# Introduction of FlutterEngine::ProcessExternalWindowMessage

> External windows should call ProcessExternalWindowMessage to be considered for application lifecycle events.




:::important
These breaking change docs are accurate, as of the release
under which they are published. Over time, the
workarounds described here might become inaccurate.
We don't, in general, keep these breaking change docs up
to date as of each release.

The [breaking change index file](/release/breaking-changes)
lists the docs created for each release.
:::


## Summary

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`.

## Who is affected

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

## Description of change

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.

## Migration guide

The following example `WndProc` procedure invokes
`FlutterEngine::ProcessExternalWindowMessage`:

```cpp
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...
}
```

## Timeline

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

## References

Relevant PRs:

* [Reintroduce Windows lifecycle with guard for posthumous OnWindowStateEvent][]

[Reintroduce Windows lifecycle with guard for posthumous OnWindowStateEvent]: https://github.com/flutter/engine/pull/44344