Converted Observatory heap-profile element

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
index 049081de046459ab7645d5eaf4b26fa08eb985f6..b6f91b96661e63c525cdf4136b9ee12abbeaf86c 100644
--- a/runtime/observatory/lib/src/elements/containers/virtual_collection.dart
+++ b/runtime/observatory/lib/src/elements/containers/virtual_collection.dart
@@ -21,6 +21,7 @@ class VirtualCollectionElement extends HtmlElement implements Renderable {
       _r.onRendered;
 
   VirtualCollectionCreateCallback _create;
+  VirtualCollectionCreateCallback _createHeader;
   VirtualCollectionUpdateCallback _update;
   double _itemHeight;
   int _top;
@@ -33,12 +34,14 @@ class VirtualCollectionElement extends HtmlElement implements Renderable {
 
   set items(Iterable value) {
     _items = new List.unmodifiable(value);
+    _top = null;
     _r.dirty();
   }
 
 
   factory VirtualCollectionElement(VirtualCollectionCreateCallback create,
       VirtualCollectionUpdateCallback update, {Iterable items: const [],
+      VirtualCollectionCreateCallback createHeader,
       RenderingQueue queue}) {
     assert(create != null);
     assert(update != null);
@@ -46,6 +49,7 @@ class VirtualCollectionElement extends HtmlElement implements Renderable {
     VirtualCollectionElement e = document.createElement(tag.name);
     e._r = new RenderingScheduler(e, queue: queue);
     e._create = create;
+    e._createHeader = createHeader;
     e._update = update;
     e._items = new List.unmodifiable(items);
     return e;
@@ -57,9 +61,8 @@ class VirtualCollectionElement extends HtmlElement implements Renderable {
   attached() {
     super.attached();
     _r.enable();
-    _top = 0;
-    _height = getBoundingClientRect().height;
-    _itemHeight = _computeItemHeight();
+    _top = null;
+    _itemHeight = null;
     _onScrollSubscription = onScroll.listen(_onScroll);
     _onResizeSubscription = window.onResize.listen(_onResize);
   }
@@ -73,6 +76,7 @@ class VirtualCollectionElement extends HtmlElement implements Renderable {
     _onResizeSubscription.cancel();
   }
 
+  final DivElement _header = new DivElement()..classes = const ['header'];
   final DivElement _scroller = new DivElement()..classes = const ['scroller'];
   final DivElement _shifter = new DivElement()..classes = const ['shifter'];
 
@@ -103,11 +107,26 @@ class VirtualCollectionElement extends HtmlElement implements Renderable {
   static const double _inverse_preload = 1 / (_preload + 2);
 
   void render() {
-    _top = (scrollTop / _itemHeight).floor();
+    if (children.isEmpty) {
+      children = [
+        _scroller
+          ..children = [
+            _shifter
+              ..children = [_create()]
+          ],
+      ];
+      if (_createHeader != null) {
+        _header.children = [_createHeader()];
+        _scroller.children.insert(0, _header);
+      }
+      _itemHeight = _shifter.children[0].getBoundingClientRect().height;
+      _height = getBoundingClientRect().height;
+    }
+    final top = (scrollTop / _itemHeight).floor();
 
+    _header.style.top = '${scrollTop}px';
     _scroller.style.height = '${_itemHeight*(_items.length)}px';
     final tail_length = (_height / _itemHeight / _preload).ceil();
-    _shifter.style.top = '${_itemHeight*(_top - tail_length)}px';
     final length = tail_length * 2 + tail_length * _preload;
 
     if (_shifter.children.length < length) {
@@ -116,38 +135,27 @@ class VirtualCollectionElement extends HtmlElement implements Renderable {
         e..style.display = 'hidden';
         _shifter.children.add(e);
       }
-      children = [
-        _scroller
-          ..children = [_shifter]
-      ];
+      _top = null; // force update;
     }
 
-    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';
+    if ((_top == null) || ((top - _top).abs() >= tail_length)) {
+      _shifter.style.top = '${_itemHeight*(top-tail_length)}px';
+      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++;
       }
-      i++;
+      _top = top;
     }
   }
 
-  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();
-    }
+    _r.dirty();
   }
 
   void _onResize(_) {