1 // Copyright 2014 The Chromium Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 5 // CancelableTaskTracker posts tasks (in the form of a Closure) to a 6 // TaskRunner, and is able to cancel the task later if it's not needed 7 // anymore. On destruction, CancelableTaskTracker will cancel all 8 // tracked tasks. 9 // 10 // Each cancelable task can be associated with a reply (also a Closure). After 11 // the task is run on the TaskRunner, |reply| will be posted back to 12 // originating TaskRunner. 13 // 14 // NOTE: 15 // 16 // CancelableCallback (base/cancelable_callback.h) and WeakPtr binding are 17 // preferred solutions for canceling a task. However, they don't support 18 // cancelation from another thread. This is sometimes a performance critical 19 // requirement. E.g. We need to cancel database lookup task on DB thread when 20 // user changes inputed text. If it is performance critical to do a best effort 21 // cancelation of a task, then CancelableTaskTracker is appropriate, 22 // otherwise use one of the other mechanisms. 23 // 24 // THREAD-SAFETY: 25 // 26 // 1. CancelableTaskTracker objects are not thread safe. They must 27 // be created, used, and destroyed on the originating thread that posts the 28 // task. It's safe to destroy a CancelableTaskTracker while there 29 // are outstanding tasks. This is commonly used to cancel all outstanding 30 // tasks. 31 // 32 // 2. Both task and reply are deleted on the originating thread. 33 // 34 // 3. IsCanceledCallback is thread safe and can be run or deleted on any 35 // thread. 36 #ifndef BASE_TASK_CANCELABLE_TASK_TRACKER_H_ 37 #define BASE_TASK_CANCELABLE_TASK_TRACKER_H_ 38 39 #include <stdint.h> 40 41 #include "base/base_export.h" 42 #include "base/callback.h" 43 #include "base/containers/hash_tables.h" 44 #include "base/macros.h" 45 #include "base/memory/weak_ptr.h" 46 #include "base/task_runner_util.h" 47 #include "base/threading/thread_checker.h" 48 49 namespace tracked_objects { 50 class Location; 51 } // namespace tracked_objects 52 53 namespace base { 54 55 class CancellationFlag; 56 class TaskRunner; 57 58 class BASE_EXPORT CancelableTaskTracker { 59 public: 60 // All values except kBadTaskId are valid. 61 typedef int64_t TaskId; 62 static const TaskId kBadTaskId; 63 64 typedef base::Callback<bool()> IsCanceledCallback; 65 66 CancelableTaskTracker(); 67 68 // Cancels all tracked tasks. 69 ~CancelableTaskTracker(); 70 71 TaskId PostTask(base::TaskRunner* task_runner, 72 const tracked_objects::Location& from_here, 73 const base::Closure& task); 74 75 TaskId PostTaskAndReply(base::TaskRunner* task_runner, 76 const tracked_objects::Location& from_here, 77 const base::Closure& task, 78 const base::Closure& reply); 79 80 template <typename TaskReturnType, typename ReplyArgType> PostTaskAndReplyWithResult(base::TaskRunner * task_runner,const tracked_objects::Location & from_here,const base::Callback<TaskReturnType (void)> & task,const base::Callback<void (ReplyArgType)> & reply)81 TaskId PostTaskAndReplyWithResult( 82 base::TaskRunner* task_runner, 83 const tracked_objects::Location& from_here, 84 const base::Callback<TaskReturnType(void)>& task, 85 const base::Callback<void(ReplyArgType)>& reply) { 86 TaskReturnType* result = new TaskReturnType(); 87 return PostTaskAndReply( 88 task_runner, 89 from_here, 90 base::Bind(&base::internal::ReturnAsParamAdapter<TaskReturnType>, 91 task, 92 base::Unretained(result)), 93 base::Bind(&base::internal::ReplyAdapter<TaskReturnType, ReplyArgType>, 94 reply, 95 base::Owned(result))); 96 } 97 98 // Creates a tracked TaskId and an associated IsCanceledCallback. Client can 99 // later call TryCancel() with the returned TaskId, and run |is_canceled_cb| 100 // from any thread to check whether the TaskId is canceled. 101 // 102 // The returned task ID is tracked until the last copy of 103 // |is_canceled_cb| is destroyed. 104 // 105 // Note. This function is used to address some special cancelation requirement 106 // in existing code. You SHOULD NOT need this function in new code. 107 TaskId NewTrackedTaskId(IsCanceledCallback* is_canceled_cb); 108 109 // After calling this function, |task| and |reply| will not run. If the 110 // cancelation happens when |task| is running or has finished running, |reply| 111 // will not run. If |reply| is running or has finished running, cancellation 112 // is a noop. 113 // 114 // Note. It's OK to cancel a |task| for more than once. The later calls are 115 // noops. 116 void TryCancel(TaskId id); 117 118 // It's OK to call this function for more than once. The later calls are 119 // noops. 120 void TryCancelAll(); 121 122 // Returns true iff there are in-flight tasks that are still being 123 // tracked. 124 bool HasTrackedTasks() const; 125 126 private: 127 void Track(TaskId id, base::CancellationFlag* flag); 128 void Untrack(TaskId id); 129 130 base::hash_map<TaskId, base::CancellationFlag*> task_flags_; 131 132 TaskId next_id_; 133 base::ThreadChecker thread_checker_; 134 135 base::WeakPtrFactory<CancelableTaskTracker> weak_factory_; 136 137 DISALLOW_COPY_AND_ASSIGN(CancelableTaskTracker); 138 }; 139 140 } // namespace base 141 142 #endif // BASE_TASK_CANCELABLE_TASK_TRACKER_H_ 143