Added new implementations for virtual containers

runtime/observatory/lib/src/elements/containers/virtual_collection.dart

Index: runtime/observatory/lib/src/elements/containers/virtual_collection.dart
diff --git a/runtime/observatory/lib/src/elements/containers/virtual_collection.dart b/runtime/observatory/lib/src/elements/containers/virtual_collection.dart
new file mode 100644
index 0000000000000000000000000000000000000000..36e682c2852c436a7e0e650594652dc06e4e3b39
--- /dev/null
+++ b/runtime/observatory/lib/src/elements/containers/virtual_collection.dart
@@ -0,0 +1,158 @@
+// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+// for details. 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:async';
+import 'dart:html';
+import 'package:observatory/src/elements/helpers/rendering_scheduler.dart';
+import 'package:observatory/src/elements/helpers/tag.dart';
+
+typedef HtmlElement VirtualCollectionCreateCallback();
+typedef void VirtualCollectionUpdateCallback(HtmlElement el, dynamic item,
+    int index);
+
+class VirtualCollectionElement extends HtmlElement implements Renderable {
+  static const tag =
+      const Tag<VirtualCollectionElement>('virtual-collection');
+
+  RenderingScheduler<VirtualCollectionElement> _r;
+
+  Stream<RenderedEvent<VirtualCollectionElement>> get onRendered =>
+      _r.onRendered;
+
+  VirtualCollectionCreateCallback _create;
+  VirtualCollectionUpdateCallback _update;
+  double _itemHeight;
+  int _top;
+  int _height;
+  List _items;
+  StreamSubscription _onScrollSubscription;
+  StreamSubscription _onResizeSubscription;
+
+  List get items => _items;
+
+  set items(Iterable value) {
+    _items = new List.unmodifiable(value);
+    _r.dirty();
+  }
+
+
+  factory VirtualCollectionElement(VirtualCollectionCreateCallback create,
+      VirtualCollectionUpdateCallback update, {Iterable items: const [],
+      RenderingQueue queue}) {
+    assert(create != null);
+    assert(update != null);
+    assert(items != null);
+    VirtualCollectionElement e = document.createElement(tag.name);
+    e._r = new RenderingScheduler(e, queue: queue);
+    e._create = create;
+    e._update = update;
+    e._items = new List.unmodifiable(items);
+    return e;
+  }
+
+  VirtualCollectionElement.created() : super.created();
+
+  @override
+  attached() {
+    super.attached();
+    _r.enable();
+    _top = 0;
+    _height = getBoundingClientRect().height;
+    _itemHeight = _computeItemHeight();
+    _onScrollSubscription = onScroll.listen(_onScroll);
+    _onResizeSubscription = window.onResize.listen(_onResize);
+  }
+
+ @override
+  detached() {
+    super.detached();
+    _r.disable(notify: true);
+    children = const [];
+    _onScrollSubscription.cancel();
+    _onResizeSubscription.cancel();
+  }
+
+  final DivElement _scroller = new DivElement()..classes = const ['scroller'];
+  final DivElement _shifter = new DivElement()..classes = const ['shifter'];
+
+  dynamic getItemFromElement(HtmlElement element) {
+    final el_index = _shifter.children.indexOf(element);
+    if (el_index < 0) {
+      return null;
+    }
+    final item_index =
+      _top + el_index - (_shifter.children.length * _inverse_preload).floor();
+    if (0 <= item_index && item_index < items.length) {
+      return _items[item_index];
+    }
+    return null;
+  }
+
+  /// The preloaded element before and after the visible area are:
+  /// 1/preload_size of the number of items in the visble area.
+  /// See shared.css for the "top:-25%;".
+  static const int _preload = 2;
+  /// L = length of all the elements loaded
+  /// l = length of the visible area
+  ///
+  /// L = l + 2 * l / _preload
+  /// l = L * _preload / (_preload + 2)
+  ///
+  /// tail = l / _preload = L * 1 / (_preload + 2) = L * _inverse_preload
+  static const double _inverse_preload = 1 / (_preload + 2);
+
+  void render() {
+    _top = (scrollTop / _itemHeight).floor();
+
+    _scroller.style.height = '${_itemHeight*(_items.length)}px';
+    _shifter.style.top = '${_itemHeight*_top}px';
+    final tail_length = (_height / _itemHeight / _preload).ceil();
+    final length = tail_length * 2 + tail_length * _preload;
+
+    if (_shifter.children.length < length) {
+      while (_shifter.children.length != length) {
+        var e = _create();
+        e..style.display = 'hidden';
+        _shifter.children.add(e);
+      }
+      _shifter.style.height = '${_itemHeight*length}px';
+      children = [
+        _scroller
+          ..children = [_shifter]
+      ];
+    }
+
+    int i = _top - tail_length;
+    for (final HtmlElement e in _shifter.children) {
+      if (0 <= i && i < _items.length) {
+        e..style.display = null;
+        _update(e, _items[i], i);
+      } else {
+        e.style.display = 'hidden';
+      }
+      i++;
+    }
+  }
+
+  double _computeItemHeight() {
+    final c = children;
+    children = [_create()];
+    final height = children[0].getBoundingClientRect().height;
+    children = c;
+    return height;
+  }
+
+  void _onScroll(_) {
+    if(_r.isDirty) return;
+    if ((scrollTop - _top * _itemHeight).abs() >=
+         _shifter.children.length * _inverse_preload * _itemHeight) {
+      _r.dirty();
+    }
+  }
+
+  void _onResize(_) {
+    _height = getBoundingClientRect().height;
+    _r.dirty();
+  }
+}