Windows lifecycle

Windows applications built against Flutter versions after 3.13 that open non-Flutter windows.

When adding a non-Flutter window to a Flutter Windows app, it will not be part of the logic for application lifecycle state updates by default. For example, this means that when the external window is shown or hidden, the app lifecycle state won't appropriately update to inactive or hidden. As a result, the app might receive incorrect lifecycle state changes through WidgetsBindingObserver.didChangeAppLifecycle.

What do I need to do?

To add the external window to this application logic, the window's WndProc procedure must invoke FlutterEngine::ProcessExternalWindowMessage.

To achieve this, add the following code to the message handler function of your custom external window. In C++ wrappers for the Win32 API, this is often a class method called from the window's WndProc. The exact file and class name depend on your app's implementation.

LRESULT MyExternalWindow::MessageHandler(HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam) {
    std::optional<LRESULT> result = flutter_controller_->engine()->ProcessExternalWindowMessage(hwnd, msg, wparam, lparam);
    if (result.has_value()) {
        return *result;
    }
    // Original contents of WndProc...
}