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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.
// 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 CHROME_COMMON_CANCELABLE_TASK_TRACKER_H_
#define CHROME_COMMON_CANCELABLE_TASK_TRACKER_H_
#include "base/basictypes.h"
#include "base/callback.h"
#include "base/containers/hash_tables.h"
#include "base/memory/weak_ptr.h"
#include "base/threading/thread_checker.h"
namespace base {
class CancellationFlag;
class TaskRunner;
} // namespace base
namespace tracked_objects {
class Location;
} // namespace tracked_objects
class 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);
// 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);
};
#endif // CHROME_COMMON_CANCELABLE_TASK_TRACKER_H_
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