Block should work inside LazyBlock (#3546)

Previously we were locking down the state even when calling layout in
LazyBlock. Now we lock only when building children. Making this work well
involved moving the catch out of lockState and into the few callers who
actually wanted it.

Fixes #3534

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

@@ -285,7 +285,7 @@ class RenderObjectToWidgetAdapter<T extends RenderObject> extends RenderObjectWi
       } else {
         element.update(this);
       }
-    }, building: true, context: 'while attaching root widget to rendering tree');
+    }, building: true);
     return element;
   }
 

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

@@ -710,7 +710,7 @@ class BuildOwner {
   ///
   /// The context argument is used to describe the scope in case an exception is
   /// caught while invoking the callback.
-  void lockState(void callback(), { bool building: false, String context }) {
+  void lockState(void callback(), { bool building: false }) {
     bool debugPreviouslyBuilding;
     assert(_debugStateLockLevel >= 0);
     assert(() {
@@ -723,8 +723,6 @@ class BuildOwner {
     });
     try {
       callback();
-    } catch (e, stack) {
-      _debugReportException(context, e, stack);
     } finally {
       assert(() {
         _debugStateLockLevel -= 1;
@@ -758,6 +756,7 @@ class BuildOwner {
     if (_dirtyElements.isEmpty)
       return;
     Timeline.startSync('Build');
+    try {
       lockState(() {
         _dirtyElements.sort(_elementSort);
         int dirtyCount = _dirtyElements.length;
@@ -771,11 +770,12 @@ class BuildOwner {
           }
         }
         assert(!_dirtyElements.any((BuildableElement element) => element.dirty));
+      }, building: true);
+    } finally {
       _dirtyElements.clear();
-    }, building: true, context: 'while rebuilding dirty elements');
-    assert(_dirtyElements.isEmpty);
       Timeline.finishSync();
     }
+  }
 
   /// Complete the element build pass by unmounting any elements that are no
   /// longer active.
@@ -789,13 +789,18 @@ class BuildOwner {
   /// about changes to global keys will run.
   void finalizeTree() {
     Timeline.startSync('Finalize tree');
+    try {
       lockState(() {
         _inactiveElements._unmountAll();
-    }, context: 'while finalizing the widget tree');
+      });
       assert(GlobalKey._debugCheckForDuplicates);
       scheduleMicrotask(GlobalKey._notifyListeners);
+    } catch (e, stack) {
+      _debugReportException('while finalizing the widget tree', e, stack);
+    } finally {
       Timeline.finishSync();
     }
+  }
 
   /// Cause the entire subtree rooted at the given [Element] to
   /// be entirely rebuilt. This is used by development tools when

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

@@ -494,7 +494,6 @@ class _LazyBlockElement extends RenderObjectElement {
   void _layout(BoxConstraints constraints) {
     final double blockExtent = _getMainAxisExtent(renderObject.size);
 
-    owner.lockState(() {
     final IndexedBuilder builder = widget.delegate.buildItem;
     final double startLogicalOffset = widget.startOffset;
     final double endLogicalOffset = startLogicalOffset + blockExtent;
@@ -524,10 +523,24 @@ class _LazyBlockElement extends RenderObjectElement {
 
       while (currentLogicalIndex > 0 && currentLogicalOffset > startLogicalOffset) {
         currentLogicalIndex -= 1;
+        Element newElement;
+        owner.lockState(() {
+          // TODO(abarth): Handle exceptions from builder gracefully.
           Widget newWidget = builder(this, currentLogicalIndex);
-          assert(newWidget != null);
+          if (newWidget == null) {
+            throw new FlutterError(
+              'buildItem must not return null after returning non-null.\n'
+              'If buildItem for a LazyBlockDelegate returns a non-null widget for a given '
+              'index, it must return non-null widgets for every smaller index as well. The '
+              'buildItem function for ${widget.delegate.runtimeType} returned null for '
+              'index $currentLogicalIndex after having returned a non-null value for index '
+              '${currentLogicalIndex - 1}.'
+            );
+          }
           newWidget = new RepaintBoundary.wrap(newWidget, currentLogicalIndex);
-          newChildren.add(inflateWidget(newWidget, null));
+          newElement = inflateWidget(newWidget, null);
+        }, building: true);
+        newChildren.add(newElement);
         RenderBox child = block.firstChild;
         assert(child == newChildren.last.renderObject);
         child.layout(innerConstraints, parentUsesSize: true);
@@ -592,12 +605,19 @@ class _LazyBlockElement extends RenderObjectElement {
       int physicalIndex = currentLogicalIndex - _firstChildLogicalIndex;
       if (physicalIndex >= _children.length) {
         assert(physicalIndex == _children.length);
+        Element newElement;
+        owner.lockState(() {
+          // TODO(abarth): Handle exceptions from builder gracefully.
           Widget newWidget = builder(this, currentLogicalIndex);
           if (newWidget == null)
-            break;
+            return;
           newWidget = new RepaintBoundary.wrap(newWidget, currentLogicalIndex);
           Element previousChild = _children.isEmpty ? null : _children.last;
-          _children.add(inflateWidget(newWidget, previousChild));
+          newElement = inflateWidget(newWidget, previousChild);
+        }, building: true);
+        if (newElement == null)
+          return;
+        _children.add(newElement);
       }
       child = _getNextWithin(block, child);
       assert(child != null);
@@ -652,7 +672,6 @@ class _LazyBlockElement extends RenderObjectElement {
     }
 
     _updatePaintOffset();
-    }, building: true, context: 'during $runtimeType layout');
 
     LazyBlockExtentsChangedCallback onExtentsChanged = widget.onExtentsChanged;
     if (onExtentsChanged != null) {

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

@@ -164,7 +164,7 @@ abstract class VirtualViewportElement extends RenderObjectElement {
     assert(startOffsetBase != null);
     assert(startOffsetLimit != null);
     _updatePaintOffset();
-    owner.lockState(_materializeChildren, building: true, context: 'during $runtimeType layout');
+    owner.lockState(_materializeChildren, building: true);
   }
 
   void _materializeChildren() {

packages/flutter/test/widget/lazy_block_test.dart

@@ -0,0 +1,34 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/material.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('Block inside LazyBlock', () {
+    testWidgets((WidgetTester tester) {
+      tester.pumpWidget(new LazyBlock(
+        delegate: new LazyBlockChildren(
+          children: <Widget>[
+            new Block(
+              children: <Widget>[
+                new Text('1'),
+                new Text('2'),
+                new Text('3'),
+              ]
+            ),
+            new Block(
+              children: <Widget>[
+                new Text('4'),
+                new Text('5'),
+                new Text('6'),
+              ]
+            ),
+          ]
+        )
+      ));
+    });
+  });
+}