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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 <stddef.h>
#include "base/base_export.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_.task_runner()->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_
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