Task Scheduler.

base/task_scheduler/post_task.h

+// Copyright 2016 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_TASK_SCHEDULER_POST_TASK_H_
+#define BASE_TASK_SCHEDULER_POST_TASK_H_
+
+#include "base/base_export.h"
+#include "base/callback_forward.h"
+#include "base/memory/ref_counted.h"
+#include "base/task_runner.h"
+#include "base/task_scheduler/task_traits.h"
+
+namespace tracked_objects {
+class Location;
+}
+
+namespace base {
+// The functions below forward posted tasks to the task scheduler.
+// A TaskScheduler must have been initialized via
+// TaskScheduler::Initialize() before these are valid.
+//
+// To post a simple one-off task:
+//     PostTask(FROM_HERE, Bind(...));
+//
+// To post a high priority one-off task to respond to a user interaction:
+//     PostTaskWithTraits(
+//         FROM_HERE,
+//         TaskTraits().WithPriority(TaskPriority::USER_BLOCKING),
+//         Bind(...));
+//
+// To post tasks that must run in sequence:
+//     scoped_refptr<TaskRunner> task_runner = CreateTaskRunnerWithTraits(
+//         TaskTraits(), ExecutionMode::SEQUENCED);
+//     task_runner.PostTask(FROM_HERE, Bind(...));
+//     task_runner.PostTask(FROM_HERE, Bind(...));
+//
+// To post file I/O tasks that must run in sequence and can be skipped on
+// shutdown:
+//     scoped_refptr<TaskRunner> task_runner =
+//         CreateTaskRunnerWithTraits(
+//             TaskTraits().WithFileIO().WithShutdownBehavior(
+//                 TaskShutdownBehavior::SKIP_ON_SHUTDOWN),
+//             ExecutionMode::SEQUENCED);
+//     task_runner.PostTask(FROM_HERE, Bind(...));
+//     task_runner.PostTask(FROM_HERE, Bind(...));
+//
+//
+// The default TaskTraits apply to tasks that:
+//     (1) don't need to do I/O,
+//     (2) don't affect user interaction and/or visible elements, and
+//     (3) can either block shutdown or be skipped on shutdown
+//         (barring current TaskScheduler default).
+// If those loose requirements are sufficient for your task, use
+// PostTask[AndReply], otherwise override these with explicit traits via
+// PostTaskWithTraits[AndReply].
+
+// Posts |task| to the TaskScheduler. Calling this is equivalent to
+// calling PostTaskWithTraits with plain TaskTraits.
+BASE_EXPORT void PostTask(const tracked_objects::Location& from_here,
+                          const Closure& task);
+
+// Posts |task| to the TaskScheduler and posts |reply| on the
+// caller's execution context (i.e. same Sequence or MessageLoop)
+// when |task| completes. Calling this is equivalent to calling
+// PostTaskWithTraitsAndReply with plain TaskTraits.
+BASE_EXPORT void PostTaskAndReply(const tracked_objects::Location& from_here,
+                                  const Closure& task,
+                                  const Closure& reply);
+
+// Posts |task| with specific |traits| to the TaskScheduler.
+BASE_EXPORT void PostTaskWithTraits(const tracked_objects::Location& from_here,
+                                    TaskTraits traits,
+                                    const Closure& task);
+
+// Posts |task| with specific |traits| to the TaskScheduler and posts
+// |reply| on the caller's execution context (i.e. same Sequence or
+// MessageLoop).
+BASE_EXPORT void PostTaskWithTraitsAndReply(
+    const tracked_objects::Location& from_here,
+    TaskTraits traits,
+    const Closure& task,
+    const Closure& reply);
+
+// Returns a TaskRunner whose PostTask invocations will result in scheduling
+// tasks using |traits| which will be executed according to |execution_mode|.
+BASE_EXPORT scoped_refptr<TaskRunner> CreateTaskRunnerWithTraits(
+    TaskTraits traits,
+    ExecutionMode execution_mode);
+
+// Returns a TaskRunner akin to one created through CreateTaskRunnerWithTraits()
+// except that it will inherit |parent_task_runner|'s sequence instead of
+// creating its own. Useful for posting tasks with different traits to the same
+// sequence. |parent_task_runner| must be a SEQUENCED TaskRunner returned from
+// CreateTaskRunnerWithTraits are supported.

[fdoray, 2016/02/11 17:30:33]

remove "are supported"

[fdoray, 2016/02/12 04:16:19]

Done.

+BASE_EXPORT scoped_refptr<TaskRunner> CreateChildTaskRunnerWithTraits(
+    TaskRunner* parent_task_runner,
+    TaskTraits traits);

[fdoray, 2016/02/11 17:30:33]

How will we know that |parent_task_runner| is SEQUENCED?

[robliao, 2016/02/11 22:49:30]

That's a good question. We currently don't have a good way to introspect into
this pointer and enforce that the TaskRunners are actually our task runners.
We could have either all task runners we create extend from a TaskRunnerType
class that provides one virtual method to return the TaskRunnerType.

Alternatively, we could create an unordered_map of all TaskRunners we've handed
out. We would have to use WeakPtrs for this and it would probably be pretty
ugly.

[fdoray, 2016/02/12 04:16:19]

Solution 1 seems good but... ref-counted classes can't use multiple inheritance
and our TaskRunners already inherit from (Sequenced|SingleThread)TaskRunner.
Maybe we could have all our TaskRunners inherit from SchedulerTaskRunner which
would itself inherit from TaskRunner (no more
SequencedTaskRunner/SingleThreadTaskRunner inheritance). We would need special
TaskRunners for (Thread|Sequenced)TaskRunnerHandle.

Solution 2 is indeed ugly.

+
+}  // namespace base
+
+#endif  // BASE_TASK_SCHEDULER_POST_TASK_H_