetFromWindowId](https://docs.microsoft.com/windows/windows-app-sdk/api/winrt/Microsoft.UI.Windowing.AppWindow.GetFromWindowId)
+is first called can potentially have a behavioral side effect. To ensure this is predictable, the AppWindow
+for Xaml's hwnd will be created during the Xaml Window's construction. Note that this could potentially be
+an observable behavior change from the behavior before the introduction of this API.
+
+### Sample example
+```c#
+var xamlWindow = WindowHelper.GetWindowForElement(this);   
+auto windowSize = xamlWindow.AppWindow.Size;  
+```
+
+# API Details
+
+```c# (but really MIDL3)
+namespace Microsoft.UI.Xaml
+{
+   unsealed runtimeclass Window
+  {
+      ...
+      Microsoft.UI.Windowing.AppWindow AppWindow{ get; };
+  }
+}
+```
+
+# Appendix
+- Window.AppWindow api returns reference to the same appwindow object every time. It gets created
+  during xaml window object's creation.
+
+- Since AppWindow is just an abstraction for HWND, its lifetime will be same as that of HWND. The
+  api cannot return null as AppWindow will always be present if Window object is there. If called after
+  window has been closed, it will fail similarly to other winui apis and return a failure HRESULT. 
+
+- There will be just one appwindow object per top level HWND, created during Window object creation.
+  No new appwindow objects are created for child windows. 
+
+- Future scope : Since this api is limited to top level HWND, there should be a way to get app window object
+  for top level HWND from one of its nested children HWND which doesn’t require app developer writing a lot of code. 
+