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+// 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_TASK_RUNNER_H_
+#define BASE_TASK_RUNNER_H_
+
+#include "base/base_export.h"
+#include "base/basictypes.h"
+#include "base/callback_forward.h"
+#include "base/memory/ref_counted.h"
+#include "base/time/time.h"
+
+namespace tracked_objects {
+class Location;
+} // namespace tracked_objects
+
+namespace base {
+
+struct TaskRunnerTraits;
+
+// A TaskRunner is an object that runs posted tasks (in the form of
+// Closure objects). The TaskRunner interface provides a way of
+// decoupling task posting from the mechanics of how each task will be
+// run. TaskRunner provides very weak guarantees as to how posted
+// tasks are run (or if they're run at all). In particular, it only
+// guarantees:
+//
+// - Posting a task will not run it synchronously. That is, no
+// Post*Task method will call task.Run() directly.
+//
+// - Increasing the delay can only delay when the task gets run.
+// That is, increasing the delay 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.
+//
+// TaskRunner does not guarantee the order in which posted tasks are
+// run, whether tasks overlap, or whether they're run on a particular
+// thread. Also it does not guarantee a memory model for shared data
+// between tasks. (In other words, you should use your own
+// synchronization/locking primitives if you need to share data
+// between tasks.)
+//
+// Implementations of TaskRunner should be thread-safe in that all
+// methods must be safe to call on any thread. Ownership semantics
+// for TaskRunners are in general not clear, which is why the
+// interface itself is RefCountedThreadSafe.
+//
+// Some theoretical implementations of TaskRunner:
+//
+// - A TaskRunner that uses a thread pool to run posted tasks.
+//
+// - A TaskRunner that, for each task, spawns a non-joinable thread
+// to run that task and immediately quit.
+//
+// - A TaskRunner that stores the list of posted tasks and has a
+// method Run() that runs each runnable task in random order.
+class BASE_EXPORT TaskRunner
+ : public RefCountedThreadSafe<TaskRunner, TaskRunnerTraits> {
+ public:
+ // Posts the given task to be run. Returns true if the task may be
+ // run at some point in the future, and false if the task definitely
+ // will not be run.
+ //
+ // Equivalent to PostDelayedTask(from_here, task, 0).
+ bool PostTask(const tracked_objects::Location& from_here,
+ const Closure& task);
+
+ // Like PostTask, but tries to run the posted task only after
+ // |delay_ms| has passed.
+ //
+ // It is valid for an implementation to ignore |delay_ms|; that is,
+ // to have PostDelayedTask behave the same as PostTask.
+ virtual bool PostDelayedTask(const tracked_objects::Location& from_here,
+ const Closure& task,
+ base::TimeDelta delay) = 0;
+
+ // Returns true if the current thread is a thread on which a task
+ // may be run, and false if no task will be run on the current
+ // thread.
+ //
+ // It is valid for an implementation to always return true, or in
+ // general to use 'true' as a default value.
+ virtual bool RunsTasksOnCurrentThread() const = 0;
+
+ // Posts |task| on the current TaskRunner. On completion, |reply|
+ // is posted to the thread that called PostTaskAndReply(). Both
+ // |task| and |reply| are guaranteed to be deleted on the thread
+ // from which PostTaskAndReply() is invoked. This allows objects
+ // that must be deleted on the originating thread to be bound into
+ // the |task| and |reply| Closures. In particular, it can be useful
+ // to use WeakPtr<> in the |reply| Closure so that the reply
+ // operation can be canceled. See the following pseudo-code:
+ //
+ // class DataBuffer : public RefCountedThreadSafe<DataBuffer> {
+ // public:
+ // // Called to add data into a buffer.
+ // void AddData(void* buf, size_t length);
+ // ...
+ // };
+ //
+ //
+ // class DataLoader : public SupportsWeakPtr<DataLoader> {
+ // public:
+ // void GetData() {
+ // scoped_refptr<DataBuffer> buffer = new DataBuffer();
+ // target_thread_.message_loop_proxy()->PostTaskAndReply(
+ // FROM_HERE,
+ // base::Bind(&DataBuffer::AddData, buffer),
+ // base::Bind(&DataLoader::OnDataReceived, AsWeakPtr(), buffer));
+ // }
+ //
+ // private:
+ // void OnDataReceived(scoped_refptr<DataBuffer> buffer) {
+ // // Do something with buffer.
+ // }
+ // };
+ //
+ //
+ // Things to notice:
+ // * Results of |task| are shared with |reply| by binding a shared argument
+ // (a DataBuffer instance).
+ // * The DataLoader object has no special thread safety.
+ // * The DataLoader object can be deleted while |task| is still running,
+ // and the reply will cancel itself safely because it is bound to a
+ // WeakPtr<>.
+ bool PostTaskAndReply(const tracked_objects::Location& from_here,
+ const Closure& task,
+ const Closure& reply);
+
+ protected:
+ friend struct TaskRunnerTraits;
+
+ // Only the Windows debug build seems to need this: see
+ // http://crbug.com/112250.
+ friend class RefCountedThreadSafe<TaskRunner, TaskRunnerTraits>;
+
+ TaskRunner();
+ virtual ~TaskRunner();
+
+ // Called when this object should be destroyed. By default simply
+ // deletes |this|, but can be overridden to do something else, like
+ // delete on a certain thread.
+ virtual void OnDestruct() const;
+};
+
+struct BASE_EXPORT TaskRunnerTraits {
+ static void Destruct(const TaskRunner* task_runner);
+};
+
+} // namespace base
+
+#endif // BASE_TASK_RUNNER_H_
