Test to demonstrate rememberAnsweringNavigator result handling (#1582)

Adding this as an example for testing result handing for
#1565

backstack/src/commonMain/kotlin/com/slack/circuit/backstack/SaveableBackStack.kt

@@ -112,12 +112,15 @@ internal constructor(
   }
 
   override fun pop(result: PopResult?): Record? {
-    val popped = entryList.removeFirstOrNull()
-    if (result != null) {
-      // Send the pending result to our new top record, but only if it's expecting one
-      topRecord?.apply { if (expectingResult()) sendResult(result) }
+    // Run in a snapshot to ensure the sendResult doesn't get missed.
+    return Snapshot.withMutableSnapshot {
+      val popped = entryList.removeFirstOrNull()
+      if (result != null) {
+        // Send the pending result to our new top record, but only if it's expecting one
+        topRecord?.apply { if (expectingResult()) sendResult(result) }
+      }
+      popped
     }
-    return popped
   }
 
   override fun saveState() {

circuit-foundation/src/jvmTest/kotlin/com/slack/circuit/foundation/AnsweringNavigatorTest.kt

@@ -0,0 +1,98 @@
+// Copyright (C) 2024 Slack Technologies, LLC
+// SPDX-License-Identifier: Apache-2.0
+package com.slack.circuit.foundation
+
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.SideEffect
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.test.junit4.ComposeContentTestRule
+import androidx.compose.ui.test.junit4.createComposeRule
+import app.cash.turbine.Turbine
+import com.slack.circuit.backstack.SaveableBackStack
+import com.slack.circuit.backstack.rememberSaveableBackStack
+import com.slack.circuit.runtime.CircuitUiState
+import com.slack.circuit.runtime.Navigator
+import com.slack.circuit.runtime.presenter.Presenter
+import com.slack.circuit.runtime.screen.PopResult
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlinx.coroutines.test.runTest
+import org.junit.Rule
+import org.junit.runner.RunWith
+
+/**
+ * Test verifying that a [PopResult] is returned to a [rememberAnsweringNavigator] across a
+ * navigation event.
+ */
+@RunWith(ComposeUiTestRunner::class)
+class AnsweringNavigatorTest {
+
+  @get:Rule val composeTestRule = createComposeRule()
+
+  private val state1 = Turbine<State>()
+  private val state2 = Turbine<State>()
+
+  private val circuit =
+    Circuit.Builder()
+      .addPresenter<TestScreen, State> { _, navigator, _ -> AnsweringPresenter(navigator) }
+      .addPresenter<TestScreen2, State> { _, navigator, _ -> PopPresenter(navigator) }
+      .addUi<TestScreen, State> { state, _ -> SideEffect { state1.add(state) } }
+      .addUi<TestScreen2, State> { state, _ -> SideEffect { state2.add(state) } }
+      .build()
+
+  @Test
+  fun `verify pop result is returned to the answering navigator`() = runTest {
+    with(composeTestRule) {
+      val backStack = setCircuitContent(circuit)
+      assertEquals(listOf(TestScreen), backStack.screens)
+      // Go to next screen
+      state1.awaitItem().eventSink()
+      assertEquals(listOf(TestScreen2, TestScreen), backStack.screens)
+      waitForIdle()
+      // Pop back to the previous screen
+      state2.awaitItem().eventSink()
+      assertEquals(listOf(TestScreen), backStack.screens)
+      waitForIdle()
+      assertEquals(TestPopResult, state1.expectMostRecentItem().resultCount)
+    }
+  }
+}
+
+private fun ComposeContentTestRule.setCircuitContent(circuit: Circuit): SaveableBackStack {
+  lateinit var backStack: SaveableBackStack
+  setContent {
+    CircuitCompositionLocals(circuit) {
+      val saveableBackStack = rememberSaveableBackStack(TestScreen) { backStack = this }
+      val navigator = rememberCircuitNavigator(backStack = backStack, onRootPop = {})
+      NavigableCircuitContent(navigator = navigator, backStack = saveableBackStack)
+    }
+  }
+  return backStack
+}
+
+private val SaveableBackStack.screens
+  get() = map { it.screen }
+
+private data class State(val resultCount: TestPopResult? = null, val eventSink: () -> Unit) :
+  CircuitUiState
+
+private class AnsweringPresenter(private val navigator: Navigator) : Presenter<State> {
+
+  @Composable
+  override fun present(): State {
+    var result by remember { mutableStateOf<TestPopResult?>(null) }
+    val answeringNavigator = rememberAnsweringNavigator<TestPopResult>(navigator) { result = it }
+    return State(result) { answeringNavigator.goTo(TestScreen2) }
+  }
+}
+
+private class PopPresenter(private val navigator: Navigator) : Presenter<State> {
+
+  @Composable
+  override fun present(): State {
+    return State { navigator.pop(TestPopResult) }
+  }
+}