1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
5 #ifndef BASE_SEQUENCED_TASK_RUNNER_H_
6 #define BASE_SEQUENCED_TASK_RUNNER_H_
10 #include "base/base_export.h"
11 #include "base/callback.h"
12 #include "base/sequenced_task_runner_helpers.h"
13 #include "base/task_runner.h"
17 // A SequencedTaskRunner is a subclass of TaskRunner that provides
18 // additional guarantees on the order that tasks are started, as well
19 // as guarantees on when tasks are in sequence, i.e. one task finishes
20 // before the other one starts.
24 // Non-nested tasks with the same delay will run one by one in FIFO
27 // Detailed guarantees
28 // -------------------
30 // SequencedTaskRunner also adds additional methods for posting
31 // non-nestable tasks. In general, an implementation of TaskRunner
32 // may expose task-running methods which are themselves callable from
33 // within tasks. A non-nestable task is one that is guaranteed to not
34 // be run from within an already-running task. Conversely, a nestable
35 // task (the default) is a task that can be run from within an
36 // already-running task.
38 // The guarantees of SequencedTaskRunner are as follows:
40 // - Given two tasks T2 and T1, T2 will start after T1 starts if:
42 // * T2 is posted after T1; and
43 // * T2 has equal or higher delay than T1; and
44 // * T2 is non-nestable or T1 is nestable.
46 // - If T2 will start after T1 starts by the above guarantee, then
47 // T2 will start after T1 finishes and is destroyed if:
49 // * T2 is non-nestable, or
50 // * T1 doesn't call any task-running methods.
52 // - If T2 will start after T1 finishes by the above guarantee, then
53 // all memory changes in T1 and T1's destruction will be visible
56 // - If T2 runs nested within T1 via a call to the task-running
57 // method M, then all memory changes in T1 up to the call to M
58 // will be visible to T2, and all memory changes in T2 will be
59 // visible to T1 from the return from M.
61 // Note that SequencedTaskRunner does not guarantee that tasks are run
62 // on a single dedicated thread, although the above guarantees provide
63 // most (but not all) of the same guarantees. If you do need to
64 // guarantee that tasks are run on a single dedicated thread, see
65 // SingleThreadTaskRunner (in single_thread_task_runner.h).
67 // Some corollaries to the above guarantees, assuming the tasks in
68 // question don't call any task-running methods:
70 // - Tasks posted via PostTask are run in FIFO order.
72 // - Tasks posted via PostNonNestableTask are run in FIFO order.
74 // - Tasks posted with the same delay and the same nestable state
75 // are run in FIFO order.
77 // - A list of tasks with the same nestable state posted in order of
78 // non-decreasing delay is run in FIFO order.
80 // - A list of tasks posted in order of non-decreasing delay with at
81 // most a single change in nestable state from nestable to
82 // non-nestable is run in FIFO order. (This is equivalent to the
83 // statement of the first guarantee above.)
85 // Some theoretical implementations of SequencedTaskRunner:
87 // - A SequencedTaskRunner that wraps a regular TaskRunner but makes
88 // sure that only one task at a time is posted to the TaskRunner,
89 // with appropriate memory barriers in between tasks.
91 // - A SequencedTaskRunner that, for each task, spawns a joinable
92 // thread to run that task and immediately quit, and then
93 // immediately joins that thread.
95 // - A SequencedTaskRunner that stores the list of posted tasks and
96 // has a method Run() that runs each runnable task in FIFO order
97 // that can be called from any thread, but only if another
98 // (non-nested) Run() call isn't already happening.
99 class BASE_EXPORT SequencedTaskRunner : public TaskRunner {
101 // The two PostNonNestable*Task methods below are like their
102 // nestable equivalents in TaskRunner, but they guarantee that the
103 // posted task will not run nested within an already-running task.
105 // A simple corollary is that posting a task as non-nestable can
106 // only delay when the task gets run. That is, posting a task as
107 // non-nestable may not affect when the task gets run, or it could
108 // make it run later than it normally would, but it won't make it
109 // run earlier than it normally would.
111 // TODO(akalin): Get rid of the boolean return value for the methods
114 bool PostNonNestableTask(const Location& from_here, OnceClosure task);
116 virtual bool PostNonNestableDelayedTask(const Location& from_here,
118 base::TimeDelta delay) = 0;
120 // Submits a non-nestable task to delete the given object. Returns
121 // true if the object may be deleted at some point in the future,
122 // and false if the object definitely will not be deleted.
124 bool DeleteSoon(const Location& from_here, const T* object) {
125 return DeleteOrReleaseSoonInternal(from_here, &DeleteHelper<T>::DoDelete,
130 bool DeleteSoon(const Location& from_here, std::unique_ptr<T> object) {
131 return DeleteSoon(from_here, object.release());
134 // Submits a non-nestable task to release the given object.
136 // ReleaseSoon makes sure that the object it the scoped_refptr points to gets
137 // properly released on the correct thread.
138 // We apply ReleaseSoon to the rvalue as the side-effects can be unclear to
139 // the caller if an lvalue is used. That being so, the scoped_refptr should
140 // always be std::move'd.
143 // scoped_refptr<T> foo_scoped_refptr;
145 // task_runner->ReleaseSoon(std::move(foo_scoped_refptr));
147 void ReleaseSoon(const Location& from_here, scoped_refptr<T>&& object) {
151 DeleteOrReleaseSoonInternal(from_here, &ReleaseHelper<T>::DoRelease,
155 // Returns true iff tasks posted to this TaskRunner are sequenced
159 // - Returns true if this is a SequencedTaskRunner to which the
160 // current task was posted.
161 // - Returns true if this is a SequencedTaskRunner bound to the
162 // same sequence as the SequencedTaskRunner to which the current
164 // - Returns true if this is a SingleThreadTaskRunner bound to
165 // the current thread.
166 virtual bool RunsTasksInCurrentSequence() const = 0;
169 ~SequencedTaskRunner() override = default;
172 bool DeleteOrReleaseSoonInternal(const Location& from_here,
173 void (*deleter)(const void*),
177 // Sample usage with std::unique_ptr :
178 // std::unique_ptr<Foo, base::OnTaskRunnerDeleter> ptr(
179 // new Foo, base::OnTaskRunnerDeleter(my_task_runner));
181 // For RefCounted see base::RefCountedDeleteOnSequence.
182 struct BASE_EXPORT OnTaskRunnerDeleter {
183 explicit OnTaskRunnerDeleter(scoped_refptr<SequencedTaskRunner> task_runner);
184 ~OnTaskRunnerDeleter();
186 OnTaskRunnerDeleter(OnTaskRunnerDeleter&&);
187 OnTaskRunnerDeleter& operator=(OnTaskRunnerDeleter&&);
189 // For compatibility with std:: deleters.
190 template <typename T>
191 void operator()(const T* ptr) {
193 task_runner_->DeleteSoon(FROM_HERE, ptr);
196 scoped_refptr<SequencedTaskRunner> task_runner_;
201 #endif // BASE_SEQUENCED_TASK_RUNNER_H_