Add support/enablers for custom/in-app virtual keyboards

[jpnurmi]

Use case

Flutter should make it possible to override the default text input control.

The default text input control is hard-wired to a method channel that is used to communicate with the platform plugin. However, on embedded systems that do not have a system-wide virtual keyboard, it should be possible for an in-app virtual keyboard to register itself so that it can take over the text input communication.

Proposal

In essence, an in-app VKB should be allowed to:
a) intercept text input calls (respective to 'TextInput.xxx') from Flutter, and
b) provide text input responses (respective to 'TextInputClient.xxx') back to Flutter.

This would be possible if there was a way to register a custom text input control that would override the default text input control. Notice, however, that even with custom text input controls it is important to keep some of the text input method channel communication intact to ensure that text input with e.g. physical keyboards continues to work.

Related packages

There is a basic virtual_keyboard package on pub.dev. Notice that it's just a widget with a callback for key presses. Since it has no way to integrate with Flutter and specifically TextField, the example is simply collecting key presses into a Text label and has a toggle button to tell the VKB whether it should present a textual or a numeric mode. While this approach "works" for a trivial single page example, it does not really scale that well for real-life apps with multiple screens and input fields.

Proof of concept

I have made a proof of concept implementation based on the above proposal and the virtual_keyboard package. Here's a video that shows it in action, running with flutter-pi on a Raspberry Pi 4: https://www.youtube.com/watch?v=zgiiThQCU1E

[pedromassangocode]

Cc @justinmc @LongCatIsLooong

[justinmc]

This is really awesome, thanks for the proposal! It never crossed my mind that there might be a reason to write an in-app keyboard in Flutter, but this definitely makes sense for IoT platforms with no system keyboard.

Would you be willing to turn your proof of concept into a PR? I'm happy to help with reviews or anything else.

CC @gspencergoog who has worked on Linux keyboard support.

[jpnurmi]

I'm glad you like the proposal! I can start cleaning up the experimental patches I have lying around.

[jpnurmi]

I have opened a PR: #69146