cc: Add UniqueNotifier to cc/base.

cc/base/unique_notifier.h

Index: cc/base/unique_notifier.h
diff --git a/cc/base/unique_notifier.h b/cc/base/unique_notifier.h
new file mode 100644
index 0000000000000000000000000000000000000000..65a663feb2767cf53aeb1b994fc213c4c18d520e
--- /dev/null
+++ b/cc/base/unique_notifier.h
@@ -0,0 +1,52 @@
+// Copyright 2014 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 CC_BASE_UNIQUE_NOTIFIER_H_
+#define CC_BASE_UNIQUE_NOTIFIER_H_
+
+#include "base/callback.h"
+#include "base/memory/weak_ptr.h"
+#include "cc/base/cc_export.h"
+
+namespace base {
+class SequencedTaskRunner;
+}  // namespace base
+
+namespace cc {
+
+class CC_EXPORT UniqueNotifier {
+ public:
+  // Configure this notifier to issue the |closure| notification in |delay| time
+  // from Schedule() call. Note that 0 delay is also acceptable, which will post
+  // the task to be run as soon as possible.
+  UniqueNotifier(const scoped_refptr<base::SequencedTaskRunner>& task_runner,

[reveman, 2014/05/21 17:03:37]

no need to pass ownership here, base::SequencedTaskRunner* please

[vmpstr, 2014/05/21 17:41:02]

Done.

+                 const base::Closure& closure,
+                 const base::TimeDelta& delay);

[reveman, 2014/05/21 17:03:37]

I would prefer if we had two classes (UniqueNotifier, DelayedUniqueNotifier)
instead of special casing delay=0. I think this will both make the usage and the
implementation a bit cleaner.

Maybe DelayedUniqueNotifier could derive from UniqueNotifier class and
Schedule() could be virtual.

[vmpstr, 2014/05/21 17:41:02]

I don't think deriving from each other would make much sense, since the classes
wouldn't share much implementation aside from the conceptual thing they do. 

I'll split this into two different classes for now. If in the future we want to
pass a delayed unique notifier pointer as a unique notifier, changing it to a
hierarchy should be straightforward.

+
+  // Destroying the notifier will ensure that no further notifications will
+  // happen from this class.
+  ~UniqueNotifier();
+
+  // Schedule a notification to be run. If another notification is already
+  // pending, then it will happen in given delay from now. That is, if delay is
+  // 16ms and a notification has been scheduled 10ms ago (ie, it should trigger
+  // in 6ms), then calling schedule will ensure that the only notification that
+  // arrives will happen in 16ms from now.
+  void Schedule();
+
+ private:
+  void NotifyIfTime();
+
+  scoped_refptr<base::SequencedTaskRunner> task_runner_;
+  base::Closure closure_;
+  base::TimeDelta delay_;
+  base::TimeTicks next_notification_time_;
+  bool notification_pending_;
+
+  base::WeakPtrFactory<UniqueNotifier> weak_ptr_factory_;
+};
+
+}  // namespace cc
+
+#endif  // CC_BASE_UNIQUE_NOTIFIER_H_