Add getWindowHandle (#548)

Co-authored-by: copywith9 <copywith9@example.com>

Author: copywith9

packages/window_manager/example/lib/pages/home.dart

@@ -180,6 +180,14 @@ class _HomePageState extends State<HomePage> with TrayListener, WindowListener {
                 print('isFocused: ${await windowManager.isFocused()}');
               },
             ),
+            if(Platform.isWindows)
+            PreferenceListItem(
+              title: const Text('getWindowHandle'),
+              onTap: () async {
+                final result = await windowManager.getWindowHandle();
+                print('HWND:$result');
+              },
+            ),
             PreferenceListItem(
               title: const Text('show / hide'),
               onTap: () async {

packages/window_manager/lib/src/window_manager.dart

@@ -104,6 +104,10 @@ class WindowManager {
     await _channel.invokeMethod('ensureInitialized');
   }
 
+  Future<int> getWindowHandle()async{
+    return await _channel.invokeMethod('getWindowHandle') as int;
+  }
+
   /// You can call this to remove the window frame (title bar, outline border, etc), which is basically everything except the Flutter view, also can call setTitleBarStyle(TitleBarStyle.normal) or setTitleBarStyle(TitleBarStyle.hidden) to restore it.
   Future<void> setAsFrameless() async {
     await _channel.invokeMethod('setAsFrameless');

packages/window_manager/windows/window_manager_plugin.cpp

@@ -350,7 +350,11 @@ void WindowManagerPlugin::HandleMethodCall(
     window_manager->native_window =
         ::GetAncestor(registrar->GetView()->GetNativeWindow(), GA_ROOT);
     result->Success(flutter::EncodableValue(true));
-  } else if (method_name.compare("waitUntilReadyToShow") == 0) {
+  }
+  else if (method_name.compare("getWindowHandle") == 0) {
+    result->Success(flutter::EncodableValue(reinterpret_cast<__int64>(window_manager->GetMainWindow())));
+  }
+  else if (method_name.compare("waitUntilReadyToShow") == 0) {
     window_manager->WaitUntilReadyToShow();
     result->Success(flutter::EncodableValue(true));
   } else if (method_name.compare("setAsFrameless") == 0) {