CodeKeyboard Part 3: Composing, Selection, Recompose

Why composing

InputConnection.setComposingText(text, 1) shows underlined in-progress text in many apps. commitText finalizes. Mid-word suggestions need a buffer the IME owns so a tap can replace the whole partial word. Without composing you only insert characters and scrape getTextBeforeCursor forever.

ComposingBuffer

File: ComposingBuffer.kt

class ComposingBuffer {
    val text: String
    fun append(char: String): String
    fun backspace(): Boolean  // false if empty
    fun flush(): String       // return copy and clear
    fun setText(s: String)    // clear then append (recompose)
    fun clear()
}

No Android types. Unit-testable alone (ComposingBufferTest.kt).

Character input (supportsComposing true)

In handleKey else-branch for character keys (simplified):

1. text = kbState.resolveLabel(key)
2. If single char and meta active (ctrl/alt/meta): flush composing, send key events with meta, onCharCommitted, return
3. If text == ";": append, setComposingText, snippet suggestions
4. Else if length 1 and not punctuation: append, setComposingText, dictionary suggestions with context = prevCommittedWord
5. Else: flush, commitText
6. Always (for char path): kbState.onCharCommitted() and refresh key draw state

Punctuation set is a fixed char set in CodeKeyboardIME (period, comma, operators, quotes, etc.).

Space

"space" -> {
    flushComposing(ic)
    ic?.commitText(" ", 1)
    val next = bigramModel.nextWords(prevCommittedWord, n = 5)
    if (next.isNotEmpty()) suggestionBar.update("", next)
}

flushComposing

val word = composing.flush()
if (word.isNotEmpty()) {
    ic?.commitText(word, 1)
    wordLearner.learnFromFlush(word)
    kbState.onCharCommitted()
    // metrics...
    if (prevCommittedWord.isNotEmpty())
        bigramModel.recordTransition(prevCommittedWord, word)
    prevCommittedWord = word
}
keystrokesSinceCommit = 0
suggestionBar.clear()  // space path may refill immediately after

Note: the space handler calls flush (which clears the bar) then may show bigrams.

onUpdateSelection

Problem: user taps elsewhere; composing region is stale; next char would replace the wrong span.

Also: IME calls that change selection must not look like user taps.

val imedriven = SystemClock.uptimeMillis() < expectSelectionUpdateBy
if (composing.text.isEmpty()) return
if (imedriven) return
// if cursor outside candidatesStart..candidatesEnd (or candidates unset):
ic.finishComposingText()
composing.clear()
suggestionBar.clear()

Recompose after backspace into committed text

When composing is empty and backspace deletes one committed character:

1. deleteSurroundingText(1, 0) (or DEL key event fallback)
2. recomposeWordAtCursor(ic)

val before = ic.getTextBeforeCursor(RECOMPOSE_SCAN_CHARS, 0) // 50
val fragment = before.takeLastWhile { it.isLetterOrDigit() || it == '\'' }
// get absolute cursor via ExtractedText
expectSelectionUpdateBy = now + 500L
ic.beginBatchEdit()
ic.finishComposingText()
ic.setComposingRegion(absCursor - fragment.length, absCursor)
composing.setText(fragment)
ic.endBatchEdit()
suggestionBar.update(fragment, suggestionStrategy.suggest(fragment, 5, prevCommittedWord))

Tests: BackspaceRecomposeTest.kt. Debug logs used tag CKB_COMPOSE during development.

Backspace order

1. Non-empty selection: batch finish composing, clear buffer, commitText("") to delete selection
2. Else if composing.backspace(): update composing text + suggestions
3. Else: delete surrounding + recompose

Enter

flushComposing(ic)
// if IME_FLAG_NO_ENTER_ACTION or action NONE/UNSPECIFIED -> KEYCODE_ENTER
// else performEditorAction(action), fallback KEYCODE_ENTER

Search/done fields need performEditorAction.

onStartInput / onFinishInput

Start: set supportsComposing, clear composing and prevCommittedWord, finishComposingText, clear bar.
Finish: finish composing, clear buffer/bar, scheduleUserTrieFlush() saves user.trie.

Verification (part 3)