Introduction of FlutterEngine::ProcessExternalWindowMessage

When you add any external windows to your Flutter app, you need to include them in the Window's app lifecycle logic. To include the window, its WndProc function should invoke FlutterEngine::ProcessExternalWindowMessage.

Windows applications built against Flutter 3.13 or newer that open non-Flutter windows.

Implementing application lifecycle on Windows involves listening for Window messages in order to update the lifecycle state. In order for additional non-Flutter windows to affect the lifecycle state, they must forward their window messages to FlutterEngine::ProcessExternalWindowMessage from their WndProc functions. This function returns an std::optional<LRESULT>, which is std::nullopt when the message is received, but not consumed. When the returned result has a value, the message has been consumed, and further processing in WndProc should cease.

The following example WndProc procedure invokes FlutterEngine::ProcessExternalWindowMessage:

LRESULT Window::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...
}

Landed in version: 3.14.0-3.0.pre
In stable release: 3.16

Relevant PRs:

• Reintroduce Windows lifecycle with guard for posthumous OnWindowStateEvent

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