Add unit tests for OnBackPressedDispatcher (#1648)

redwood-treehouse-host/src/commonTest/kotlin/app/cash/redwood/treehouse/EventLog.kt

@@ -41,4 +41,11 @@ class EventLog {
       "expected no events but was ${received.getOrNull()}"
     }
   }
+
+  /** Discard all events. */
+  fun clear() {
+    while (true) {
+      if (!events.tryReceive().isSuccess) break
+    }
+  }
 }

redwood-treehouse-host/src/commonTest/kotlin/app/cash/redwood/treehouse/FakeOnBackPressedDispatcher.kt

@@ -20,10 +20,26 @@ import app.cash.redwood.ui.OnBackPressedCallback
 import app.cash.redwood.ui.OnBackPressedDispatcher
 
 internal class FakeOnBackPressedDispatcher : OnBackPressedDispatcher {
+  private val mutableCallbacks = mutableListOf<OnBackPressedCallback>()
+
+  val callbacks: List<OnBackPressedCallback>
+    get() = mutableCallbacks.toList()
+
   override fun addCallback(onBackPressedCallback: OnBackPressedCallback): Cancellable {
+    mutableCallbacks += onBackPressedCallback
+
     return object : Cancellable {
       override fun cancel() {
+        mutableCallbacks -= onBackPressedCallback
       }
     }
   }
+
+  fun onBack() {
+    // Only one callback should handle each back press.
+    val callbackToNotify = mutableCallbacks.lastOrNull {
+      it.isEnabled
+    }
+    callbackToNotify?.handleOnBackPressed()
+  }
 }

redwood-treehouse-host/src/commonTest/kotlin/app/cash/redwood/treehouse/FakeZiplineTreehouseUi.kt

@@ -38,6 +38,7 @@ class FakeZiplineTreehouseUi(
   private var nextWidgetId = 1
 
   private lateinit var host: ZiplineTreehouseUi.Host
+  private val extraServicesToClose = mutableListOf<CancellableService>()
 
   override fun start(host: ZiplineTreehouseUi.Host) {
     eventLog += "$name.start()"
@@ -59,6 +60,22 @@ class FakeZiplineTreehouseUi(
     eventLog += "$name.sendEvent($event)"
   }
 
+  fun addBackHandler(isEnabled: Boolean): CancellableService {
+    val result = host.addOnBackPressedCallback(object : OnBackPressedCallbackService {
+      override var isEnabled = isEnabled
+
+      override fun handleOnBackPressed() {
+        eventLog += "$name.onBackPressed()"
+      }
+    })
+
+    // Keep track of services produced by this ZiplineTreehouseUi so we can close them when this
+    // service is itself closed. In production code the ZiplineScope does this automatically.
+    extraServicesToClose += result
+
+    return result
+  }
+
   @Suppress("OVERRIDE_DEPRECATION")
   override fun start(
     changesSink: ChangesSinkService,
@@ -79,6 +96,9 @@ class FakeZiplineTreehouseUi(
   }
 
   override fun close() {
+    for (cancellableService in extraServicesToClose) {
+      cancellableService.close()
+    }
     eventLog += "$name.close()"
   }
 }

redwood-treehouse-host/src/commonTest/kotlin/app/cash/redwood/treehouse/TreehouseAppContentTest.kt

@@ -17,7 +17,9 @@ package app.cash.redwood.treehouse
 
 import app.cash.redwood.ui.UiConfiguration
 import assertk.assertThat
+import assertk.assertions.isEmpty
 import assertk.assertions.isEqualTo
+import assertk.assertions.isNotEmpty
 import kotlin.test.AfterTest
 import kotlin.test.Test
 import kotlinx.coroutines.CoroutineScope
@@ -225,6 +227,72 @@ class TreehouseAppContentTest {
     eventLog.takeEvent("codeSessionA.app.uis[1].close()")
   }
 
+  @Test
+  fun addBackHandler_receives_back_presses_until_canceled() = runTest {
+    val content = treehouseAppContent()
+
+    val view1 = FakeTreehouseView("view1")
+    content.bind(view1)
+    codeHost.session = FakeCodeSession("codeSessionA", eventLog)
+    eventLog.clear()
+
+    val backCancelable = codeHost.session!!.appService.uis.single().addBackHandler(true)
+    view1.onBackPressedDispatcher.onBack()
+    eventLog.takeEvent("codeSessionA.app.uis[0].onBackPressed()")
+
+    view1.onBackPressedDispatcher.onBack()
+    eventLog.takeEvent("codeSessionA.app.uis[0].onBackPressed()")
+
+    backCancelable.cancel()
+    view1.onBackPressedDispatcher.onBack()
+    eventLog.assertNoEvents()
+
+    content.unbind()
+    eventLog.takeEvent("codeSessionA.app.uis[0].close()")
+  }
+
+  @Test
+  fun addBackHandler_receives_no_back_presses_if_disabled() = runTest {
+    val content = treehouseAppContent()
+
+    val view1 = FakeTreehouseView("view1")
+    content.bind(view1)
+    codeHost.session = FakeCodeSession("codeSessionA", eventLog)
+    eventLog.clear()
+
+    val backCancelable = codeHost.session!!.appService.uis.single().addBackHandler(false)
+    view1.onBackPressedDispatcher.onBack()
+    eventLog.assertNoEvents()
+
+    backCancelable.cancel()
+
+    content.unbind()
+    eventLog.takeEvent("codeSessionA.app.uis[0].close()")
+  }
+
+  @Test
+  fun backHandlers_cleared_when_session_changes() = runTest {
+    val content = treehouseAppContent()
+
+    val view1 = FakeTreehouseView("view1")
+    content.bind(view1)
+    val codeSessionA = FakeCodeSession("codeSessionA", eventLog)
+    codeHost.session = codeSessionA
+
+    codeSessionA.appService.uis.single().addBackHandler(true)
+    assertThat(view1.onBackPressedDispatcher.callbacks).isNotEmpty()
+
+    val codeSessionB = FakeCodeSession("codeSessionB", eventLog)
+    codeHost.session = codeSessionB
+
+    // When we close codeSessionA, its back handlers are released with it.
+    assertThat(view1.onBackPressedDispatcher.callbacks).isEmpty()
+    eventLog.clear()
+
+    content.unbind()
+    eventLog.takeEvent("codeSessionB.app.uis[0].close()")
+  }
+
   private fun TestScope.treehouseAppContent(): TreehouseAppContent<FakeAppService> {
     return TreehouseAppContent(
       codeHost = codeHost,