Scheduler breaks keyboard behavior for Compose UI consumers

Compose UI’s CoreTextField is very picky about getting immediate updates to its values. The common (but useless) example seen in the docs, where the text state is held in a by remember { mutableStateOf("") }, has no problems updating the text field when typing on the keyboard quickly. However, if you provide the text value from something that doesn’t provide updates synchronously from onValueChange, the text field starts behaving erratically; characters randomly disappear, the cursor moves around in its own, and other weird stuff.

This issue made us realize that mapping UI state on a background thread will never work. So we’ve been running our presenter composition on the main thread, using an immediate monotonic frame clock:

private object ImmediateMonotonicFrameClock : MonotonicFrameClock {
  override suspend fun <R> withFrameNanos(onFrame: (frameTimeNanos: Long) -> R): R {
    return onFrame(System.nanoTime())
  }
}

This completely solved the keyboard glitchiness.

We’re now looking to migrate to Molecule. But in doing so, we’ve seen the keyboard glitchiness return! I somewhat narrowed it down to behaviors that depend on the combination of the Molecule’s Dispatcher and MonotonicFrameClock:

Dispatcher	MonotonicFrameClock	Behavior
AndroidUiDispatcher	Choreographer	Bad
AndroidUiDispatcher	ImmediateMonotonicFrameClock	Less bad (but still bad)
Dispatchers.Main	Choreographer	Bad
Dispatchers.Main	ImmediateMonotonicFrameClock	Good

More specifics on the glitchiness:

Try spamming a character quickly. Occasionally, some will be dropped. When that happens, this appears in logcat:

getSurroundingText on inactive InputConnection
beginBatchEdit on inactive InputConnection
getTextBeforeCursor on inactive InputConnection
getTextAfterCursor on inactive InputConnection
getSelectedText on inactive InputConnection
endBatchEdit on inactive InputConnection

An easy way to check if the issue is happening is to hold the backspace button when there’s some text present. With the issue, the backspace will randomly “stop working”; characters will stop being deleted even though you’re still pressing backspace.

Honestly, I’m not positive as to whether this is Molecule’s or CoreTextField’s fault. Input appreciated.

Repro project with those dispatcher/frameclock modes

[chris-horner]

I put together a sample that allows for easy demoing of the issue and switching between the two clock types. RecompositionClock.Immediate does seem to fix the weird behaviour of input fields.

https://user-images.githubusercontent.com/1245751/182053296-14df5da3-e45b-4747-aa10-fd49e74f3a54.mp4

[JakeWharton]

I have no had a chance to look, no. I can try to look when I’m back working on this library as part of the Kotlin 1.6.20 upgrade to Compose.