This updates the Win32 desktop embedder to support input method (abbreviated IM
or IME) composing regions.
In contrast to languages such as English, where keyboard input is
managed keystroke-by-keystroke, languages such as Japanese require a
multi-step input process wherein the user begins a composing sequence,
during which point their keystrokes are captured by a system input
method and converted into a text sequence. During composing, the user is
able to edit the composing range and manage the conversion from keyboard
input to text before eventually committing the text to the underlying
text input field.
To illustrate this, in Japanese, this sequence might look something like
the following:
1. User types 'k'. The character 'k' is added to the composing region.
Typically, the text 'k' will be inserted inline into the underlying
text field but the composing range will be highlighted in some manner,
frequently with a highlight or underline.
2. User types 'a'. The composing range is replaced with the phonetic
kana character 'か' (ka). The composing range continues to be
highlighted.
3. User types 'k'. The character 'k' is appended to the composing
range such that the highlighted text is now 'かｋ'
4. User types 'u'. The trailing 'k' is replaced with the phonetic kana
character 'く' (ku) such that the composing range now reads 'かく'
The composing range continues to be highlighted.
5. The user presses the space bar to convert the kana characters to
kanji. The composing range is replaced with '書く' (kaku: to write).
6. The user presses the space bar again to show other conversions. The
user's configured input method (for example, ibus) pops up a
completions menu populated with alternatives such as 各 (kaku:
every), 描く (kaku: to draw), 核 (kaku: pit of a fruit, nucleus), 角
(kaku: angle), etc.
7. The user uses the arrow keys to navigate the completions menu and
select the alternative to input. As they do, the inline composing
region in the text field is updated. It continues to be highlighted
or underlined.
8. The user hits enter to commit the composing region. The text is
committed to the underlying text field and the visual highlighting is
removed.
9. If the user presses another key, a new composing sequence begins.
If a selection is present when composing begins, it is preserved until
the first keypress of input is received, at which point the selection is
deleted. If a composing sequence is aborted before the first keypress,
the selection is preserved. Creating a new selection (with the mouse,
for example) aborts composing and the composing region is automatically
committed. A composing range and selection, both with an extent, are
not permitted to co-exist.
During composing, keyboard navigation via the arrow keys, or home and
end (or equivalent shortcuts) is restricted to the composing range, as
are deletions via backspace and the delete key. This patch adds two new
private convenience methods, `editing_range` and `text_range`. The
former returns the range for which editing is currently active -- the
composing range, if composing, otherwise the full range of the text. The
latter, returns a range from position 0 (inclusive) to `text_.length()`
exclusive.
Windows IME support revolves around two main UI windows: the composition window
and the candidate window. The composition window is a system window overlaid
within the current window bounds which renders the composing string. Flutter
already renders this string itself, so we request that this window be hidden.
The candidate window is a system-rendered dropdown that displays all possible
conversions for the text in the composing region. Since the contents of this
window are specific to the particular IME in use, and because the user may have
installed one or more third-party IMEs, Flutter does not attempt to render this
as a widget itself, but rather delegates to the system-rendered window.
The lifecycle of IME composing begins follows the following event order:
1. WM_IME_SETCONTEXT: on window creation this event is received. We strip the
ISC_SHOWUICOMPOSITIONWINDOW bit from the event lparam before passing it to
DefWindowProc() in order to hide the composition window, which Flutter
already renders itself.
2. WM_IME_STARTCOMPOSITION: triggered whenever the user begins inputting new
text. We use this event to set Flutter's TextInputModel into composing mode.
3. WM_IME_COMPOSITION: triggered on each keypress as the user adds, replaces,
or deletes text in the composing region, navigates with their cursor within
the composing region, or selects a new conversion candidate from the
candidates list.
4. WM_IME_ENDCOMPOSITION: triggered when the user has finished editing the text
in the composing region and decides to commit or abort the composition.
Additionally, the following IME-related events are emitted but not yet handled:
* WM_INPUTLANGCHANGE: triggered whenever the user selects a new language using
the system language selection menu. Since there some language-specific
behaviours to IMEs, we may want to make use of this in the future.
* WM_IME_NOTIFY: triggered to notify of various status events such as opening
or closing the candidate window, setting the conversion mode, etc. None of
these are relevant to Flutter at the moment.
* WM_IME_REQUEST: triggered to notify of various commands/requests such as
triggering reconversion of text, which should begin composition mode, insert
the selected text into the composing region, and allow the user to select new
alternative candidates for the text in question before re-committing their
new selection. This patch doesn't support this feature, but it's an important
feature that we should support in future.
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 framework handles arrow keys, delete, and backspace (and with better
unicode support), so we shouldn't handle them at the embedding level.
Fixes#69202
Replaces selection_base() and selection_extent() with selection() and
SetSelection(int, int) with SetSelection(range).
This also adds the following convenience methods to TextRange:
* reversed()
* Contains(size_t position)
* Contains(const TextRange& range)
as well as operator== for use in unit tests. When Flutter migrates to
C++20, we can replace that method with a default declaration.
Previously, TextInputModel's SetEditingState method was a 1:1 mapping of
the underlying protocol used on the text input channel between the
framework and the engine. This breaks it up into two methods, which
allows the selection to be updated independently of the text, and avoids
tying the API the the underlying protocol.
This will become more important when we add additional state to support
composing regions for multi-step input methods such as those used for
Japanese.
SetText resets the selection rather than making a best-efforts attempt
to preserve it. This choice was primarily to keep the code simple and
make the API easier to reason about. An alternative would have been to
make a best-effort attempt to preserve the selection, potentially
clamping one or both to the end of the new string. In all cases where an
embedder resets the string, it is expected that they also have the
selection, so can call SetSelection with an updated selection if needed.
The Windows, Linux, and GLFW embeddings (which all share a common code
ancestry) pass TextInput.setEditingState selection base and extents
straight through to the shared text model class. The model expects those
values to be valid, but the framework sends -1/-1 for "invalid"
selections, which happen for some empty text cases (e.g.,
TextFieldController.clear()).
This translates those invalid selection values to an empty selection at
the start of the string, as expected by the model.
Fixes https://github.com/flutter/flutter/issues/59140
The existing logic incorrectly factored out a check that there were arguments too early, applying it to any message not already handled (including unhandled methods, such as methods added after the initial implementation) and thus failing if any unhandled message had no arguments.
Fixes https://github.com/flutter/flutter/issues/55653
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.
The JSON codec is awkward to use in the wrapper (since the client has to build and link one of the JSON libraries to do so). Since it would be very cumbersome to wrap in a C API, and there's essentially no reason to use it instead of the standard codec, this removes it from the wrapper entirely.
Since some system channels (internal to the engine) still use it, it's moved into common/cpp instead of being eliminated entirely. Internally we always use RapidJSON though, so the jsoncpp implementation is removed. Also adds some unit test coverage, since there wasn't any.
Fixes#30669
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.
