Observatory: Add view of dominator tree with siblings merged by class.

runtime/observatory/lib/src/elements/heap_snapshot.dart

Index: runtime/observatory/lib/src/elements/heap_snapshot.dart
diff --git a/runtime/observatory/lib/src/elements/heap_snapshot.dart b/runtime/observatory/lib/src/elements/heap_snapshot.dart
index 2fd6e7882247f5d014aa80cb3cb8b53602f3f691..03e981ce09dbdc1c12d33b1efb9732e0df5787c7 100644
--- a/runtime/observatory/lib/src/elements/heap_snapshot.dart
+++ b/runtime/observatory/lib/src/elements/heap_snapshot.dart
@@ -24,7 +24,7 @@ import 'package:observatory/src/elements/nav/top_menu.dart';
 import 'package:observatory/src/elements/nav/vm_menu.dart';
 import 'package:observatory/utils.dart';
 
-enum HeapSnapshotTreeMode { dominatorTree, groupByClass }
+enum HeapSnapshotTreeMode { dominatorTree, mergedDominatorTree, groupByClass }
 
 class HeapSnapshotElement extends HtmlElement implements Renderable {
   static const tag =
@@ -266,6 +266,31 @@ class HeapSnapshotElement extends HtmlElement implements Renderable {
           _tree
         ]);
         break;
+      case HeapSnapshotTreeMode.mergedDominatorTree:
+        _tree = new VirtualTreeElement(
+            _createMergedDominator, _updateMergedDominator,
+            _getChildrenMergedDominator,
+            items: _getChildrenMergedDominator(_snapshot.mergedDominatorTree),
+            queue: _r.queue);
+        _tree.expand(_snapshot.mergedDominatorTree);
+        final text = 'In a heap dominator tree, an object X is a parent of '
+            'object Y if every path from the root to Y goes through '
+            'X. This allows you to find "choke points" that are '
+            'holding onto a lot of memory. If an object becomes '
+            'garbage, all its children in the dominator tree become '
+            'garbage as well. '
+            'The retained size of an object is the sum of the '
+            'retained sizes of its children in the dominator tree '

[Cutch, 2016/11/14 17:24:13]

This text should explain what "merge" means in more detail.

[rmacnak, 2016/11/15 00:44:15]

Done.

+            'plus its own shallow size, and is the amount of memory '
+            'that would be freed if the object became garbage.';
+        report.addAll([
+          new DivElement()
+            ..classes = ['content-centered-big', 'explanation']
+            ..text = text
+            ..title = text,
+          _tree
+        ]);
+        break;
       case HeapSnapshotTreeMode.groupByClass:
         final items = _snapshot.classReferences.toList();
         items.sort((a, b) => b.shallowSize - a.shallowSize);
@@ -299,6 +324,24 @@ class HeapSnapshotElement extends HtmlElement implements Renderable {
       ];
   }
 
+  static Element _createMergedDominator(toggle) {
+    return new DivElement()
+      ..classes = ['tree-item']
+      ..children = [
+        new SpanElement()
+          ..classes = ['size']
+          ..title = 'retained size',
+        new SpanElement()..classes = ['lines'],
+        new ButtonElement()
+          ..classes = ['expander']
+          ..onClick.listen((_) => toggle(autoToggleSingleChildNodes: true)),
+        new SpanElement()
+          ..classes = ['percentage']
+          ..title = 'percentage of heap being retained',
+        new SpanElement()..classes = ['name']
+      ];
+  }
+
   static Element _createGroup(toggle) {
     return new DivElement()
       ..classes = ['tree-item']
@@ -327,6 +370,13 @@ class HeapSnapshotElement extends HtmlElement implements Renderable {
         .where((child) => child.retainedSize >= kMinRetainedSize)
         .take(kMaxChildren);
   }
+  static _getChildrenMergedDominator(M.HeapSnapshotMergedDominatorNode node) {
+    final list = node.children.toList();
+    list.sort((a, b) => b.retainedSize - a.retainedSize);
+    return list
+        .where((child) => child.retainedSize >= kMinRetainedSize)
+        .take(kMaxChildren);
+  }
 
   static _getChildrenGroup(item) {
     if (item is M.HeapSnapshotClassReferences) {
@@ -361,6 +411,31 @@ class HeapSnapshotElement extends HtmlElement implements Renderable {
     });
   }
 
+  void _updateMergedDominator(
+      HtmlElement element, M.HeapSnapshotMergedDominatorNode node, int depth) {
+    element.children[0].text = Utils.formatSize(node.retainedSize);
+    _updateLines(element.children[1].children, depth);
+    if (_getChildrenMergedDominator(node).isNotEmpty) {
+      element.children[2].text = _tree.isExpanded(node) ? '▼' : '►';
+    } else {
+      element.children[2].text = '';
+    }
+    element.children[3].text =
+        Utils.formatPercentNormalized(node.retainedSize * 1.0 / _snapshot.size);
+    final wrapper = new SpanElement()
+      ..classes = ['name']
+      ..text = 'Loading...';
+    element.children[4] = wrapper;
+    node.klass.then((klass) {
+      wrapper
+        ..text = ''
+        ..children = [
+          new SpanElement()..text = '${node.instanceCount} instances of ',
+          anyRef(_isolate, klass, _instances, queue: _r.queue)
+        ];
+    });
+  }
+
   void _updateGroup(HtmlElement element, item, int depth) {
     _updateLines(element.children[1].children, depth);
     if (item is M.HeapSnapshotClassReferences) {
@@ -424,6 +499,8 @@ class HeapSnapshotElement extends HtmlElement implements Renderable {
     switch (mode) {
       case HeapSnapshotTreeMode.dominatorTree:
         return 'Dominator tree';
+      case HeapSnapshotTreeMode.mergedDominatorTree:
+        return 'Dominator tree (merged siblings by class)';
       case HeapSnapshotTreeMode.groupByClass:
         return 'Group by class';
     }