Support slotchange events

third_party/WebKit/Source/core/dom/Node.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 26 matching lines @@
 #include "core/dom/ChildNodeList.h"
 #include "core/dom/DOMNodeIds.h"
 #include "core/dom/Document.h"
 #include "core/dom/DocumentFragment.h"
 #include "core/dom/DocumentType.h"
 #include "core/dom/Element.h"
 #include "core/dom/ElementRareData.h"
 #include "core/dom/ElementTraversal.h"
 #include "core/dom/ExceptionCode.h"
 #include "core/dom/LayoutTreeBuilderTraversal.h"
+#include "core/dom/Microtask.h"
 #include "core/dom/NodeRareData.h"
 #include "core/dom/NodeTraversal.h"
 #include "core/dom/ProcessingInstruction.h"
 #include "core/dom/Range.h"
 #include "core/dom/StaticNodeList.h"
 #include "core/dom/StyleEngine.h"
 #include "core/dom/TemplateContentDocumentFragment.h"
 #include "core/dom/Text.h"
 #include "core/dom/TreeScopeAdopter.h"
 #include "core/dom/UserActionElementSet.h"
@@ skipping 626 matching lines @@
 {
     ScriptForbiddenScope forbidScriptDuringRawIteration;
     for (Node* node = parentOrShadowHostNode(); node && !node->childNeedsStyleInvalidation(); node = node->parentOrShadowHostNode())
         node->setChildNeedsStyleInvalidation();
     document().scheduleLayoutTreeUpdateIfNeeded();
 }
 
 void Node::markAncestorsWithChildNeedsDistributionRecalc()
 {
     ScriptForbiddenScope forbidScriptDuringRawIteration;
+    if (inDocument() && !document().childNeedsDistributionRecalc()) {
+        // TODO(hayato): Support a non-document composed tree.
+        // TODO(hayato): Enqueue a task only if a 'slotchange' event listner is registered in the document composed tree.
+        Microtask::enqueueMicrotask(WTF::bind(&Document::updateDistribution, PassRefPtrWillBeRawPtr<Document>(&document())));

[esprehn, 2016/02/18 17:30:43]

this means we force a distribution after every DOM mutation even if no one is
listening which is just wasted battery. Also this seems pretty bad, it means if
you do:

1) micro task that mutates the dom
2) microtask that dispatches an event (causes distribution), then mutates the
dom
3) microtask that dispatches another event (causes distribution), then mutates
the dom
4) now run *3* updateDistribution microtasks.

[esprehn, 2016/02/18 18:07:37]

This is also adding an extra microtask every time an input element or other
thing with a UA shadowroot is inserted since all of those also mark the document
for distribution.

ex.

.appendChild(input);
.dispatchEvent(...);
.appendChild(input);
.dispatchEvent(...);
.appendChild(input);
.dispatchEvent(...);
.appendChild(input);
// Now 3 no-op Document::updateDistribution microtasks run.

This grows the microtask list unbounded if you interleave creating elements that
use shadow roots and doing anything that forces distribution (events,
getComputedStyle, layout, ...)

That's actually exactly what <iron-list> in polymer does, they appendChild and
then measure this means your patch causes an extra 10 microtasks on the chrome
downloads page.

+    }
     for (Node* node = this; node && !node->childNeedsDistributionRecalc(); node = node->parentOrShadowHostNode())
         node->setChildNeedsDistributionRecalc();
     document().scheduleLayoutTreeUpdateIfNeeded();
 }
 
 inline void Node::setStyleChange(StyleChangeType changeType)
 {
     m_nodeFlags = (m_nodeFlags & ~StyleChangeMask) | changeType;
 }
 
@@ skipping 1722 matching lines @@
 
 void showNodePath(const blink::Node* node)
 {
     if (node)
         node->showNodePathForThis();
     else
         fprintf(stderr, "Cannot showNodePath for (nil)\n");
 }
 
 #endif