PulseView.kt

package com.example

import android.content.Context
import android.graphics.BitmapFactory
import android.graphics.Canvas
import android.graphics.Paint
import android.graphics.PointF
import android.util.AttributeSet
import android.view.MotionEvent
import android.view.View
import android.view.animation.DecelerateInterpolator

class PulseView @JvmOverloads constructor(
    context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0
) : View(context, attrs, defStyleAttr) {

    private val paint = Paint(Paint.ANTI_ALIAS_FLAG).apply { color = 0xff6DA9D7.toInt() }
    private var button = BitmapFactory.decodeResource(resources, R.drawable.button)
    private var touch = PointF(0.0f, 0.0f)
    private var startTime = System.currentTimeMillis()
    private val size = SIZE_DP * resources.displayMetrics.density
    private val interpolator = DecelerateInterpolator()

    override fun dispatchTouchEvent(event: MotionEvent): Boolean {
        touch = PointF(event.x, event.y)
        return true
    }

    private fun lerp(a: Float, b: Float, t: Float) = a * (1 - t) + b * t

    override fun onDraw(canvas: Canvas) {
        super.onDraw(canvas)
        val time = System.currentTimeMillis()
        for (r in 0..RIPPLE_COUNT) {
            val offset = INTERVAL_MS * r / RIPPLE_COUNT
            val t = (time - startTime + offset) % INTERVAL_MS / INTERVAL_MS
            val interpolatedT = interpolator.getInterpolation(t)
            paint.alpha = (255 - interpolatedT * 255).toInt()
            canvas.drawCircle(
                lerp(touch.x, width / 2.0f, t),
                lerp(touch.y, height / 2.0f, t),
                interpolatedT * size,
                paint
            )
        }
        paint.alpha = 255
        canvas.drawBitmap(button, touch.x - button.width / 2, touch.y - button.height / 2, paint)
        postInvalidate()
    }

    companion object {
        const val RIPPLE_COUNT = 2
        const val INTERVAL_MS = 1000.0f
        const val SIZE_DP = 100.0f
    }
}