Allow snapshotFlow to be used without Compose.

ComposeStateTestRule.kt

import androidx.compose.runtime.snapshots.Snapshot
import kotlinx.coroutines.CoroutineScope
import kotlinx.coroutines.DelicateCoroutinesApi
import kotlinx.coroutines.Job
import kotlinx.coroutines.SupervisorJob
import kotlinx.coroutines.coroutineScope
import kotlinx.coroutines.delay
import kotlinx.coroutines.flow.flow
import kotlinx.coroutines.flow.launchIn
import kotlinx.coroutines.isActive
import kotlinx.coroutines.newSingleThreadContext
import org.junit.rules.TestWatcher
import org.junit.runner.Description

/**
 * To be able to use snapshotFlow in test without Compose, We have to take snapshots.
 * So this rule takes snapshots regularly.
 *
 * What's about snapshotFlow:
 * https://developer.android.com/jetpack/compose/side-effects#snapshotFlow
 */
class ComposeStateTestRule(
    snapshotIntervalMilliSec: Long = 1L
) : TestWatcher() {

    @OptIn(DelicateCoroutinesApi::class)
    private val dispatcher = newSingleThreadContext(name = "snapshot")
    private val scope = CoroutineScope(SupervisorJob() + dispatcher)
    private var job: Job? = null

    private val snapshotTaker = flow<Unit> {
        coroutineScope {
            while(isActive) {
                delay(snapshotIntervalMilliSec)
                Snapshot.takeSnapshot { }
            }
        }
    }

    override fun starting(description: Description?) {
        job = snapshotTaker.launchIn(scope)
    }

    override fun finished(description: Description?) {
        job?.cancel()
        job = null
    }
}