Avoid Stack Overflows during VirtualTree expansion

runtime/observatory/tests/observatory_ui/virtual_tree/element_test.dart

Index: runtime/observatory/tests/observatory_ui/virtual_tree/element_test.dart
diff --git a/runtime/observatory/tests/observatory_ui/virtual_tree/element_test.dart b/runtime/observatory/tests/observatory_ui/virtual_tree/element_test.dart
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+// Copyright (c) 2017, 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:html';
+import 'package:unittest/unittest.dart';
+import 'package:observatory/src/elements/containers/virtual_collection.dart';
+import 'package:observatory/src/elements/containers/virtual_tree.dart';
+
+main() {
+  VirtualTreeElement.tag.ensureRegistration();
+
+  final cTag = VirtualCollectionElement.tag.name;
+
+  var container;
+  setUp(() {
+    container = document.body.getElementsByClassName('test_container').first;
+  });
+  group('instantiation', () {
+    test('default', () {
+      final e = new VirtualTreeElement((_) {}, (_1, _2, _3) {}, (_) {});
+      expect(e, isNotNull, reason: 'element correctly created');
+      expect(e.items, isNotNull, reason: 'items not null');
+      expect(e.items, isEmpty, reason: 'no items');
+    });
+    test('items: []', () {
+      final items = ["1", 2, {}];
+      final e =
+          new VirtualTreeElement((_) {}, (_1, _2, _3) {}, (_) {}, items: items);
+      expect(e, isNotNull, reason: 'element correctly created');
+      expect(e.items, isNot(same(items)), reason: 'avoid side effect');
+      expect(e.items, equals(items), reason: 'same items');
+    });
+  });
+  test('elements created after attachment', () async {
+    final create = (toggle) => new DivElement()..classes = ['test_item'];
+    final update = (HtmlElement el, item, depth) {
+      el.text = item.toString();
+    };
+    final children = (item) => [];
+    final items = ["1", 2, {}];
+    final e = new VirtualTreeElement(create, update, children);
+    container.append(e);
+    await e.onRendered.first;
+    expect(e.children.length, isNonZero, reason: 'has elements');
+    expect(e.querySelectorAll(cTag).length, same(1));
+    e.remove();
+    await e.onRendered.first;
+    expect(e.children.length, isZero, reason: 'is empty');
+  });
+  test('expand single child', () async {
+    const max_depth = 100000;
+    final create = (toggle) => new DivElement()..classes = ['test_item'];
+    final update = (HtmlElement el, item, depth) {
+      el.text = item.toString();
+    };
+    final children = (item) => item >= max_depth ? [] : [item + 1];
+    final items = [0];
+    final e = new VirtualTreeElement(create, update, children, items: items);
+    container.append(e);
+    await e.onRendered.first;
+    expect(e.children.length, isNonZero, reason: 'has elements');
+    final VirtualCollectionElement collection = e.querySelectorAll(cTag).first;
+    expect(collection.items.length, equals(1), reason: 'begin');
+    e.expand(0, autoExpandSingleChildNodes: true);
+    await e.onRendered.first;
+    expect(collection.items.length, equals(max_depth + 1), reason: 'expanded');
+    e.collapse(0, autoCollapseSingleChildNodes: true);
+    await e.onRendered.first;
+    expect(collection.items.length, equals(1), reason: 'collapsed');
+    e.remove();
+    await e.onRendered.first;
+    expect(e.children.length, isZero, reason: 'is empty');
+  });
+
+  test('expand whole tree', () async {
+    const max_depth = 100000;
+    final create = (toggle) => new DivElement()..classes = ['test_item'];
+    final update = (HtmlElement el, item, depth) {
+      el.text = item.toString();
+    };
+    // We want to generated a tree that doesn't collapse to a chain of items
+    // while avoiding to generate an exponential number of items
+    final children = (item) {
+      if (item < 2 * max_depth) {
+        if (item % 200 == 0) {
+          return [item + 1, item + 2];
+        } else if (item % 2 == 0) {
+          return [item + 2];
+        }
+      }
+      return [];
+    };
+    final items = [0];
+    final e = new VirtualTreeElement(create, update, children, items: items);
+    container.append(e);
+    await e.onRendered.first;
+    expect(e.children.length, isNonZero, reason: 'has elements');
+    final VirtualCollectionElement collection = e.querySelectorAll(cTag).first;
+    expect(collection.items.length, equals(1), reason: 'begin');
+    e.expand(0, autoExpandWholeTree: true);
+    await e.onRendered.first;
+    expect(collection.items.length, equals(max_depth + max_depth / 100 + 1),
+        reason: 'expanded');
+    e.collapse(0, autoCollapseWholeTree: true);
+    await e.onRendered.first;
+    expect(collection.items.length, equals(1), reason: 'collapsed');
+    e.remove();
+    await e.onRendered.first;
+    expect(e.children.length, isZero, reason: 'is empty');
+  });
+}