Underflowing LazyBlock should relayout for additional children (#5956)

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

@@ -497,6 +497,9 @@ class _LazyBlockElement extends RenderObjectElement {
   /// The largest start offset (exclusive) that can be displayed properly with the items currently represented in [_children].
   double _startOffsetUpperLimit = 0.0;
 
+  /// True if the children don't fill the viewport.
+  bool _underflow = false;
+
   int _lastReportedFirstChildLogicalIndex;
   int _lastReportedLastChildLogicalIndex;
   double _lastReportedFirstChildLogicalOffset;
@@ -597,7 +600,10 @@ class _LazyBlockElement extends RenderObjectElement {
   void performRebuild() {
     IndexedWidgetBuilder builder = widget.delegate.buildItem;
     List<Widget> widgets = <Widget>[];
-    for (int i = 0; i < _children.length; i += 1) {
+    // If the most recent layout didn't fill the viewport but an additional child
+    // is now available, add it to the widgets list which will force a layout.
+    int buildChildCount = _underflow ? _children.length + 1 : _children.length;
+    for (int i = 0; i < buildChildCount; ++i) {
       int logicalIndex = _firstChildLogicalIndex + i;
       Widget childWidget = _callBuilder(builder, logicalIndex);
       if (childWidget == null)
@@ -701,8 +707,8 @@ class _LazyBlockElement extends RenderObjectElement {
       _startOffsetLowerLimit = currentLogicalOffset;
     }
 
-    // Materialize new children until we fill the viewport (or run out of
-    // children to materialize).
+    // Materialize new children until we fill the viewport or run out of
+    // children to materialize. If we run out then _underflow is true.
 
     RenderBox child;
     while (currentLogicalOffset < endLogicalOffset) {
@@ -737,7 +743,8 @@ class _LazyBlockElement extends RenderObjectElement {
     // viewport. The currentLogicalIndex is the index of the first child that
     // we don't need.
 
-    if (currentLogicalOffset < endLogicalOffset) {
+    _underflow = currentLogicalOffset < endLogicalOffset;
+    if (_underflow) {
       // The last element is visible. We can scroll as far as they want, there's
       // nothing more to paint.
       _startOffsetUpperLimit = double.INFINITY;

packages/flutter/test/widget/lazy_block_test.dart

@@ -28,4 +28,131 @@ void main() {
       )
     ));
   });
+
+  testWidgets('Underflowing LazyBlock should relayout for additional children', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/5950
+
+    await tester.pumpWidget(new LazyBlock(
+      delegate: new LazyBlockChildren(
+        children: <Widget>[
+          new SizedBox(height: 100.0, child: new Text('100')),
+        ]
+      )
+    ));
+
+    await tester.pumpWidget(new LazyBlock(
+      delegate: new LazyBlockChildren(
+        children: <Widget>[
+          new SizedBox(height: 100.0, child: new Text('100')),
+          new SizedBox(height: 200.0, child: new Text('200')),
+        ]
+      )
+    ));
+
+    expect(find.text('200'), findsOneWidget);
+  });
+
+
+  testWidgets('Underflowing LazyBlock contentExtent should track additional children ', (WidgetTester tester) async {
+    await tester.pumpWidget(new LazyBlock(
+      delegate: new LazyBlockChildren(
+        children: <Widget>[
+          new SizedBox(height: 100.0, child: new Text('100')),
+        ]
+      )
+    ));
+
+    StatefulElement statefulElement = tester.element(find.byType(Scrollable));
+    ScrollableState scrollable = statefulElement.state;
+    OverscrollWhenScrollableBehavior scrollBehavior = scrollable.scrollBehavior;
+    expect(scrollBehavior.contentExtent, equals(100.0));
+
+    await tester.pumpWidget(new LazyBlock(
+      delegate: new LazyBlockChildren(
+        children: <Widget>[
+          new SizedBox(height: 100.0, child: new Text('100')),
+          new SizedBox(height: 200.0, child: new Text('200')),
+        ]
+      )
+    ));
+    expect(scrollBehavior.contentExtent, equals(300.0));
+
+    await tester.pumpWidget(new LazyBlock(
+      delegate: new LazyBlockChildren(
+        children: <Widget>[
+        ]
+      )
+    ));
+    expect(scrollBehavior.contentExtent, equals(0.0));
+  });
+
+
+  testWidgets('Overflowing LazyBlock should relayout for missing children', (WidgetTester tester) async {
+    await tester.pumpWidget(new LazyBlock(
+      delegate: new LazyBlockChildren(
+        children: <Widget>[
+          new SizedBox(height: 300.0, child: new Text('300')),
+          new SizedBox(height: 400.0, child: new Text('400')),
+        ]
+      )
+    ));
+
+    expect(find.text('300'), findsOneWidget);
+    expect(find.text('400'), findsOneWidget);
+
+    await tester.pumpWidget(new LazyBlock(
+      delegate: new LazyBlockChildren(
+        children: <Widget>[
+          new SizedBox(height: 300.0, child: new Text('300')),
+        ]
+      )
+    ));
+
+    expect(find.text('300'), findsOneWidget);
+    expect(find.text('400'), findsNothing);
+
+    await tester.pumpWidget(new LazyBlock(
+      delegate: new LazyBlockChildren(
+        children: <Widget>[
+        ]
+      )
+    ));
+
+    expect(find.text('300'), findsNothing);
+    expect(find.text('400'), findsNothing);
+  });
+
+  testWidgets('Overflowing LazyBlock should not relayout for additional children', (WidgetTester tester) async {
+    await tester.pumpWidget(new LazyBlock(
+      delegate: new LazyBlockChildren(
+        children: <Widget>[
+          new SizedBox(height: 300.0, child: new Text('300')),
+          new SizedBox(height: 400.0, child: new Text('400')),
+        ]
+      )
+    ));
+
+    expect(find.text('300'), findsOneWidget);
+    expect(find.text('400'), findsOneWidget);
+
+    await tester.pumpWidget(new LazyBlock(
+      delegate: new LazyBlockChildren(
+        children: <Widget>[
+          new SizedBox(height: 300.0, child: new Text('300')),
+          new SizedBox(height: 400.0, child: new Text('400')),
+          new SizedBox(height: 100.0, child: new Text('100')),
+        ]
+      )
+    ));
+
+    expect(find.text('300'), findsOneWidget);
+    expect(find.text('400'), findsOneWidget);
+    expect(find.text('100'), findsNothing);
+
+    StatefulElement statefulElement = tester.element(find.byType(Scrollable));
+    ScrollableState scrollable = statefulElement.state;
+    OverscrollWhenScrollableBehavior scrollBehavior = scrollable.scrollBehavior;
+    expect(scrollBehavior.contentExtent, equals(700.0));
+  });
+
 }