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 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.
Changes include:
- File structure
- Header guards
- Include paths
- Namespaces
- Integration with the engine's GN build
- Conversion from jsoncpp to rapidjson
- Style and clang-format adjustment to match engine repository
