Autosizing EditText kotlin extension.

AutoSizeEditText.kt

fun EditText.setAutoSizing() {
    initialTextSize = textSize
    afterTextChanged { autoSizeText() }
}

private fun EditText.autoSizeText() {
    if(text.isEmpty()) {
        setTextSize(TypedValue.COMPLEX_UNIT_PX, initialTextSize)
        return
    }
    val length = paint.measureText(text.toString())
    paint.getTextBounds(text.toString(), 0, text.length, autoSizeBounds)
    val widthSize = textSize * width / length
    val heightSize = textSize * height / autoSizeBounds.height()

    setTextSize(TypedValue.COMPLEX_UNIT_PX, arrayOf(widthSize, heightSize, initialTextSize).min()!!)
}

fun EditText.afterTextChanged(afterTextChanged: (String) -> Unit) {
    this.addTextChangedListener(object : TextWatcher {
        override fun beforeTextChanged(p0: CharSequence?, p1: Int, p2: Int, p3: Int) { /* nop */
        }

        override fun onTextChanged(p0: CharSequence?, p1: Int, p2: Int, p3: Int) { /* nop */
        }

        override fun afterTextChanged(editable: Editable?) {
            afterTextChanged.invoke(editable.toString())
        }
    })
}

private var EditText.initialTextSize: Float
    get() = getTag(INITIAL_TEXT_SIZE) as Float
    set(value) {
        setTag(INITIAL_TEXT_SIZE, value)
    }

private val autoSizeBounds = Rect()
private const val INITIAL_TEXT_SIZE = 72846234

The above gist automatically adjust font size to fit within the bounds of the EditText. It requires the text to be one-line only and the editText to have pre-set width (that means no wrap_content. It's not optimized but a single edittext shouldn't throttle even the lowest-end devices.

Also worth noting - it breaks heavily for very small font sizes. Probably something related to floating point error, but I can't be bothered with fixing it since it isn't a real world use-case.