Still try to sync even if a node has been removed from the tree.

This should handle a case like a stateful component inside a Container
inside another Container having one of those Containers removed while
still keeping that stateful component around with its state.

The problem of how to handle the Container then being reinserted is a
separate issue not handled by this patch.

packages/flutter/lib/widgets/framework.dart

@@ -246,6 +246,19 @@ abstract class Widget {
   /// chain being correctly configured at this point.
   void walkChildren(WidgetTreeWalker walker) { }
 
+  /// If the object has a single child, return it. Override this if
+  /// you define a new child model with only one child.
+  Widget get singleChild => null;
+
+  /// Detaches the single child of this object from this object,
+  /// without calling remove() on that child.
+  /// Only called when singleChild returns a non-null node.
+  /// Override this if you override singleChild to return non-null.
+  void takeChild() {
+    assert(singleChild != null);
+    throw '${runtimeType} does not define a "takeChild()" method';
+  }
+
   static void _notifyMountStatusChanged() {
     try {
       sky.tracing.begin("Widget._notifyMountStatusChanged");
@@ -357,7 +370,31 @@ abstract class Widget {
     }
 
     if (oldNode != null) {
-      if (_canSync(newNode, oldNode)) {
+      if (!_canSync(newNode, oldNode)) {
+        assert(oldNode.mounted);
+        // We want to handle the case where there is a removal of zero
+        // or more widgets. In this case, we should be able to sync
+        // ourselves with a Widget that is a descendant of the
+        // oldNode, skipping the nodes in between. Let's try that.
+        Widget deadNode = oldNode;
+        Widget candidate = oldNode.singleChild;
+        assert(candidate == null || candidate.parent == oldNode);
+        oldNode = null;
+        while (candidate != null) {
+          if (_canSync(newNode, candidate)) {
+            assert(candidate.parent != null);
+            assert(candidate.parent.singleChild == candidate);
+            candidate.parent.takeChild();
+            oldNode = candidate;
+            break;
+          }
+          assert(candidate.singleChild == null || candidate.singleChild.parent == candidate);
+          candidate = candidate.singleChild;
+        }
+        deadNode.detachRenderObject();
+        deadNode.remove();
+      }
+      if (oldNode != null) {
         if (oldNode.retainStatefulNodeIfPossible(newNode)) {
           assert(oldNode.mounted);
           assert(!newNode.mounted);
@@ -368,11 +405,6 @@ abstract class Widget {
         } else {
           oldNode.setParent(null);
         }
-      } else {
-        assert(oldNode.mounted);
-        oldNode.detachRenderObject();
-        oldNode.remove();
-        oldNode = null;
       }
     }
 
@@ -442,13 +474,13 @@ bool _canSync(Widget a, Widget b) {
 abstract class TagNode extends Widget {
 
   TagNode({ Key key, Widget child })
-    : this.child = child, super(key: key) {
+    : child = child, super(key: key) {
     assert(child != null);
   }
 
   // TODO(jackson): Remove this workaround for limitation of Dart mixins
   TagNode._withKey(Widget child, Key key)
-    : this.child = child, super._withKey(key);
+    : child = child, super._withKey(key);
 
   Widget child;
 
@@ -457,6 +489,14 @@ abstract class TagNode extends Widget {
       walker(child);
   }
 
+  Widget get singleChild => child;
+
+  void takeChild() {
+    assert(singleChild == child);
+    assert(child != null);
+    child = null;
+  }
+
   void _sync(Widget old, dynamic slot) {
     Widget oldChild = old == null ? null : (old as TagNode).child;
     child = syncChild(child, oldChild, slot);
@@ -636,29 +676,33 @@ abstract class Component extends Widget {
   bool _dirty = true;
 
   Widget _child;
-  dynamic _slot; // cached slot from the last time we were synced
-
-  void updateSlot(dynamic newSlot) {
-    _slot = newSlot;
-    if (_child != null)
-      _child.updateSlot(newSlot);
-  }
 
   void walkChildren(WidgetTreeWalker walker) {
     if (_child != null)
       walker(_child);
   }
 
-  void remove() {
+  Widget get singleChild => _child;
+
+  bool _debugChildTaken = false;
+
+  void takeChild() {
+    assert(!_debugChildTaken);
+    assert(singleChild == _child);
     assert(_child != null);
-    assert(renderObject != null);
+    _child = null;
+    _renderObject = null;
+    assert(() { _debugChildTaken = true; return true; });
+  }
+
+  void remove() {
+    assert(_debugChildTaken || (_child != null && _renderObject != null));
     super.remove();
     _child = null;
   }
 
   void detachRenderObject() {
-    assert(_child != null);
-    assert(renderObject != null);
+    if (_child != null)
       _child.detachRenderObject();
   }
 
@@ -667,6 +711,14 @@ abstract class Component extends Widget {
     _scheduleBuild();
   }
 
+  dynamic _slot; // cached slot from the last time we were synced
+
+  void updateSlot(dynamic newSlot) {
+    _slot = newSlot;
+    if (_child != null)
+      _child.updateSlot(newSlot);
+  }
+
   // order corresponds to _build_ order, not depth in the tree.
   // All the Components built by a particular other Component will have the
   // same order, regardless of whether one is subsequently inserted
@@ -711,7 +763,9 @@ abstract class Component extends Widget {
     _child = build();
     _currentOrder = lastOrder;
     assert(_child != null);
+    assert(() { _debugChildTaken = false; return true; });
     _child = syncChild(_child, oldChild, slot);
+    assert(!_debugChildTaken); // we shouldn't be able to lose our child when we're syncing it!
     assert(_child != null);
     assert(_child.parent == this);
 
@@ -1195,6 +1249,14 @@ abstract class OneChildRenderObjectWrapper extends RenderObjectWrapper {
       walker(child);
   }
 
+  Widget get singleChild => _child;
+
+  void takeChild() {
+    assert(singleChild == child);
+    assert(child != null);
+    _child = null;
+  }
+
   void syncRenderObject(RenderObjectWrapper old) {
     super.syncRenderObject(old);
     Widget oldChild = old == null ? null : (old as OneChildRenderObjectWrapper).child;

packages/unit/test/widget/syncing_test.dart

+import 'package:sky/widgets.dart';
+import 'package:test/test.dart';
+
+import 'widget_tester.dart';
+
+class TestState extends StatefulComponent {
+  TestState({this.child, this.state});
+  Widget child;
+  int state;
+  int syncs = 0;
+  void syncConstructorArguments(TestState source) {
+    child = source.child;
+    // we explicitly do NOT sync the state from the new instance
+    // because we're using that to track whether we got recreated
+    syncs += 1;
+  }
+  Widget build() {
+    return child;
+  }
+}
+
+void main() {
+
+  test('no change', () {
+
+    WidgetTester tester = new WidgetTester();
+
+    tester.pumpFrame(() {
+      return new Container(
+        child: new Container(
+          child: new TestState(
+            state: 1,
+            child: new Container()
+          )
+        )
+      );
+    });
+
+    TestState stateWidget = tester.findWidget((widget) => widget is TestState);
+
+    expect(stateWidget.state, equals(1));
+    expect(stateWidget.syncs, equals(0));
+
+    tester.pumpFrame(() {
+      return new Container(
+        child: new Container(
+          child: new TestState(
+            state: 2,
+            child: new Container()
+          )
+        )
+      );
+    });
+
+    expect(stateWidget.state, equals(1));
+    expect(stateWidget.syncs, equals(1));
+
+  });
+
+  test('remove one', () {
+
+    WidgetTester tester = new WidgetTester();
+
+    tester.pumpFrame(() {
+      return new Container(
+        child: new Container(
+          child: new TestState(
+            state: 10,
+            child: new Container()
+          )
+        )
+      );
+    });
+
+    TestState stateWidget = tester.findWidget((widget) => widget is TestState);
+
+    expect(stateWidget.state, equals(10));
+    expect(stateWidget.syncs, equals(0));
+
+    tester.pumpFrame(() {
+      return new Container(
+        child: new TestState(
+          state: 11,
+          child: new Container()
+        )
+      );
+    });
+
+    expect(stateWidget.state, equals(10));
+    expect(stateWidget.syncs, equals(1));
+
+  });
+
+}