Typing a Text

Composable.kt

ComposableFunction() {
  DisposableEffect(Unit) {
        typingTimer = TypingTimer().apply {
            setup(
                baseMessage = welcomingMessage,
                finishCallback = { showFooter = true },
                currentMessageUpdateCallback = { 
                  // here if you are using viewbinding it could be textView.text = currentMessage
                  currentMessage = it 
                }
            )
        }

        onDispose {
            releaseTimer()
        }
    }
}

TypingTimer.kt

package com.matematicasimples.presentation.common.components

import android.os.CountDownTimer
import kotlin.math.roundToInt

private const val TYPING_TOTAL_TIME = 10_000L
private const val TYPING_TICK = 100L

class TypingTimer : CountDownTimer(
    TYPING_TOTAL_TIME, TYPING_TICK
) {

    private var baseMessage: String = ""
    private var currentMessageUpdateCallback: ((String) -> Unit)? = null
    private var finishCallback: (() -> Unit)? = null

    var isRunning: Boolean = false
        private set

    fun setup(
        baseMessage: String,
        currentMessageUpdateCallback: ((String) -> Unit),
        finishCallback: (() -> Unit)
    ) {
        this.baseMessage = baseMessage
        this.currentMessageUpdateCallback = currentMessageUpdateCallback
        this.finishCallback = finishCallback

        start()
        isRunning = true
    }

    override fun onTick(p0: Long) {
        val currentTimestamp = TYPING_TOTAL_TIME - p0
        val currentPercentageTimestamp = currentTimestamp.div(TYPING_TOTAL_TIME.toFloat())

        val messageMaxIndex = (currentPercentageTimestamp * baseMessage.length).roundToInt()
        val message = baseMessage.substring(0, messageMaxIndex)

        currentMessageUpdateCallback?.invoke(message)
    }

    override fun onFinish() {
        isRunning = false
        currentMessageUpdateCallback?.invoke(baseMessage)
        finishCallback?.invoke()
    }
}