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 sequence. 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, otherwise 22 // use one of the other mechanisms. 23 // 24 // THREAD-SAFETY: 25 // 26 // 1. A CancelableTaskTracker object must be created, used, and destroyed on a 27 // single sequence. 28 // 29 // 2. It's safe to destroy a CancelableTaskTracker while there are outstanding 30 // tasks. This is commonly used to cancel all outstanding tasks. 31 // 32 // 3. The task is deleted on the target sequence, and the reply are deleted on 33 // the originating sequence. 34 // 35 // 4. IsCanceledCallback can be run or deleted on any sequence. 36 #ifndef BASE_TASK_CANCELABLE_TASK_TRACKER_H_ 37 #define BASE_TASK_CANCELABLE_TASK_TRACKER_H_ 38 39 #include <stdint.h> 40 41 #include <utility> 42 43 #include "base/base_export.h" 44 #include "base/bind.h" 45 #include "base/callback.h" 46 #include "base/containers/small_map.h" 47 #include "base/macros.h" 48 #include "base/memory/weak_ptr.h" 49 #include "base/post_task_and_reply_with_result_internal.h" 50 #include "base/sequence_checker.h" 51 52 namespace base { 53 54 class CancellationFlag; 55 class Location; 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 Callback<bool()> IsCanceledCallback; 65 66 CancelableTaskTracker(); 67 68 // Cancels all tracked tasks. 69 ~CancelableTaskTracker(); 70 71 TaskId PostTask(TaskRunner* task_runner, 72 const Location& from_here, 73 OnceClosure task); 74 75 TaskId PostTaskAndReply(TaskRunner* task_runner, 76 const Location& from_here, 77 OnceClosure task, 78 OnceClosure reply); 79 80 template <typename TaskReturnType, typename ReplyArgType> PostTaskAndReplyWithResult(TaskRunner * task_runner,const Location & from_here,OnceCallback<TaskReturnType ()> task,OnceCallback<void (ReplyArgType)> reply)81 TaskId PostTaskAndReplyWithResult(TaskRunner* task_runner, 82 const Location& from_here, 83 OnceCallback<TaskReturnType()> task, 84 OnceCallback<void(ReplyArgType)> reply) { 85 TaskReturnType* result = new TaskReturnType(); 86 return PostTaskAndReply( 87 task_runner, from_here, 88 BindOnce(&internal::ReturnAsParamAdapter<TaskReturnType>, 89 std::move(task), Unretained(result)), 90 BindOnce(&internal::ReplyAdapter<TaskReturnType, ReplyArgType>, 91 std::move(reply), Owned(result))); 92 } 93 94 // Callback version of PostTaskWithTraitsAndReplyWithResult above. 95 // Though RepeatingCallback is convertible to OnceCallback, we need this since 96 // we can not use template deduction and object conversion at once on the 97 // overload resolution. 98 // TODO(tzik): Update all callers of the Callback version to use OnceCallback. 99 template <typename TaskReturnType, typename ReplyArgType> PostTaskAndReplyWithResult(TaskRunner * task_runner,const Location & from_here,Callback<TaskReturnType ()> task,Callback<void (ReplyArgType)> reply)100 TaskId PostTaskAndReplyWithResult(TaskRunner* task_runner, 101 const Location& from_here, 102 Callback<TaskReturnType()> task, 103 Callback<void(ReplyArgType)> reply) { 104 return PostTaskAndReplyWithResult( 105 task_runner, from_here, 106 static_cast<OnceCallback<TaskReturnType()>>(std::move(task)), 107 static_cast<OnceCallback<void(ReplyArgType)>>(std::move(reply))); 108 } 109 110 // Creates a tracked TaskId and an associated IsCanceledCallback. Client can 111 // later call TryCancel() with the returned TaskId, and run |is_canceled_cb| 112 // from any thread to check whether the TaskId is canceled. 113 // 114 // The returned task ID is tracked until the last copy of 115 // |is_canceled_cb| is destroyed. 116 // 117 // Note. This function is used to address some special cancelation requirement 118 // in existing code. You SHOULD NOT need this function in new code. 119 TaskId NewTrackedTaskId(IsCanceledCallback* is_canceled_cb); 120 121 // After calling this function, |task| and |reply| will not run. If the 122 // cancelation happens when |task| is running or has finished running, |reply| 123 // will not run. If |reply| is running or has finished running, cancellation 124 // is a noop. 125 // 126 // Note. It's OK to cancel a |task| for more than once. The later calls are 127 // noops. 128 void TryCancel(TaskId id); 129 130 // It's OK to call this function for more than once. The later calls are 131 // noops. 132 void TryCancelAll(); 133 134 // Returns true iff there are in-flight tasks that are still being 135 // tracked. 136 bool HasTrackedTasks() const; 137 138 private: 139 void Track(TaskId id, CancellationFlag* flag); 140 void Untrack(TaskId id); 141 142 // Typically the number of tasks are 0-2 and occationally 3-4. But since 143 // this is a general API that could be used in unexpected ways, use a 144 // small_map instead of a flat_map to avoid falling over if there are many 145 // tasks. 146 small_map<std::map<TaskId, CancellationFlag*>, 4> task_flags_; 147 148 TaskId next_id_; 149 SequenceChecker sequence_checker_; 150 151 WeakPtrFactory<CancelableTaskTracker> weak_factory_; 152 153 DISALLOW_COPY_AND_ASSIGN(CancelableTaskTracker); 154 }; 155 156 } // namespace base 157 158 #endif // BASE_TASK_CANCELABLE_TASK_TRACKER_H_ 159