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_TASKRUNNER_H_
6 #define BASE_SEQUENCED_TASKRUNNER_H_
8 #include "base/base_export.h"
9 #include "base/sequenced_task_runner_helpers.h"
10 #include "base/task_runner.h"
14 // A SequencedTaskRunner is a subclass of TaskRunner that provides
15 // additional guarantees on the order that tasks are started, as well
16 // as guarantees on when tasks are in sequence, i.e. one task finishes
17 // before the other one starts.
21 // Barring delayed/non-nestable tasks, tasks posted will run one by
24 // Detailed guarantees
25 // -------------------
27 // SequencedTaskRunner also adds additional methods for posting
28 // non-nestable tasks. In general, an implementation of TaskRunner
29 // may expose task-running methods which are themselves callable from
30 // within tasks. A non-nestable task is one that is guaranteed to not
31 // be run from within an already-running task. Conversely, a nestable
32 // task (the default) is a task that can be run from within an
33 // already-running task.
35 // The guarantees of SequencedTaskRunner are as follows:
37 // - Given two tasks T2 and T1, T2 will start after T1 starts if:
39 // * T2 is posted after T1;
40 // * T2 has equal or higher delay than T1; and
41 // * T2 is non-nestable or T1 is nestable.
43 // - If T2 will start after T1 starts by the above guarantee, then
44 // T2 will start after T1 finishes if:
46 // * T2 is non-nestable, or
47 // * T1 doesn't call any task-running methods.
49 // - If T2 will start after T1 finishes by the above guarantee, then
50 // all memory changes in T1 will be visible to T2.
52 // - If T2 runs nested within T1 via a call to the task-running
53 // method M, then all memory changes in T1 up to the call to M
54 // will be visible to T2, and all memory changes in T2 will be
55 // visible to T1 from the return from M.
57 // Note that SequencedTaskRunner does not guarantee that tasks are run
58 // on a single dedicated thread, although the above guarantees provide
59 // most (but not all) of the same guarantees. If you do need to
60 // guarantee that tasks are run on a single dedicated thread, see
61 // SingleThreadTaskRunner (in single_thread_task_runner.h).
63 // Some corollaries to the above guarantees, assuming the tasks in
64 // question don't call any task-running methods:
66 // - Tasks posted via PostTask are run in FIFO order.
68 // - Tasks posted via PostNonNestableTask are run in FIFO order.
70 // - Tasks posted with the same delay and the same nestable state
71 // are run in FIFO order.
73 // - A list of tasks with the same nestable state posted in order of
74 // non-decreasing delay is run in FIFO order.
76 // - A list of tasks posted in order of non-decreasing delay with at
77 // most a single change in nestable state from nestable to
78 // non-nestable is run in FIFO order. (This is equivalent to the
79 // statement of the first guarantee above.)
81 // Some theoretical implementations of SequencedTaskRunner:
83 // - A SequencedTaskRunner that wraps a regular TaskRunner but makes
84 // sure that only one task at a time is posted to the TaskRunner,
85 // with appropriate memory barriers in between tasks.
87 // - A SequencedTaskRunner that, for each task, spawns a joinable
88 // thread to run that task and immediately quit, and then
89 // immediately joins that thread.
91 // - A SequencedTaskRunner that stores the list of posted tasks and
92 // has a method Run() that runs each runnable task in FIFO order
93 // that can be called from any thread, but only if another
94 // (non-nested) Run() call isn't already happening.
95 class BASE_EXPORT SequencedTaskRunner
: public TaskRunner
{
97 // The two PostNonNestable*Task methods below are like their
98 // nestable equivalents in TaskRunner, but they guarantee that the
99 // posted task will not run nested within an already-running task.
101 // A simple corollary is that posting a task as non-nestable can
102 // only delay when the task gets run. That is, posting a task as
103 // non-nestable may not affect when the task gets run, or it could
104 // make it run later than it normally would, but it won't make it
105 // run earlier than it normally would.
107 // TODO(akalin): Get rid of the boolean return value for the methods
110 bool PostNonNestableTask(const tracked_objects::Location
& from_here
,
111 const Closure
& task
);
113 virtual bool PostNonNestableDelayedTask(
114 const tracked_objects::Location
& from_here
,
116 base::TimeDelta delay
) = 0;
118 // Submits a non-nestable task to delete the given object. Returns
119 // true if the object may be deleted at some point in the future,
120 // and false if the object definitely will not be deleted.
122 bool DeleteSoon(const tracked_objects::Location
& from_here
,
125 subtle::DeleteHelperInternal
<T
, bool>::DeleteViaSequencedTaskRunner(
126 this, from_here
, object
);
129 // Submits a non-nestable task to release the given object. Returns
130 // true if the object may be released at some point in the future,
131 // and false if the object definitely will not be released.
133 bool ReleaseSoon(const tracked_objects::Location
& from_here
,
136 subtle::ReleaseHelperInternal
<T
, bool>::ReleaseViaSequencedTaskRunner(
137 this, from_here
, object
);
141 virtual ~SequencedTaskRunner() {}
144 template <class T
, class R
> friend class subtle::DeleteHelperInternal
;
145 template <class T
, class R
> friend class subtle::ReleaseHelperInternal
;
147 bool DeleteSoonInternal(const tracked_objects::Location
& from_here
,
148 void(*deleter
)(const void*),
151 bool ReleaseSoonInternal(const tracked_objects::Location
& from_here
,
152 void(*releaser
)(const void*),
158 #endif // BASE_SEQUENCED_TASKRUNNER_H_