Merge pull request #709 from Hixie/canPop

Navigator.canPop(context)

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

@@ -37,6 +37,9 @@ abstract class Route<T> {
   /// responsibility of the Route to later call dispose().
   bool didPop(T result) => true;
 
+  /// Whether calling didPop() would return false.
+  bool get willHandlePopInternally => false;
+
   /// The given route has been pushed onto the navigator after this route.
   void didPushNext(Route nextRoute) { }
 
@@ -129,6 +132,11 @@ class Navigator extends StatefulComponent {
     return returnValue;
   }
 
+  static bool canPop(BuildContext context) {
+    NavigatorState navigator = context.ancestorStateOfType(NavigatorState);
+    return navigator.canPop();
+  }
+
   static void popAndPushNamed(BuildContext context, String routeName, { Set<Key> mostValuableKeys }) {
     openTransaction(context, (NavigatorTransaction transaction) {
       transaction.pop();
@@ -280,7 +288,10 @@ class NavigatorState extends State<Navigator> {
     assert(() { _debugLocked = true; return true; });
     Route route = _history.last;
     assert(route._navigator == this);
+    bool debugPredictedWouldPop;
+    assert(() { debugPredictedWouldPop = !route.willHandlePopInternally; return true; });
     if (route.didPop(result)) {
+      assert(debugPredictedWouldPop);
       if (_history.length > 1) {
         setState(() {
           // We use setState to guarantee that we'll rebuild, since the routes
@@ -295,6 +306,8 @@ class NavigatorState extends State<Navigator> {
         assert(() { _debugLocked = false; return true; });
         return false;
       }
+    } else {
+      assert(!debugPredictedWouldPop);
     }
     assert(() { _debugLocked = false; return true; });
     return true;
@@ -306,6 +319,11 @@ class NavigatorState extends State<Navigator> {
       _pop();
   }
 
+  bool canPop() {
+    assert(_history.length > 0);
+    return _history.length > 1 || _history[0].willHandlePopInternally;
+  }
+
   bool _hadTransaction = true;
 
   bool openTransaction(NavigatorTransactionCallback callback) {

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

@@ -236,6 +236,9 @@ abstract class LocalHistoryRoute<T> extends Route<T> {
     }
     return super.didPop(result);
   }
+  bool get willHandlePopInternally {
+    return _localHistory != null && _localHistory.length > 0;
+  }
 }
 
 class _ModalScopeStatus extends InheritedWidget {

packages/unit/test/widget/page_transitions_test.dart

@@ -29,7 +29,9 @@ void main() {
       expect(tester.findText('Settings'), isNull);
       expect(tester.findText('Overlay'), isNull);
 
+      expect(Navigator.canPop(containerKey1.currentContext), isFalse);
       Navigator.pushNamed(containerKey1.currentContext, '/settings');
+      expect(Navigator.canPop(containerKey1.currentContext), isTrue);
 
       tester.pump();
 
@@ -63,6 +65,7 @@ void main() {
       expect(tester.findText('Settings'), isOnStage);
       expect(tester.findText('Overlay'), isOnStage);
 
+      expect(Navigator.canPop(containerKey2.currentContext), isTrue);
       Navigator.pop(containerKey2.currentContext);
       tester.pump();
 
@@ -76,6 +79,7 @@ void main() {
       expect(tester.findText('Settings'), isOnStage);
       expect(tester.findText('Overlay'), isNull);
 
+      expect(Navigator.canPop(containerKey2.currentContext), isTrue);
       Navigator.pop(containerKey2.currentContext);
       tester.pump();
 
@@ -89,6 +93,8 @@ void main() {
       expect(tester.findText('Settings'), isNull);
       expect(tester.findText('Overlay'), isNull);
 
+      expect(Navigator.canPop(containerKey1.currentContext), isFalse);
+
     });
   });
 }