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* Copyright 2016 The WebRTC Project Authors. All rights reserved.
* Use of this source code is governed by a BSD-style license
* that can be found in the LICENSE file in the root of the source
* tree. An additional intellectual property rights grant can be found
* in the file PATENTS. All contributing project authors may
* be found in the AUTHORS file in the root of the source tree.
#include <memory>
#include <type_traits>
#include <utility>
#include "absl/memory/memory.h"
#include "rtc_base/constructormagic.h"
#include "rtc_base/scoped_ref_ptr.h"
#include "rtc_base/system/rtc_export.h"
#include "rtc_base/thread_annotations.h"
namespace rtc {
// Base interface for asynchronously executed tasks.
// The interface basically consists of a single function, Run(), that executes
// on the target queue. For more details see the Run() method and TaskQueue.
class QueuedTask {
QueuedTask() {}
virtual ~QueuedTask() {}
// Main routine that will run when the task is executed on the desired queue.
// The task should return |true| to indicate that it should be deleted or
// |false| to indicate that the queue should consider ownership of the task
// having been transferred. Returning |false| can be useful if a task has
// re-posted itself to a different queue or is otherwise being re-used.
virtual bool Run() = 0;
// Simple implementation of QueuedTask for use with rtc::Bind and lambdas.
template <class Closure>
class ClosureTask : public QueuedTask {
explicit ClosureTask(Closure&& closure)
: closure_(std::forward<Closure>(closure)) {}
bool Run() override {
return true;
typename std::remove_const<
typename std::remove_reference<Closure>::type>::type closure_;
// Extends ClosureTask to also allow specifying cleanup code.
// This is useful when using lambdas if guaranteeing cleanup, even if a task
// was dropped (queue is too full), is required.
template <class Closure, class Cleanup>
class ClosureTaskWithCleanup : public ClosureTask<Closure> {
ClosureTaskWithCleanup(Closure&& closure, Cleanup&& cleanup)
: ClosureTask<Closure>(std::forward<Closure>(closure)),
cleanup_(std::forward<Cleanup>(cleanup)) {}
~ClosureTaskWithCleanup() { cleanup_(); }
typename std::remove_const<
typename std::remove_reference<Cleanup>::type>::type cleanup_;
// Convenience function to construct closures that can be passed directly
// to methods that support std::unique_ptr<QueuedTask> but not template
// based parameters.
template <class Closure>
static std::unique_ptr<QueuedTask> NewClosure(Closure&& closure) {
return absl::make_unique<ClosureTask<Closure>>(
template <class Closure, class Cleanup>
static std::unique_ptr<QueuedTask> NewClosure(Closure&& closure,
Cleanup&& cleanup) {
return absl::make_unique<ClosureTaskWithCleanup<Closure, Cleanup>>(
std::forward<Closure>(closure), std::forward<Cleanup>(cleanup));
// Implements a task queue that asynchronously executes tasks in a way that
// guarantees that they're executed in FIFO order and that tasks never overlap.
// Tasks may always execute on the same worker thread and they may not.
// To DCHECK that tasks are executing on a known task queue, use IsCurrent().
// Here are some usage examples:
// 1) Asynchronously running a lambda:
// class MyClass {
// ...
// TaskQueue queue_("MyQueue");
// };
// void MyClass::StartWork() {
// queue_.PostTask([]() { Work(); });
// ...
// 2) Doing work asynchronously on a worker queue and providing a notification
// callback on the current queue, when the work has been done:
// void MyClass::StartWorkAndLetMeKnowWhenDone(
// std::unique_ptr<QueuedTask> callback) {
// DCHECK(TaskQueue::Current()) << "Need to be running on a queue";
// queue_.PostTaskAndReply([]() { Work(); }, std::move(callback));
// }
// ...
// my_class->StartWorkAndLetMeKnowWhenDone(
// NewClosure([]() { RTC_LOG(INFO) << "The work is done!";}));
// 3) Posting a custom task on a timer. The task posts itself again after
// every running:
// class TimerTask : public QueuedTask {
// public:
// TimerTask() {}
// private:
// bool Run() override {
// ++count_;
// TaskQueue::Current()->PostDelayedTask(
// std::unique_ptr<QueuedTask>(this), 1000);
// // Ownership has been transferred to the next occurance,
// // so return false to prevent from being deleted now.
// return false;
// }
// int count_ = 0;
// };
// ...
// queue_.PostDelayedTask(
// std::unique_ptr<QueuedTask>(new TimerTask()), 1000);
// For more examples, see
// A note on destruction:
// When a TaskQueue is deleted, pending tasks will not be executed but they will
// be deleted. The deletion of tasks may happen asynchronously after the
// TaskQueue itself has been deleted or it may happen synchronously while the
// TaskQueue instance is being deleted. This may vary from one OS to the next
// so assumptions about lifetimes of pending tasks should not be made.
// TaskQueue priority levels. On some platforms these will map to thread
// priorities, on others such as Mac and iOS, GCD queue priorities.
enum class Priority {
explicit TaskQueue(const char* queue_name,
Priority priority = Priority::NORMAL);
static TaskQueue* Current();
// Used for DCHECKing the current queue.
bool IsCurrent() const;
// TODO(tommi): For better debuggability, implement RTC_FROM_HERE.
// Ownership of the task is passed to PostTask.
void PostTask(std::unique_ptr<QueuedTask> task);
void PostTaskAndReply(std::unique_ptr<QueuedTask> task,
std::unique_ptr<QueuedTask> reply,
TaskQueue* reply_queue);
void PostTaskAndReply(std::unique_ptr<QueuedTask> task,
std::unique_ptr<QueuedTask> reply);
// Schedules a task to execute a specified number of milliseconds from when
// the call is made. The precision should be considered as "best effort"
// and in some cases, such as on Windows when all high precision timers have
// been used up, can be off by as much as 15 millseconds (although 8 would be
// more likely). This can be mitigated by limiting the use of delayed tasks.
void PostDelayedTask(std::unique_ptr<QueuedTask> task, uint32_t milliseconds);
// std::enable_if is used here to make sure that calls to PostTask() with
// std::unique_ptr<SomeClassDerivedFromQueuedTask> would not end up being
// caught by this template.
template <class Closure,
typename std::enable_if<!std::is_convertible<
std::unique_ptr<QueuedTask>>::value>::type* = nullptr>
void PostTask(Closure&& closure) {
// See documentation above for performance expectations.
template <class Closure,
typename std::enable_if<!std::is_convertible<
std::unique_ptr<QueuedTask>>::value>::type* = nullptr>
void PostDelayedTask(Closure&& closure, uint32_t milliseconds) {
PostDelayedTask(NewClosure(std::forward<Closure>(closure)), milliseconds);
template <class Closure1, class Closure2>
void PostTaskAndReply(Closure1&& task,
Closure2&& reply,
TaskQueue* reply_queue) {
NewClosure(std::forward<Closure2>(reply)), reply_queue);
template <class Closure>
void PostTaskAndReply(std::unique_ptr<QueuedTask> task, Closure&& reply) {
PostTaskAndReply(std::move(task), NewClosure(std::forward<Closure>(reply)));
template <class Closure>
void PostTaskAndReply(Closure&& task, std::unique_ptr<QueuedTask> reply) {
PostTaskAndReply(NewClosure(std::forward<Closure>(task)), std::move(reply));
template <class Closure1, class Closure2>
void PostTaskAndReply(Closure1&& task, Closure2&& reply) {
class Impl;
const scoped_refptr<Impl> impl_;
} // namespace rtc