transition-outline.kt

package com.example.outlinetransition

import android.graphics.Color
import android.graphics.Outline
import android.os.Bundle
import android.view.View
import android.view.ViewOutlineProvider
import android.widget.FrameLayout
import androidx.annotation.ColorInt
import androidx.annotation.Px
import androidx.appcompat.app.AppCompatActivity
import androidx.transition.TransitionManager
import com.google.android.material.transition.MaterialContainerTransform
import kotlinx.coroutines.MainScope
import kotlinx.coroutines.delay
import kotlinx.coroutines.launch

class MainActivity : AppCompatActivity() {
    
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        val red = circleLayout(Color.BLUE, 300)
        val blue = circleLayout(Color.RED, 600)

        suspend fun swap(view: View) {
            delay(2000)
            val transition = MaterialContainerTransform()
            TransitionManager.beginDelayedTransition(findViewById(android.R.id.content), transition)
            setContentView(view)
        }

        MainScope().launch {
            while (true) {
                swap(red)
                swap(blue)
            }
        }
    }

    private fun circleLayout(@ColorInt color: Int, @Px size: Int): View {
        val circle = View(this).apply {
            transitionName = "circle"
            setBackgroundColor(color)
            outlineProvider = object : ViewOutlineProvider() {
                override fun getOutline(view: View, outline: Outline) {
                    outline.setOval(0, 0, view.measuredWidth, view.measuredHeight)
                }
            }
            clipToOutline = true
        }
        return FrameLayout(this).apply {
            addView(circle, size, size)
        }
    }
}