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,
+    this.itemCount

[Hixie, 2015/07/10 20:55:11]

Requiring that we pass this in implies this is a finite variable height
scrollable, not an infinite variable height scrollable.

[hansmuller, 2015/07/10 22:42:50]

Done.

+  }) : super(key: key);
+
+  IndexedBuilder builder;
+  Object token;
+  int itemCount;
+
+  void syncFields(VariableHeightScrollable source) {
+    builder = source.builder;
+    token = source.token;
+    itemCount = source.itemCount;
+    super.syncFields(source);
+  }
+
+  ScrollBehavior createScrollBehavior() => new OverscrollBehavior();
+  OverscrollBehavior get scrollBehavior => super.scrollBehavior;
+
+  void _handleSizeChanged(Size newSize) {
+    setState(() {
+      scrollBehavior.containerHeight = newSize.height;
+    });
+  }
+
+  void _handleLayoutChanged(int startIndex, int childCount, UnmodifiableListView<double> offsets) {
+    assert(offsets.length > 0);
+
+    double contentsHeight = offsets.last;
+    int contentsCount = offsets.length - 1;
+    scrollBehavior.contentsHeight = contentsHeight;
+
+    // We don't know the height of all items yet. Grow the ScrollBehavior's

[Hixie, 2015/07/10 20:55:11]

This isn't the right way to do this IMHO. We should make the ScrollBehaviour
know about the "unknown height" state and do the right thing with that state.

[hansmuller, 2015/07/10 22:42:50]

I agree. I've updated OverScrollBehavior to handle contentsHeight == null.
There's no over-scrolling past the maxScrollOffset in this case.

+    // contentsHeight a little, to accomodate scrolling further.
+    if (contentsCount < itemCount)

[Hixie, 2015/07/10 20:55:12]

Rather than using itemCount and figuring out if we happen to know the offset of
the last item, we should just have the underlying class expose a bit regarding
whether it knows that it's reached the bottom or not. (It knows that because
when it asks the builder for the N+1th item, the builder returns null.)

[hansmuller, 2015/07/10 22:42:50]

Good point. I've added a parameter to the onLayoutChanged callback that
indicates if the end has been seen.

+      scrollBehavior.contentsHeight += scrollBehavior.containerHeight;
+
+    if (scrollOffset > scrollBehavior.maxScrollOffset)
+      settleScrollOffset();
+  }
+
+  Widget buildContent() {
+    return new SizeObserver(
+      callback: _handleSizeChanged,
+      child: new BlockViewport(
+        builder: builder,
+        onLayoutChanged: _handleLayoutChanged,
+        startOffset: scrollOffset,
+        token: token
+      )
+    );
+  }
+}