cancelable_task_tracker.h

// Copyright 2014 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.

// CancelableTaskTracker posts tasks (in the form of a Closure) to a
// TaskRunner, and is able to cancel the task later if it's not needed
// anymore.  On destruction, CancelableTaskTracker will cancel all
// tracked tasks.
//
// Each cancelable task can be associated with a reply (also a Closure). After
// the task is run on the TaskRunner, |reply| will be posted back to
// originating TaskRunner.
//
// NOTE:
//
// CancelableCallback (base/cancelable_callback.h) and WeakPtr binding are
// preferred solutions for canceling a task. However, they don't support
// cancelation from another thread. This is sometimes a performance critical
// requirement. E.g. We need to cancel database lookup task on DB thread when
// user changes inputed text. If it is performance critical to do a best effort
// cancelation of a task, then CancelableTaskTracker is appropriate,
// otherwise use one of the other mechanisms.
//
// THREAD-SAFETY:
//
// 1. CancelableTaskTracker objects are not thread safe. They must
// be created, used, and destroyed on the originating thread that posts the
// task. It's safe to destroy a CancelableTaskTracker while there
// are outstanding tasks. This is commonly used to cancel all outstanding
// tasks.
//
// 2. Both task and reply are deleted on the originating thread.
//
// 3. IsCanceledCallback is thread safe and can be run or deleted on any
// thread.
#ifndef BASE_TASK_CANCELABLE_TASK_TRACKER_H_
#define BASE_TASK_CANCELABLE_TASK_TRACKER_H_

#include "base/base_export.h"
#include "base/basictypes.h"
#include "base/callback.h"
#include "base/containers/hash_tables.h"
#include "base/memory/weak_ptr.h"
#include "base/task_runner_util.h"
#include "base/threading/thread_checker.h"

namespace tracked_objects {
class Location;
}  // namespace tracked_objects

namespace base {

class CancellationFlag;
class TaskRunner;

class BASE_EXPORT CancelableTaskTracker {
 public:
  // All values except kBadTaskId are valid.
  typedef int64 TaskId;
  static const TaskId kBadTaskId;

  typedef base::Callback<bool()> IsCanceledCallback;

  CancelableTaskTracker();

  // Cancels all tracked tasks.
  ~CancelableTaskTracker();

  TaskId PostTask(base::TaskRunner* task_runner,
                  const tracked_objects::Location& from_here,
                  const base::Closure& task);

  TaskId PostTaskAndReply(base::TaskRunner* task_runner,
                          const tracked_objects::Location& from_here,
                          const base::Closure& task,
                          const base::Closure& reply);

  template <typename TaskReturnType, typename ReplyArgType>
  TaskId PostTaskAndReplyWithResult(
      base::TaskRunner* task_runner,
      const tracked_objects::Location& from_here,
      const base::Callback<TaskReturnType(void)>& task,
      const base::Callback<void(ReplyArgType)>& reply) {
    TaskReturnType* result = new TaskReturnType();
    return PostTaskAndReply(
        task_runner,
        from_here,
        base::Bind(&base::internal::ReturnAsParamAdapter<TaskReturnType>,
                   task,
                   base::Unretained(result)),
        base::Bind(&base::internal::ReplyAdapter<TaskReturnType, ReplyArgType>,
                   reply,
                   base::Owned(result)));
  }

  // Creates a tracked TaskId and an associated IsCanceledCallback. Client can
  // later call TryCancel() with the returned TaskId, and run |is_canceled_cb|
  // from any thread to check whether the TaskId is canceled.
  //
  // The returned task ID is tracked until the last copy of
  // |is_canceled_cb| is destroyed.
  //
  // Note. This function is used to address some special cancelation requirement
  // in existing code. You SHOULD NOT need this function in new code.
  TaskId NewTrackedTaskId(IsCanceledCallback* is_canceled_cb);

  // After calling this function, |task| and |reply| will not run. If the
  // cancelation happens when |task| is running or has finished running, |reply|
  // will not run. If |reply| is running or has finished running, cancellation
  // is a noop.
  //
  // Note. It's OK to cancel a |task| for more than once. The later calls are
  // noops.
  void TryCancel(TaskId id);

  // It's OK to call this function for more than once. The later calls are
  // noops.
  void TryCancelAll();

  // Returns true iff there are in-flight tasks that are still being
  // tracked.
  bool HasTrackedTasks() const;

 private:
  void Track(TaskId id, base::CancellationFlag* flag);
  void Untrack(TaskId id);

  base::hash_map<TaskId, base::CancellationFlag*> task_flags_;
  base::WeakPtrFactory<CancelableTaskTracker> weak_factory_;

  TaskId next_id_;
  base::ThreadChecker thread_checker_;

  DISALLOW_COPY_AND_ASSIGN(CancelableTaskTracker);
};

}  // namespace base

#endif  // BASE_TASK_CANCELABLE_TASK_TRACKER_H_