Move VariableHeightScrollable to sdk/lib/widgets

sky/sdk/lib/widgets/variable_height_scrollable.dart

+// Copyright 2015 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 'dart:collection';
+
+import '../animation/scroll_behavior.dart';
+import 'basic.dart';
+import 'block_viewport.dart';
+import 'scrollable.dart';
+import 'widget.dart';
+
+class VariableHeightScrollable extends Scrollable {
+  VariableHeightScrollable({
+    String key,
+    this.builder,
+    this.token
+  }) : super(key: key);
+
+  IndexedBuilder builder;
+  Object token;
+
+  void syncFields(VariableHeightScrollable source) {
+    builder = source.builder;
+    token = source.token;
+    super.syncFields(source);
+  }
+
+  ScrollBehavior createScrollBehavior() => new OverscrollBehavior();
+  OverscrollBehavior get scrollBehavior => super.scrollBehavior;
+
+  void _handleSizeChanged(Size newSize) {
+    setState(() {
+      scrollBehavior.containerSize = newSize.height;
+    });
+  }
+
+  void _handleLayoutChanged(
+      int firstVisibleChildIndex, 
+      int visibleChildCount,
+      UnmodifiableListView<double> childOffsets,
+      bool didReachLastChild) {
+    assert(childOffsets.length > 0);
+    scrollBehavior.contentsSize = didReachLastChild ? childOffsets.last : double.INFINITY;
+    if (didReachLastChild && scrollOffset > scrollBehavior.maxScrollOffset)
+      settleScrollOffset();
+  }
+
+  Widget buildContent() {
+    return new SizeObserver(
+      callback: _handleSizeChanged,
+      child: new BlockViewport(
+        builder: builder,
+        onLayoutChanged: _handleLayoutChanged,
+        startOffset: scrollOffset,
+        token: token
+      )
+    );
+  }
+}