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Recomposition Highlighter

Author: Google Play

The Google Play team were among the first internal teams at Google to leverage Jetpack Compose. They worked very closely with the Compose team and even wrote a case study describing their experience migrating to Compose. One of the golden nuggets from that post was a Modifier that they developed to visualize recompositions.

Let's look at what it looks like in action. Our example has a simple composable function that has a button which increments a counter when its clicked. We are using the recomposeHighlighter modifier in two places - the root of the MyButtonComponent composable itself and the MyTextComponent composable which is a slot for the button itself.

fun MyButtomComponent(
    modifier: Modifier = Modifier.recomposeHighlighter()
) {
    var counter by remember { mutableStateOf(0) }

        onClick = { counter++ },
        modifier = modifier,
    ) {
            text = "Counter: $counter",
            modifier = Modifier.clickable {

fun MyTextComponent(
    text: String,
    modifier: Modifier = Modifier,
) {
        text = text,
        modifier = modifier

On running this example, we notice that the button and the text inside the button both have a blue bounding box initially. This makes sense as that's the first composition and it corresponds to the two places where we are using the recomposeHighlighter() Modifier. When we click on the button, we notice the bounding box only around the text inside the button and not the button itself. This is because Compose is smart about recompositions and it doesn't need to recompose the entire button - just the composable that depends on the counter value that was updated.

Recompose Highlighter Modifier Demo

recomposeHighlighter Modifier in action

Using this modifier, we were able to visualize how recompositions are happening in our Composable functions. This is a really powerful tool and I can imagine this being extended in really creative ways.

* Copyright 2022 Google Inc.
* Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file
* except in compliance with the License. You may obtain a copy of the License at
* http://www.apache.org/licenses/LICENSE-2.0
* Unless required by applicable law or agreed to in writing, software distributed under the
* KIND, either express or implied. See the License for the specific language governing
* permissions and limitations under the License.
package com.example.android.compose.recomposehighlighter
import androidx.compose.runtime.LaunchedEffect
import androidx.compose.runtime.Stable
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.remember
import androidx.compose.ui.Modifier
import androidx.compose.ui.composed
import androidx.compose.ui.draw.drawWithCache
import androidx.compose.ui.geometry.Offset
import androidx.compose.ui.geometry.Size
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.graphics.SolidColor
import androidx.compose.ui.graphics.drawscope.Fill
import androidx.compose.ui.graphics.drawscope.Stroke
import androidx.compose.ui.graphics.lerp
import androidx.compose.ui.platform.debugInspectorInfo
import androidx.compose.ui.unit.dp
import androidx.compose.ui.util.lerp
import kotlin.math.min
import kotlinx.coroutines.delay
* A [Modifier] that draws a border around elements that are recomposing. The border increases in
* size and interpolates from red to green as more recompositions occur before a timeout.
fun Modifier.recomposeHighlighter(): Modifier = this.then(recomposeModifier)
// Use a single instance + @Stable to ensure that recompositions can enable skipping optimizations
// Modifier.composed will still remember unique data per call site.
private val recomposeModifier =
Modifier.composed(inspectorInfo = debugInspectorInfo { name = "recomposeHighlighter" }) {
// The total number of compositions that have occurred. We're not using a State<> here be
// able to read/write the value without invalidating (which would cause infinite
// recomposition).
val totalCompositions = remember { arrayOf(0L) }
// The value of totalCompositions at the last timeout.
val totalCompositionsAtLastTimeout = remember { mutableStateOf(0L) }
// Start the timeout, and reset everytime there's a recomposition. (Using totalCompositions
// as the key is really just to cause the timer to restart every composition).
LaunchedEffect(totalCompositions[0]) {
totalCompositionsAtLastTimeout.value = totalCompositions[0]
Modifier.drawWithCache {
onDrawWithContent {
// Draw actual content.
// Below is to draw the highlight, if necessary. A lot of the logic is copied from
// Modifier.border
val numCompositionsSinceTimeout =
totalCompositions[0] - totalCompositionsAtLastTimeout.value
val hasValidBorderParams = size.minDimension > 0f
if (!hasValidBorderParams || numCompositionsSinceTimeout <= 0) {
val (color, strokeWidthPx) =
when (numCompositionsSinceTimeout) {
// We need at least one composition to draw, so draw the smallest border
// color in blue.
1L -> Color.Blue to 1f
// 2 compositions is _probably_ okay.
2L -> Color.Green to 2.dp.toPx()
// 3 or more compositions before timeout may indicate an issue. lerp the
// color from yellow to red, and continually increase the border size.
else -> {
Color.Yellow.copy(alpha = 0.8f),
Color.Red.copy(alpha = 0.5f),
min(1f, (numCompositionsSinceTimeout - 1).toFloat() / 100f)
) to numCompositionsSinceTimeout.toInt().dp.toPx()
val halfStroke = strokeWidthPx / 2
val topLeft = Offset(halfStroke, halfStroke)
val borderSize = Size(size.width - strokeWidthPx, size.height - strokeWidthPx)
val fillArea = (strokeWidthPx * 2) > size.minDimension
val rectTopLeft = if (fillArea) Offset.Zero else topLeft
val size = if (fillArea) size else borderSize
val style = if (fillArea) Fill else Stroke(strokeWidthPx)
brush = SolidColor(color),
topLeft = rectTopLeft,
size = size,
style = style

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