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Debug State Observation

Ever scratched your head wondering why your Composable is recomposing due to a state read? Even with the Recomposition count tracker in the Layout Inspector, it can be a bit of a guessing game. Well, no more guessing games, because Andrei Shikov has your back! His code snippet lets you pinpoint exactly which state is causing recompositions, thanks to the logs it emits. It's like a GPS for your Compose state issues, and it's super easy to use

import androidx.compose.runtime.Composable
import androidx.compose.runtime.DisposableEffect
import androidx.compose.runtime.remember
import androidx.compose.runtime.snapshots.MutableSnapshot
import androidx.compose.runtime.snapshots.Snapshot
import java.util.WeakHashMap
@PublishedApi
internal class DebugStateObservation(private val id: String) {
private val map = WeakHashMap<Any, MutableList<Exception>>()
val readObserver: (Any) -> Unit = {
synchronized(this) {
val e = Exception()
val list = map.getOrPut(it) { mutableListOf() }
list += e
}
}
fun print(changes: Set<Any>) {
synchronized(this) {
val affected = map.keys.intersect(changes)
if (affected.isNotEmpty()) {
affected.forEach {
printStateChange(id, it, map[it])
}
}
}
}
fun clear() {
synchronized(this) {
map.clear()
}
}
}
private fun printStateChange(id: String, state: Any, exceptions: List<Exception>?) {
val traces = exceptions?.joinToString(separator = "\n") {
// remove trace start, sample:
// at androidx.compose.foundation.demos.DebugStateObservation$readObserver$1.invoke(Test.kt:33)
// at androidx.compose.foundation.demos.DebugStateObservation$readObserver$1.invoke(Test.kt:31)
// at androidx.compose.runtime.snapshots.SnapshotKt$mergedReadObserver$1.invoke(Snapshot.kt:1771)
// at androidx.compose.runtime.snapshots.SnapshotKt$mergedReadObserver$1.invoke(Snapshot.kt:1770)
// at androidx.compose.runtime.snapshots.SnapshotKt.readable(Snapshot.kt:2003)
// at androidx.compose.runtime.SnapshotMutableIntStateImpl.getIntValue(SnapshotIntState.kt:138)
val stackTrace = it.stackTrace
buildString {
for (i in 6.. minOf(10, stackTrace.size)) {
append("\tat ${it.stackTrace[i]}")
append("\n")
}
append("...")
}
} ?: ""
println("$id might recompose because $state changed, last read at:\n$traces")
}
/**
* Records state observations inside @Composable [block] and prints to [System.out] whenever
* state mutation is applied.
*
* NOTE: This doesn't record recompositions precisely and only uses snapshot system to record state
* mutations that /might/ invalidate recomposition. Consecutive invocations might result in
* different results depending on functions that were run / skipped during each execution. To be
* used directly inside a function scope that recomposes, as Compose might skip inner scopes and
* reads/mutations are not going to be recorded.
*/
@Composable
inline fun <T> DebugStateChanges(id: String, block: @Composable () -> T): T {
val observation = remember { DebugStateObservation(id) }
val currentSnapshot = Snapshot.current
val snapshot = if (currentSnapshot is MutableSnapshot) {
currentSnapshot.takeNestedMutableSnapshot(observation.readObserver)
} else {
currentSnapshot.takeNestedSnapshot(observation.readObserver)
}
DisposableEffect(observation) {
val disposeHandle = Snapshot.registerApplyObserver { changes, _ ->
observation.print(changes)
}
onDispose {
observation.clear()
disposeHandle.dispose()
}
}
observation.clear()
return snapshot.runAndDispose { block() }
}
// Compose doesn't work with try/finally, but we don't really use it for catching things.
@PublishedApi
internal inline fun <T> Snapshot.runAndDispose(block: () -> T): T =
try {
enter(block)
} finally {
dispose()
}

Here's how you use it:

// Just wrap the composable that you need more
// information about with the following Composable

DebugStateObservation("MyCustomComposable") {
    MyCustomComposable(...)
}
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