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// Copyright 2013 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef BASE_ONE_SHOT_EVENT_H_
#define BASE_ONE_SHOT_EVENT_H_
#include <vector>
#include "base/base_export.h"
#include "base/check.h"
#include "base/functional/callback_forward.h"
#include "base/memory/weak_ptr.h"
#include "base/task/single_thread_task_runner.h"
#include "base/threading/thread_checker.h"
namespace base {
class Location;
class SingleThreadTaskRunner;
class TimeDelta;
// 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 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 BASE_EXPORT OneShotEvent {
public:
OneShotEvent();
// Use the following constructor to create an already signaled event. This is
// useful if you construct the event on a different thread from where it is
// used, in which case it is not possible to call Signal() just after
// construction.
explicit OneShotEvent(bool signaled);
~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 tasks to be posted to their
// corresponding task runners in the FIFO order. Note that tasks posted to
// different SingleThreadTaskRunners may still execute in arbitrary order.
// This method must only be called once.
void Signal();
// Scheduled |task| to be called on |runner| after is_signaled()
// becomes true. If called with |delay|, then the task will happen
// (roughly) |delay| after is_signaled(), *not* |delay| after the
// post. 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.
//
// Tasks are posted in FIFO order, however, tasks posted to different
// SingleThreadTaskRunners may still execute in an arbitrary order. 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.
void Post(const Location& from_here,
OnceClosure task,
scoped_refptr<SingleThreadTaskRunner> runner =
SingleThreadTaskRunner::GetCurrentDefault()) const;
void PostDelayed(const Location& from_here,
OnceClosure task,
const TimeDelta& delay) const;
private:
struct TaskInfo;
void PostImpl(const Location& from_here,
OnceClosure task,
scoped_refptr<SingleThreadTaskRunner> runner,
const TimeDelta& delay) const;
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 base
#endif // BASE_ONE_SHOT_EVENT_H_