Add BackHandler support on Android (#1493)

redwood-compose/src/commonMain/kotlin/app/cash/redwood/compose/BackHandler.kt

@@ -20,15 +20,15 @@ import androidx.compose.runtime.DisposableEffect
 import androidx.compose.runtime.ProvidableCompositionLocal
 import androidx.compose.runtime.ReadOnlyComposable
 import androidx.compose.runtime.SideEffect
-import androidx.compose.runtime.compositionLocalOf
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.rememberUpdatedState
+import androidx.compose.runtime.staticCompositionLocalOf
 import app.cash.redwood.ui.OnBackPressedCallback
 import app.cash.redwood.ui.OnBackPressedDispatcher
 
 public val LocalOnBackPressedDispatcher: ProvidableCompositionLocal<OnBackPressedDispatcher> =
-  compositionLocalOf {
+  staticCompositionLocalOf {
     throw AssertionError("OnBackPressedDispatcher was not provided!")
   }
 
@@ -37,6 +37,21 @@ public val OnBackPressedDispatcher.Companion.current: OnBackPressedDispatcher
   @ReadOnlyComposable
   get() = LocalOnBackPressedDispatcher.current
 
+/**
+ * An effect for handling presses of the system back button.
+ *
+ * Calling this in your composable adds the given lambda to the [OnBackPressedDispatcher] of the
+ * [LocalOnBackPressedDispatcher].
+ *
+ * If this is called by nested composables, if enabled, the inner most composable will consume
+ * the call to system back and invoke its lambda. The call will continue to propagate up until it
+ * finds an enabled BackHandler.
+ *
+ * The [onBack] lambda is never invoked on platforms that don't have a system back button.
+ *
+ * @param enabled if this BackHandler should be enabled
+ * @param onBack the action invoked by pressing the system back
+ */
 // Multiplatform variant of
 // https://github.com/androidx/androidx/blob/94ae1a9fb3ce778295e8cc724ae29f1231436bcb/activity/activity-compose/src/main/java/androidx/activity/compose/BackHandler.kt#L82
 @Composable

redwood-treehouse-host/src/androidUnitTest/kotlin/app/cash/redwood/treehouse/TreehouseLayoutTest.kt

@@ -41,28 +41,25 @@ import org.junit.Test
 import org.junit.runner.RunWith
 import org.robolectric.Robolectric
 import org.robolectric.RobolectricTestRunner
-import org.robolectric.RuntimeEnvironment
 import org.robolectric.annotation.Config
 
 @RunWith(RobolectricTestRunner::class)
 @Config(sdk = [26])
 class TreehouseLayoutTest {
-  private val context = RuntimeEnvironment.getApplication()!!
+  private val activity = Robolectric.buildActivity(ComponentActivity::class.java).resume().visible().get()
 
   @Test fun widgetsAddChildViews() {
-    val activity = Robolectric.buildActivity(ComponentActivity::class.java).resume().visible().get()
-    val layout = TreehouseLayout(context, throwingWidgetSystem, activity.onBackPressedDispatcher)
+    val layout = TreehouseLayout(activity, throwingWidgetSystem, activity.onBackPressedDispatcher)
 
-    val view = View(context)
+    val view = View(activity)
     layout.children.insert(0, viewWidget(view))
     assertThat(layout.childCount).isEqualTo(1)
     assertThat(layout.getChildAt(0)).isSameAs(view)
   }
 
   @Test fun attachAndDetachSendsStateChange() {
-    val activity = Robolectric.buildActivity(ComponentActivity::class.java).resume().visible().get()
     val parent = activity.findViewById<ViewGroup>(android.R.id.content)
-    val layout = TreehouseLayout(context, throwingWidgetSystem, activity.onBackPressedDispatcher)
+    val layout = TreehouseLayout(activity, throwingWidgetSystem, activity.onBackPressedDispatcher)
     val listener = CountingReadyForContentChangeListener<View>()
 
     layout.readyForContentChangeListener = listener
@@ -76,46 +73,42 @@ class TreehouseLayoutTest {
   }
 
   @Test fun resetClearsUntrackedChildren() {
-    val activity = Robolectric.buildActivity(ComponentActivity::class.java).resume().visible().get()
-    val layout = TreehouseLayout(context, throwingWidgetSystem, activity.onBackPressedDispatcher)
+    val layout = TreehouseLayout(activity, throwingWidgetSystem, activity.onBackPressedDispatcher)
 
-    layout.addView(View(context))
+    layout.addView(View(activity))
     assertThat(layout.childCount).isEqualTo(1)
 
     layout.reset()
     assertThat(layout.childCount).isEqualTo(0)
   }
 
   @Test fun resetClearsTrackedWidgets() {
-    val activity = Robolectric.buildActivity(ComponentActivity::class.java).resume().visible().get()
-    val layout = TreehouseLayout(context, throwingWidgetSystem, activity.onBackPressedDispatcher)
+    val layout = TreehouseLayout(activity, throwingWidgetSystem, activity.onBackPressedDispatcher)
 
     // Needed to access internal state which cannot be reasonably observed through the public API.
     val children = layout.children as ViewGroupChildren
 
-    children.insert(0, viewWidget(View(context)))
+    children.insert(0, viewWidget(View(activity)))
     assertThat(children.widgets).hasSize(1)
 
     layout.reset()
     assertThat(children.widgets).hasSize(0)
   }
 
   @Test fun uiConfigurationReflectsInitialUiMode() {
-    val newConfig = Configuration(context.resources.configuration)
+    val newConfig = Configuration(activity.resources.configuration)
     newConfig.uiMode = (newConfig.uiMode and UI_MODE_NIGHT_MASK.inv()) or UI_MODE_NIGHT_YES
-    val newContext = context.createConfigurationContext(newConfig) // Needs API 26.
-    val activity = Robolectric.buildActivity(ComponentActivity::class.java).resume().visible().get()
+    val newContext = activity.createConfigurationContext(newConfig) // Needs API 26.
     val layout = TreehouseLayout(newContext, throwingWidgetSystem, activity.onBackPressedDispatcher)
     assertThat(layout.uiConfiguration.value).isEqualTo(UiConfiguration(darkMode = true))
   }
 
   @Test fun uiConfigurationEmitsUiModeChanges() = runTest {
-    val activity = Robolectric.buildActivity(ComponentActivity::class.java).resume().visible().get()
-    val layout = TreehouseLayout(context, throwingWidgetSystem, activity.onBackPressedDispatcher)
+    val layout = TreehouseLayout(activity, throwingWidgetSystem, activity.onBackPressedDispatcher)
     layout.uiConfiguration.test {
       assertThat(awaitItem()).isEqualTo(UiConfiguration(darkMode = false))
 
-      val newConfig = Configuration(context.resources.configuration)
+      val newConfig = Configuration(activity.resources.configuration)
       newConfig.uiMode = (newConfig.uiMode and UI_MODE_NIGHT_MASK.inv()) or UI_MODE_NIGHT_YES
 
       layout.dispatchConfigurationChanged(newConfig)
@@ -124,16 +117,15 @@ class TreehouseLayoutTest {
   }
 
   @Test fun uiConfigurationEmitsSystemBarsSafeAreaInsetsChanges() = runTest {
-    val activity = Robolectric.buildActivity(ComponentActivity::class.java).resume().visible().get()
-    val layout = TreehouseLayout(context, throwingWidgetSystem, activity.onBackPressedDispatcher)
+    val layout = TreehouseLayout(activity, throwingWidgetSystem, activity.onBackPressedDispatcher)
     layout.uiConfiguration.test {
       assertThat(awaitItem()).isEqualTo(UiConfiguration(safeAreaInsets = Margin.Zero))
       val insets = Insets.of(10, 20, 30, 40)
       val windowInsets = WindowInsetsCompat.Builder()
         .setInsets(WindowInsetsCompat.Type.systemBars(), insets)
         .build()
       ViewCompat.dispatchApplyWindowInsets(layout, windowInsets)
-      val expectedInsets = with(Density(context.resources)) {
+      val expectedInsets = with(Density(activity.resources)) {
         Margin(
           start = insets.left.toDp(),
           end = insets.right.toDp(),