sequenced_task_runner.h | Explore in Territory

// Copyright 2012 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_TASK_SEQUENCED_TASK_RUNNER_H_
#define BASE_TASK_SEQUENCED_TASK_RUNNER_H_

#include <memory>

#include "base/auto_reset.h"
#include "base/base_export.h"
#include "base/functional/callback.h"
#include "base/gtest_prod_util.h"
#include "base/memory/raw_ptr_exclusion.h"
#include "base/task/delay_policy.h"
#include "base/task/delayed_task_handle.h"
#include "base/task/sequenced_task_runner_helpers.h"
#include "base/task/task_runner.h"
#include "base/types/pass_key.h"

namespace blink {
class LowPrecisionTimer;
class ScriptedIdleTaskController;
class TimerBase;
class TimerBasedTickProvider;
class WebRtcTaskQueue;
}
namespace IPC {
class ChannelAssociatedGroupController;
}  // namespace IPC
namespace media {
class AlsaPcmOutputStream;
class AlsaPcmInputStream;
class FakeAudioWorker;
}  // namespace media
namespace viz {
class ExternalBeginFrameSourceWin;
}  // namespace viz
namespace webrtc {
class ThreadWrapper;
}  // namespace webrtc

namespace base {

namespace android {
class PreFreezeBackgroundMemoryTrimmer;
}
namespace internal {
class DelayTimerBase;
class DelayedTaskManager;
}
class DeadlineTimer;
class MetronomeTimer;
class SingleThreadTaskRunner;
class TimeDelta;
class TimeTicks;

namespace subtle {

// Restricts access to PostCancelableDelayedTask*() to authorized callers.
class PostDelayedTaskPassKey { … };

// Restricts access to RunOrPostTask() to authorized callers.
class RunOrPostTaskPassKey { … };

class PostDelayedTaskPassKeyForTesting : public PostDelayedTaskPassKey { … };
class RunOrPostTaskPassKeyForTesting : public RunOrPostTaskPassKey { … };

}  // namespace subtle

// A SequencedTaskRunner is a subclass of TaskRunner that provides
// additional guarantees on the order that tasks are started, as well
// as guarantees on when tasks are in sequence, i.e. one task finishes
// before the other one starts.
//
// Summary
// -------
// Non-nested tasks with the same delay will run one by one in FIFO
// order.
//
// Detailed guarantees
// -------------------
//
// SequencedTaskRunner also adds additional methods for posting
// non-nestable tasks.  In general, an implementation of TaskRunner
// may expose task-running methods which are themselves callable from
// within tasks.  A non-nestable task is one that is guaranteed to not
// be run from within an already-running task.  Conversely, a nestable
// task (the default) is a task that can be run from within an
// already-running task.
//
// The guarantees of SequencedTaskRunner are as follows:
//
//   - Given two tasks T2 and T1, T2 will start after T1 starts if:
//
//       * T2 is posted after T1; and
//       * T2 has equal or higher delay than T1; and
//       * T2 is non-nestable or T1 is nestable.
//
//   - If T2 will start after T1 starts by the above guarantee, then
//     T2 will start after T1 finishes and is destroyed if:
//
//       * T2 is non-nestable, or
//       * T1 doesn't call any task-running methods.
//
//   - If T2 will start after T1 finishes by the above guarantee, then
//     all memory changes in T1 and T1's destruction will be visible
//     to T2.
//
//   - If T2 runs nested within T1 via a call to the task-running
//     method M, then all memory changes in T1 up to the call to M
//     will be visible to T2, and all memory changes in T2 will be
//     visible to T1 from the return from M.
//
// Note that SequencedTaskRunner does not guarantee that tasks are run
// on a single dedicated thread, although the above guarantees provide
// most (but not all) of the same guarantees.  If you do need to
// guarantee that tasks are run on a single dedicated thread, see
// SingleThreadTaskRunner (in single_thread_task_runner.h).
//
// Some corollaries to the above guarantees, assuming the tasks in
// question don't call any task-running methods:
//
//   - Tasks posted via PostTask are run in FIFO order.
//
//   - Tasks posted via PostNonNestableTask are run in FIFO order.
//
//   - Tasks posted with the same delay and the same nestable state
//     are run in FIFO order.
//
//   - A list of tasks with the same nestable state posted in order of
//     non-decreasing delay is run in FIFO order.
//
//   - A list of tasks posted in order of non-decreasing delay with at
//     most a single change in nestable state from nestable to
//     non-nestable is run in FIFO order. (This is equivalent to the
//     statement of the first guarantee above.)
//
// Some theoretical implementations of SequencedTaskRunner:
//
//   - A SequencedTaskRunner that wraps a regular TaskRunner but makes
//     sure that only one task at a time is posted to the TaskRunner,
//     with appropriate memory barriers in between tasks.
//
//   - A SequencedTaskRunner that, for each task, spawns a joinable
//     thread to run that task and immediately quit, and then
//     immediately joins that thread.
//
//   - A SequencedTaskRunner that stores the list of posted tasks and
//     has a method Run() that runs each runnable task in FIFO order
//     that can be called from any thread, but only if another
//     (non-nested) Run() call isn't already happening.
//
// SequencedTaskRunner::GetCurrentDefault() can be used while running
// a task to retrieve the default SequencedTaskRunner for the current
// sequence.
class BASE_EXPORT SequencedTaskRunner : public TaskRunner { … };

// Sample usage with std::unique_ptr :
// std::unique_ptr<Foo, base::OnTaskRunnerDeleter> ptr(
//     new Foo, base::OnTaskRunnerDeleter(my_task_runner));
//
// For RefCounted see base::RefCountedDeleteOnSequence.
struct BASE_EXPORT OnTaskRunnerDeleter { … };

}  // namespace base

#endif  // BASE_TASK_SEQUENCED_TASK_RUNNER_H_
chromium/base/task/sequenced_task_runner.h
