compositor-worker: Initialize CW machinery plumbing to compositor and fire CW rAF callbacks.

third_party/WebKit/Source/platform/graphics/CompositorMutator.h

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CompositorMutator_h
+#define CompositorMutator_h
+
+#include "platform/heap/Handle.h"
+#include "wtf/RefCounted.h"
+
+namespace blink {
+
+class CompositorMutator : public GarbageCollectedFinalized<CompositorMutator> {

[jbroman, 2016/04/29 19:42:22]

If this is an interface (it seems to be), it should probably be a
GarbageCollectedMixin; the implementation should be the one declaring whether it
needs finalization or not.

[flackr, 2016/05/03 22:24:54]

Done.

[haraken, 2016/05/09 01:37:05]

Hmm, I don't fully understand this comment. What prevents you from just making
CompositorMutator GarbageCollectedFinalized? Finalized is expected to be
specified at the base class even if the base class doesn't have a finalizer.

[jbroman, 2016/05/19 18:50:05]

CompositorMutator is a pure virtual base class (i.e. an interface). My
understanding was that GarbageCollected (or GarbageCollectedFinalized) is
expected to be derived by the concrete base class, if any, but that interfaces
would be GarbageCollectedMixin. Among other things, this lets a single class
implement multiple interfaces -- though that's not the case here.

It will, of course, work either way. Is there a reason to prefer deriving from
GarbageCollectedFinalized here?

[haraken, 2016/05/19 23:55:51]

Because GarbageCollectedMixin adds (some complicated) virtual tracing methods,
which slows down tracing, and because GarbageCollectedMixin asks people to type
a couple of more things (e.g., USING_GC_MIXIN).

So if GarbageCollectedMixin is not needed, I'd prefer not using it :)

[jbroman, 2016/05/20 00:06:52]

OK. I do see now that
https://chromium.googlesource.com/chromium/src/+/master/third_party/WebKit/So...
is careful to use the term "non-leftmost base class", though it's not obvious to
me only in retrospect why you'd like it that way. (Nor would I know how to weigh
that against the costs of finalization if, for instance, this class had had two
subclasses, one of which required finalization and the other of which did not --
as I understand it you cannot add finalization to a GarbageCollected base
class.)

+public:
+    virtual ~CompositorMutator() {}
+
+    DEFINE_INLINE_VIRTUAL_TRACE() {}
+
+    // Called from compositor thread which does not participate in GC cycle

[jbroman, 2016/04/29 19:42:22]

What does this comment mean? Also, a brief comment about what the bool means,
please.

[flackr, 2016/05/03 22:24:54]

Sorry, this comment is way out of date, updated.

+    virtual bool mutate(double monotonicTimeNow) = 0;
+};
+
+} // namespace blink
+
+#endif // CompositorMutator_h