Modifier to highlight recompositions happening on a Composable. Full credit to Google for creating this Modifier. Original link - https://github.com/android/snippets/blob/master/compose/recomposehighlighter/src/main/java/com/example/android/compose/recomposehighlighter/RecomposeHighlighter.kt

RecomposeHighlighter.kt

/*
 *  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
 * License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * 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.
 */
@Stable
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) }
        totalCompositions[0]++

        // 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]) {
            delay(3000)
            totalCompositionsAtLastTimeout.value = totalCompositions[0]
        }

        Modifier.drawWithCache {
            onDrawWithContent {
                // Draw actual content.
                drawContent()

                // 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) {
                    return@onDrawWithContent
                }

                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 -> {
                            lerp(
                                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)

                drawRect(
                    brush = SolidColor(color),
                    topLeft = rectTopLeft,
                    size = size,
                    style = style
                )
            }
        }
    }