I only ask because if it doesn't use platform text input, that is a non-starter for mobile.
It also would mean having to reimplement platform behavior for text input, like being able to hold Shift on macOS while moving the cursor to change text selection.
Self-implemented keyboard input pretty much only works for the simplest use case in english-native countries.
https://www.cmyr.net/blog/rust-gui-infra.html and the precursor https://www.cmyr.net/blog/gui-framework-ingredients.html
Browsers are actually a great example of my favorite approach to cross-platform UI, because they sprinkle platform-native widgets throughout the canvas that they render, controlled by a platform-agnostic programming language. Which reminds me that people were trying to use webrender[1] to build native apps in Rust.
Keyboard and key event input is much harder than it first appears. From multiple platforms to multiple languages to multiple keyboard interface devices to multiple layouts, and you cannot assume that the operating system provides a workable abstraction on its own.
// pseudo-interface
class TextWidget {
...
WidgetEvent HandleMouseEvent(const MouseEvent& m, ...);
WidgetEvent HandleKeyboardEvent(const KeyboardEvent& k, ...),
WidgetEvent InputEvent(const InputEvent& i, ...);
};
// client code.
PlatformMsg msg;
while (GetMessage(&msg, ...)) {
WidgetEvent event;
if (IsKeyboard(msg)) {
event=widgets.DispatchEvent(MapKeyboardMsg(msg),...);
} ... {
if (NeedsIMEInput(event)) {
OpenIME(ExtractDetails(event));
}
}
This design will basically have the same functionality but will split the responsibilities so that the widget system would not depend on the input system. This allows the two to vary independently and remain agnostic of each other thus promoting reuse and testing and simplifying the implementation (IMHO).
The question is not about how to invoke some IME, it's about how to separate these concerns and de-couple the widget system from the input system by delegating the invocation (and required details) to somewhere else. I don't see any real reason that warrants tight coupling.