This changes the Windows text handling so that keyboard events are sent to the framework first for handling, and then passed to the text input plugin, so that the framework has a chance to handle keys before they get given to the text field.
This is complicated by the async nature of the interaction with the framework, since Windows wants a synchronous response. So, in this change, I always tell Windows that the event was handled, and if the framework (eventually) responds that it wasn't, then I synthesize a new event and send it with SendEvent.
I also added support for detecting "extended" keys, since that was missing, and converted the OnKey handlers in the API to return a bool to indicate whether or not they have handled the event.
During multi-step text input composing, such as with Chinese, Japanese,
and Korean text input, the framework sends embedders cursor rect updates
in the form of two messages:
* TextInput.setMarkedTextRect: notifies the embedder the size and
position of the composing text rect (or cursor when not composing) in
local coordinates.
* TextInput.setEditableSizeAndTransform: notifies the embedder of the
size of the EditableText and 4x4 transform matrix from local to
PipelineOwner.rootNode coordinates.
On receipt of either message, we cache a local copy on the
TextInputPlugin and notify the Win32FlutterWindow of the updated cursor
rect. In a followup patch, we update Win32FlutterWindow to implement the
Win32 input manager (IMM) calls required to position the IME candidates
window while editing.
The C++ text input model used by Windows and Linux currently uses UTF-32. The intention was to facilitate handling of arrow keys, backspace/delete, etc., however since part of what is synchronized with the engine is cursor+selection offsets, and those offsets are defined in terms of UTF-16 code units, this causes very bad interactions with the framework-side model.
This converts to using UTF-16, rather than UTF-32, so that the offsets align with the framework. It also adds surrogate pair handling to the operations that adjust indexes, to avoid breaking surrogate pairs. (Arbitrary grapheme cluster handling is out of scope for this PR; while definitely desirable in the long term, surrogate pair handling is much more critical since improper handling yields invalid UTF-16, which breaks the text field).
This partially fixes https://github.com/flutter/flutter/issues/55014. A framework-side fix is also necessary (since currently both the engine and the framework attempt to handle arrow keys, which is another out-of-scope-for-this-PR issue), but even without the framework fix this dramatically improves the cursor behavior on Windows when there are surrogate pairs somewhere in the string since at least the two sides agree on what indexes mean.
Includes minor plumbing changes to the text input plumbing on Windows so that we're not pointlessly converting from UTF-16 to UTF-32 and then back to UTF-16.
Significantly improves the behavior of non-ASCII text input on Windows. Correctly
processes incoming character events as UTF-16, and for now uses UTF-32 for
the text model so that the existing index-based logic will work much more often.
Future work is still needed, but this will handle far more cases correctly.
Start work on flutter/flutter#30726 by adding an alternative win32 shell platform implementation for Windows that is not based on GLFW and that uses LIBANGLE for rendering and native win32 windowing and input. This change does not replace the GLFW implementation but rather runs side by side with it producing a secondary flutter_windows_win32.dll artifact. The following items must be added to attain parity with the GLFW implementation:
- Custom task scheduling
- Support for keyboard modifier keys
- Async texture uploads
- Correct high DPI handling on Windows versions < 1703
and will be added in subsequent changes.
