Add a non-blocking "OneShotEvent" class

extensions/common/one_shot_event.h

Index: extensions/common/one_shot_event.h
diff --git a/extensions/common/one_shot_event.h b/extensions/common/one_shot_event.h
new file mode 100644
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+++ b/extensions/common/one_shot_event.h
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+// Copyright 2013 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 EXTENSIONS_COMMON_ONE_SHOT_EVENT_H_
+#define EXTENSIONS_COMMON_ONE_SHOT_EVENT_H_
+
+#include <vector>
+
+#include "base/callback_forward.h"
+#include "base/logging.h"
+#include "base/memory/ref_counted.h"
+#include "base/memory/weak_ptr.h"
+#include "base/threading/thread_checker.h"
+
+namespace base {
+class TaskRunner;
+}
+
+namespace tracked_objects {
+class Location;
+}
+
+namespace extensions {
+
+// This class represents an event that's expected to happen once.  It
+// allows clients to guarantee that code is run after the OneShotEvent
+// is signaled.  If the OneShotEvent is destroyed before it's
+// signaled, the delayed closures are destroyed without being run.
+//
+// This class is similar to a WaitableEvent combined with several
+// WaitableEventWatchers, but using it is simpler.
+//
+// This class is not thread-safe, and must be used from a single thread.
+class OneShotEvent {
+ public:
+  OneShotEvent();
+  ~OneShotEvent();
+
+  // True if Signal has been called.  This function is mostly for
+  // migrating old code; usually calling Post() unconditionally will
+  // result in more readable code.
+  bool is_signaled() const {
+    DCHECK(thread_checker_.CalledOnValidThread());
+    return signaled_;
+  }
+
+  // Causes is_signaled() to return true and all queued tasks to be
+  // run in an arbitrary order.  This method must only be called once.
+  void Signal();
+
+  // Scheduled |task| to be called on |runner| after is_signaled()
+  // becomes true.  Inside |task|, if this OneShotEvent is still
+  // alive, CHECK(is_signaled()) will never fail (which implies that
+  // OneShotEvent::Reset() doesn't exist).
+  //
+  // If |*this| is destroyed before being released, none of these
+  // tasks will be executed.
+  //
+  // Omitting the |runner| argument indicates that |task| should run
+  // on MessageLoopProxy::current().
+  //
+  // Tasks may be run in an arbitrary order, not just FIFO.  Tasks
+  // will never be called on the current thread before this function
+  // returns.  Beware that there's no simple way to wait for all tasks
+  // on a OneShotEvent to complete, so it's almost never safe to use
+  // base::Unretained() when creating one.
+  //
+  // Const because Post() doesn't modify the logical state of this
+  // object (which is just the is_signaled() bit).
+  void Post(const tracked_objects::Location& from_here,
+            const base::Closure& task) const;
+  void Post(const tracked_objects::Location& from_here,
+            const base::Closure& task,
+            const scoped_refptr<base::TaskRunner>& runner) const;
+
+ private:
+  struct TaskInfo;
+
+  base::ThreadChecker thread_checker_;
+
+  bool signaled_;
+
+  // The task list is mutable because it's not part of the logical
+  // state of the object.  This lets us return const references to the
+  // OneShotEvent to clients that just want to run tasks through it
+  // without worrying that they'll signal the event.
+  //
+  // Optimization note: We could reduce the size of this class to a
+  // single pointer by storing |signaled_| in the low bit of a
+  // pointer, and storing the size and capacity of the array (if any)
+  // on the far end of the pointer.
+  mutable std::vector<TaskInfo> tasks_;
+};
+
+}  // namespace extensions
+
+#endif  // EXTENSIONS_COMMON_ONE_SHOT_EVENT_H_