Support slotchange events

third_party/WebKit/Source/core/html/HTMLSlotElement.cpp

Index: third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
diff --git a/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp b/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
index e6e061fd43441c0744f6756e3e144a599401666e..cd7c0b5dedf1e69c5903d587591a3a0fff74a786 100644
--- a/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
+++ b/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
@@ -31,9 +31,11 @@
 #include "core/html/HTMLSlotElement.h"
 
 #include "core/HTMLNames.h"
+#include "core/dom/Microtask.h"
 #include "core/dom/NodeTraversal.h"
 #include "core/dom/shadow/ElementShadow.h"
 #include "core/dom/shadow/InsertionPoint.h"
+#include "core/events/Event.h"
 #include "core/html/AssignedNodesOptions.h"
 
 namespace blink {
@@ -103,10 +105,23 @@ void HTMLSlotElement::appendDistributedNodesFrom(const HTMLSlotElement& other)
 void HTMLSlotElement::clearDistribution()
 {
     m_assignedNodes.clear();
+    m_oldDistributedNodes.swap(m_distributedNodes);

[kochi, 2016/02/18 03:40:56]

Is there any possibility that clearDistribution() is called twice without
updateDistribution() in between?

[hayato, 2016/02/18 03:51:56]

It could happen in the future. I'm aware the current approach is very weak to
the future change.
See TODO comment here:
https://codereview.chromium.org/1695163003/diff/100001/third_party/WebKit/Sou...

I have a plan to use RAAI approach to get rid of m_oldDistributedNodes.

     m_distributedNodes.clear();
     m_distributedIndices.clear();
 }
 
+bool HTMLSlotElement::hasSlotChangeEventListener()
+{
+    return eventTargetData() && eventTargetData()->eventListenerMap.find(EventTypeNames::slotchange);

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

this is not correct per the semantics of events, you should be able to do:

mutateDom();
Promise.resolve().then(() => {
  dom.addEventListener("slotchanged", () => {
    // this should get called.
  });
});

so you can't skip the microtask step to fire the event, the event must "always
happen", even if no one is listening.

this means events are a bad fit, it forces you to burn battery every time
there's a mutation even if no one is listening.

+}
+
+void HTMLSlotElement::dispatchSlotChangeEvent()
+{
+    RefPtrWillBeRawPtr<Event> event = Event::create(EventTypeNames::slotchange);
+    event->setTarget(this);
+    dispatchScopedEvent(event);
+}
+
 Node* HTMLSlotElement::distributedNodeNextTo(const Node& node) const
 {
     const auto& it = m_distributedIndices.find(&node);
@@ -218,12 +233,22 @@ void HTMLSlotElement::updateDistributedNodesWithFallback()
     }
 }
 
+void HTMLSlotElement::didUpdateDistribution()
+{
+    if (hasSlotChangeEventListener() && m_distributedNodes != m_oldDistributedNodes) {
+        // TODO(hayato): Do not enqueue a slotchange event for the same slot twice in the microtask queue
+        Microtask::enqueueMicrotask(WTF::bind(&HTMLSlotElement::dispatchSlotChangeEvent, PassRefPtrWillBeRawPtr<HTMLSlotElement>(this)));

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

the de-duplication here is just reinventing what MutationObserver already does
:(

+    }
+    // TODO(hayato): Call setNeedsDistributionRecalc if the distribution changes due to the fallback elements
+}
+
 DEFINE_TRACE(HTMLSlotElement)
 {
 #if ENABLE(OILPAN)
     visitor->trace(m_assignedNodes);
     visitor->trace(m_distributedNodes);
     visitor->trace(m_distributedIndices);
+    visitor->trace(m_oldDistributedNodes);
 #endif
     HTMLElement::trace(visitor);
 }