WIP A platform channel API to declare that the nav stack is empty

Author: justinmc

shell/platform/android/io/flutter/embedding/engine/systemchannels/PlatformChannel.java

@@ -136,6 +136,10 @@ public void onMethodCall(@NonNull MethodCall call, @NonNull MethodChannel.Result
                   result.error("error", exception.getMessage(), null);
                 }
                 break;
+              case "SystemNavigator.isEmpty":
+                platformMessageHandler.navigatorIsEmpty();
+                result.success(null);
+                break;
               case "SystemNavigator.pop":
                 platformMessageHandler.popSystemNavigator();
                 result.success(null);
@@ -509,6 +513,8 @@ public interface PlatformMessageHandler {
      */
     void setSystemUiOverlayStyle(@NonNull SystemChromeStyle systemUiOverlayStyle);
 
+    void navigatorIsEmpty();
+
     /**
      * The Flutter application would like to pop the top item off of the Android app's navigation
      * back stack.

shell/platform/android/io/flutter/plugin/platform/PlatformPlugin.java

@@ -109,6 +109,11 @@ public void setSystemUiOverlayStyle(
           setSystemChromeSystemUIOverlayStyle(systemUiOverlayStyle);
         }
 
+        @Override
+        public void navigatorIsEmpty() {
+          PlatformPlugin.this.navigatorIsEmpty();
+        }
+
         @Override
         public void popSystemNavigator() {
           PlatformPlugin.this.popSystemNavigator();
@@ -475,18 +480,28 @@ private void setSystemChromeSystemUIOverlayStyle(
     currentTheme = systemChromeStyle;
   }
 
+  private void navigatorIsEmpty() {
+    Log.e("justin", "navigatorIsEmpty in PlatformPlugin");
+    // TODO(justinmc): Can't quite call it like this, need FlutterActivity.
+    // activity.unregisterOnBackInvokedCallback();
+  }
+
   private void popSystemNavigator() {
     if (platformPluginDelegate != null && platformPluginDelegate.popSystemNavigator()) {
       // A custom behavior was executed by the delegate. Don't execute default behavior.
       return;
     }
 
     // TODO(justinmc): This is another use of backpresseddispatcher that is not
-    // blocking the pback gesture for me.
+    // blocking the pback gesture for me. It seems like this if is false, so the
+    // app exits here.
     if (activity instanceof OnBackPressedDispatcherOwner) {
       Log.e("justin", "calling onBackPressed in popSystemNavigator in PlatformPlugin");
       ((OnBackPressedDispatcherOwner) activity).getOnBackPressedDispatcher().onBackPressed();
     } else {
+      Log.e(
+          "justin",
+          "popSystemNavigator in PlatformPlugin finishing the activity because it's not an OnBackPressedDispatcherOwner");
       activity.finish();
     }
   }