Jetpack compose: How to detect what is causing a Composable to recompose

Question

I'm starting to notice some jank in my app and I believe the cause is composables being re-composed when they should not.

I've detected a few user interactions that trigger unnecessary recompositions but I can't put my finger into what is causing the re-composition. I've added breakpoints in every place where I modify the state and still can find what is triggering the recomposition.

Does AS provide a way to debug this kind of thing?

Answer 1

AFAIK, there's no IDE support yet. But you can the use below custom modifier to track the recomposition.

Demo

• https://twitter.com/theapache64/status/1502932832058249220

RecompositionHighlighter.kt

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
                )
            }
        }
    }

Source: https://android-developers.googleblog.com/2022/03/play-time-with-jetpack-compose.html

Answer 2

I don't know of any way to 'catch' the recomposition trigger, but basically the recomposition is triggered by an observed state that modifier. In other words, having a mutableState or mutableStateList that changes triggers the recomposition. If you want to control this, you can take a look at Side-Effects https://developer.android.com/jetpack/compose/side-effects

And particularly this: https://developer.android.com/jetpack/compose/side-effects#snapshotFlow