Distinguish canceling a route pop from bubbling the pop up to the next level (#9165)

examples/flutter_gallery/lib/demo/material/text_form_field_demo.dart

@@ -75,12 +75,12 @@ class TextFormFieldDemoState extends State<TextFormFieldDemo> {
     return null;
   }
 
-  Future<bool> _warnUserAboutInvalidData() {
+  Future<bool> _warnUserAboutInvalidData() async {
     final FormState form = _formKey.currentState;
-    if (!_formWasEdited || form.validate())
-      return new Future<bool>.value(true);
+    if (form == null || !_formWasEdited || form.validate())
+      return true;
 
-    return showDialog<bool>(
+    return await showDialog<bool>(
       context: context,
       child: new AlertDialog(
         title: new Text('This form has errors'),
@@ -96,7 +96,7 @@ class TextFormFieldDemoState extends State<TextFormFieldDemo> {
           ),
         ],
       ),
-    );
+    ) ?? false;
   }
 
   @override

packages/flutter/lib/src/widgets/navigator.dart

@@ -13,6 +13,7 @@ import 'focus_manager.dart';
 import 'focus_scope.dart';
 import 'framework.dart';
 import 'overlay.dart';
+import 'routes.dart';
 import 'ticker_provider.dart';
 
 /// An abstraction for an entry managed by a [Navigator].
@@ -72,7 +73,9 @@ abstract class Route<T> {
   /// See also:
   ///
   /// * [Form], which provides an `onWillPop` callback that uses this mechanism.
-  Future<bool> willPop() async => !isFirst;
+  Future<RoutePopDisposition> willPop() async {
+    return isFirst ? RoutePopDisposition.bubble : RoutePopDisposition.pop;
+  }
 
   /// A request was made to pop this route. If the route can handle it
   /// internally (e.g. because it has its own stack of internal state) then
@@ -969,7 +972,9 @@ class NavigatorState extends State<Navigator> with TickerProviderStateMixin {
   Future<bool> maybePop([dynamic result]) async {
     final Route<dynamic> route = _history.last;
     assert(route._navigator == this);
-    if (await route.willPop() && mounted) {
+    final RoutePopDisposition disposition = await route.willPop();
+    if (disposition != RoutePopDisposition.bubble && mounted) {
+      if (disposition == RoutePopDisposition.pop)
         pop(result);
       return true;
     }

packages/flutter/lib/src/widgets/routes.dart

@@ -317,8 +317,10 @@ abstract class LocalHistoryRoute<T> extends Route<T> {
   }
 
   @override
-  Future<bool> willPop() async {
-    return willHandlePopInternally || await super.willPop();
+  Future<RoutePopDisposition> willPop() async {
+    if (willHandlePopInternally)
+      return RoutePopDisposition.pop;
+    return await super.willPop();
   }
 
   @override
@@ -385,6 +387,28 @@ class _ModalScopeStatus extends InheritedWidget {
   }
 }
 
+/// Indicates whether the current route should be popped.
+///
+/// Used as the return value for [Route.willPop].
+enum RoutePopDisposition {
+  /// Pop the route.
+  ///
+  /// If [Route.willPop] returns [pop] then the back button will actually pop
+  /// the current route.
+  pop,
+
+  /// Do not pop the route.
+  ///
+  /// If [Route.willPop] returns [doNotPop] then the back button will be ignored.
+  doNotPop,
+
+  /// Delegate this to the next level of navigation.
+  ///
+  /// If [Route.willPop] return [bubble] then the back button will be handled
+  /// by the [SystemNavigator], which will usually close the application.
+  bubble,
+}
+
 /// Signature for a callback that verifies that it's OK to call [Navigator.pop].
 ///
 /// Used by [Form.onWillPop], [ModalRoute.addScopedWillPopCallback], and
@@ -664,12 +688,12 @@ abstract class ModalRoute<T> extends TransitionRoute<T> with LocalHistoryRoute<T
   /// * [removeScopedWillPopCallback], which removes a callback from the list
   ///   this method checks.
   @override
-  Future<bool> willPop() async {
+  Future<RoutePopDisposition> willPop() async {
     final _ModalScopeState scope = _scopeKey.currentState;
     assert(scope != null);
     for (WillPopCallback callback in new List<WillPopCallback>.from(scope._willPopCallbacks)) {
       if (!await callback())
-        return false;
+        return RoutePopDisposition.doNotPop;
     }
     return await super.willPop();
   }

packages/flutter/test/material/will_pop_test.dart

@@ -113,6 +113,12 @@ void main() {
     await tester.pump(const Duration(seconds: 1));
     expect(find.text('Sample Page'), findsOneWidget);
 
+    // Use didPopRoute() to simulate the system back button.  Check that
+    // didPopRoute() indicates that the notification was handled.
+    final dynamic widgetsAppState = tester.state(find.byType(WidgetsApp));
+    expect(await widgetsAppState.didPopRoute(), isTrue);
+    expect(find.text('Sample Page'), findsOneWidget);
+
     willPopValue = true;
     await tester.tap(find.byTooltip('Back'));
     await tester.pump();