Example implementation of a CircularProgressIndicator using Jetpack Compose https://twitter.com/alexjlockwood/status/1300599202448199681

CircularProgressIndicator.kt

package com.alexjlockwood.circularprogressindicator

import android.os.Bundle
import android.view.animation.PathInterpolator
import androidx.appcompat.app.AppCompatActivity
import androidx.compose.animation.core.*
import androidx.compose.animation.transition
import androidx.compose.foundation.Image
import androidx.compose.foundation.layout.fillMaxHeight
import androidx.compose.foundation.layout.fillMaxWidth
import androidx.compose.material.MaterialTheme
import androidx.compose.runtime.Composable
import androidx.compose.runtime.remember
import androidx.compose.ui.Modifier
import androidx.compose.ui.graphics.SolidColor
import androidx.compose.ui.graphics.StrokeCap
import androidx.compose.ui.graphics.StrokeJoin
import androidx.compose.ui.graphics.vector.VectorAssetBuilder
import androidx.compose.ui.graphics.vector.addPathNodes
import androidx.compose.ui.graphics.vector.group
import androidx.compose.ui.platform.setContent
import androidx.compose.ui.unit.dp

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            MaterialTheme {
                CircularProgressIndicator(Modifier.fillMaxWidth().fillMaxHeight())
            }
        }
    }
}

@Composable
private fun CircularProgressIndicator(modifier: Modifier = Modifier) {
    val state = transition(
        definition = CircularProgressIndicatorTransition,
        initState = 0,
        toState = 1,
    )
    Image(
        asset = VectorAssetBuilder(
            defaultWidth = 48.dp,
            defaultHeight = 48.dp,
            viewportWidth = 48f,
            viewportHeight = 48f,
        ).group(
            translationX = 24f,
            translationY = 24f,
            rotate = state[RotationProp],
        ) {
            addPath(
                pathData = remember { addPathNodes("m 0 -18 a 18 18 0 1 1 0 36 a 18 18 0 1 1 0 -36") },
                stroke = SolidColor(MaterialTheme.colors.primary),
                strokeLineCap = StrokeCap.Square,
                strokeLineJoin = StrokeJoin.Miter,
                strokeLineWidth = 4f,
                trimPathStart = state[TrimPathStartProp],
                trimPathEnd = state[TrimPathEndProp],
                trimPathOffset = state[TrimPathOffsetProp],
            )
        }.build(),
        modifier = modifier,
    )
}

private val TrimPathStartProp = FloatPropKey()
private val TrimPathEndProp = FloatPropKey()
private val TrimPathOffsetProp = FloatPropKey()
private val RotationProp = FloatPropKey()

private val TrimPathStartEasing = PathEasing(android.graphics.Path().apply {
    lineTo(0.5f, 0f)
    cubicTo(0.7f, 0f, 0.6f, 1f, 1f, 1f)
})

private val TrimPathEndEasing = PathEasing(android.graphics.Path().apply {
    cubicTo(0.2f, 0f, 0.1f, 1f, 0.5f, 0.96f)
    cubicTo(0.96f, 0.96f, 1f, 1f, 1f, 1f)
})

private class PathEasing(path: android.graphics.Path) : Easing {
    private val pathInterpolator = PathInterpolator(path)
    override fun invoke(fraction: Float) = pathInterpolator.getInterpolation(fraction)
}

private val CircularProgressIndicatorTransition = transitionDefinition<Int> {
    state(0) {
        this[TrimPathStartProp] = 0f
        this[TrimPathEndProp] = 0.03f
        this[TrimPathOffsetProp] = 0f
        this[RotationProp] = 0f
    }

    state(1) {
        this[TrimPathStartProp] = 0.75f
        this[TrimPathEndProp] = 0.78f
        this[TrimPathOffsetProp] = 0.25f
        this[RotationProp] = 720f
    }

    transition(fromState = 0, toState = 1) {
        TrimPathStartProp using repeatable(
            iterations = AnimationConstants.Infinite,
            animation = tween(durationMillis = 1333, easing = TrimPathStartEasing)
        )
        TrimPathEndProp using repeatable(
            iterations = AnimationConstants.Infinite,
            animation = tween(durationMillis = 1333, easing = TrimPathEndEasing)
        )
        TrimPathOffsetProp using repeatable(
            iterations = AnimationConstants.Infinite,
            animation = tween(durationMillis = 1333, easing = LinearEasing)
        )
        RotationProp using repeatable(
            iterations = AnimationConstants.Infinite,
            animation = tween(durationMillis = 4444, easing = LinearEasing)
        )
    }
}