Animate alhpa of each character separately

AlphaSpan.kt

package com.luciofm.playground

import android.text.TextPaint
import android.text.style.CharacterStyle
import android.text.style.UpdateAppearance

class AlphaSpan(var alpha: Int) : CharacterStyle(), UpdateAppearance {

    override fun updateDrawState(ds: TextPaint) {
        ds.alpha = alpha
    }
}

MainActivity.kt

package com.luciofm.playground

import android.animation.Animator
import android.animation.AnimatorSet
import android.animation.ObjectAnimator
import android.annotation.SuppressLint
import android.os.Bundle
import android.support.v7.app.AppCompatActivity
import android.text.SpannableString
import android.text.Spanned
import android.util.Property
import android.view.animation.BounceInterpolator
import kotlinx.android.synthetic.main.activity_main.*


class MainActivity : AppCompatActivity() {

    @SuppressLint("WrongConstant")
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        buildAnimation()
    }

    private fun buildAnimation() {
        val text = SpannableString("Hello World!")

        val spans = ArrayList<AlphaSpan>(text.length)
        text.forEachIndexed { index, c ->
            val span = AlphaSpan(0)
            spans.add(span)
            text.setSpan(span, index, index + 1, Spanned.SPAN_EXCLUSIVE_EXCLUSIVE)
        }

        var currentDelay = 0L
        val animators = ArrayList<Animator>()
        for (span in spans) {
            val anim = ObjectAnimator.ofInt(span, ALPHA_SPAN_PROPERTY, 0, 255)
            anim.interpolator = BounceInterpolator()
            anim.duration = 1200
            anim.startDelay = currentDelay
            animators.add(anim)
            anim.addUpdateListener {
                text_view.text = text
            }
            currentDelay += 120L
        }

        val anim = AnimatorSet()
        anim.playTogether(animators)
        anim.start()
    }

    private val ALPHA_SPAN_PROPERTY = object : Property<AlphaSpan, Int>(Int::class.java, "ALPHA_SPAN_PROPERTY") {
        override fun set(span: AlphaSpan, value: Int) {
            span.alpha = value
        }

        override fun get(span: AlphaSpan) = span.alpha
    }
}