| // Copyright (c) 2012 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 BASE_SEQUENCED_TASKRUNNER_H_ |
| #define BASE_SEQUENCED_TASKRUNNER_H_ |
| |
| #include "base/base_export.h" |
| #include "base/sequenced_task_runner_helpers.h" |
| #include "base/task_runner.h" |
| |
| namespace base { |
| |
| // 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. |
| class BASE_EXPORT SequencedTaskRunner : public TaskRunner { |
| public: |
| // The two PostNonNestable*Task methods below are like their |
| // nestable equivalents in TaskRunner, but they guarantee that the |
| // posted task will not run nested within an already-running task. |
| // |
| // A simple corollary is that posting a task as non-nestable can |
| // only delay when the task gets run. That is, posting a task as |
| // non-nestable may not affect when the task gets run, or it could |
| // make it run later than it normally would, but it won't make it |
| // run earlier than it normally would. |
| |
| // TODO(akalin): Get rid of the boolean return value for the methods |
| // below. |
| |
| bool PostNonNestableTask(const tracked_objects::Location& from_here, |
| const Closure& task); |
| |
| virtual bool PostNonNestableDelayedTask( |
| const tracked_objects::Location& from_here, |
| const Closure& task, |
| base::TimeDelta delay) = 0; |
| |
| // Submits a non-nestable task to delete the given object. Returns |
| // true if the object may be deleted at some point in the future, |
| // and false if the object definitely will not be deleted. |
| template <class T> |
| bool DeleteSoon(const tracked_objects::Location& from_here, |
| const T* object) { |
| return |
| subtle::DeleteHelperInternal<T, bool>::DeleteViaSequencedTaskRunner( |
| this, from_here, object); |
| } |
| |
| // Submits a non-nestable task to release the given object. Returns |
| // true if the object may be released at some point in the future, |
| // and false if the object definitely will not be released. |
| template <class T> |
| bool ReleaseSoon(const tracked_objects::Location& from_here, |
| T* object) { |
| return |
| subtle::ReleaseHelperInternal<T, bool>::ReleaseViaSequencedTaskRunner( |
| this, from_here, object); |
| } |
| |
| protected: |
| ~SequencedTaskRunner() override {} |
| |
| private: |
| template <class T, class R> friend class subtle::DeleteHelperInternal; |
| template <class T, class R> friend class subtle::ReleaseHelperInternal; |
| |
| bool DeleteSoonInternal(const tracked_objects::Location& from_here, |
| void(*deleter)(const void*), |
| const void* object); |
| |
| bool ReleaseSoonInternal(const tracked_objects::Location& from_here, |
| void(*releaser)(const void*), |
| const void* object); |
| }; |
| |
| } // namespace base |
| |
| #endif // BASE_SEQUENCED_TASKRUNNER_H_ |