text: // 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/bind.h"
  #include "base/callback.h"
  #include "base/check.h"
  #include "base/location.h"
  #include "base/memory/ref_counted.h"
  #include "base/post_task_and_reply_with_result_internal.h"
  #include "base/time/time.h"
  
  namespace base {
  
  struct TaskRunnerTraits;
  
  // A TaskRunner is an object that runs posted tasks (in the form of
  // OnceClosure 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 Location& from_here, OnceClosure task);
  
    // Like PostTask, but tries to run the posted task only after |delay_ms|
    // has passed. Implementations should use a tick clock, rather than wall-
    // clock time, to implement |delay|.
    virtual bool PostDelayedTask(const Location& from_here,
                                 OnceClosure task,
                                 base::TimeDelta delay) = 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| OnceClosures.  In particular, it can be useful
    // to use WeakPtr<> in the |reply| OnceClosure 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::BindOnce(&DataBuffer::AddData, buffer),
    //          base::BindOnce(&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 Location& from_here,
                          OnceClosure task,
                          OnceClosure reply);
  
    // When you have these methods
    //
    //   R DoWorkAndReturn();
    //   void Callback(const R& result);
    //
    // and want to call them in a PostTaskAndReply kind of fashion where the
    // result of DoWorkAndReturn is passed to the Callback, you can use
    // PostTaskAndReplyWithResult as in this example:
    //
    // PostTaskAndReplyWithResult(
    //     target_thread_.task_runner(),
    //     FROM_HERE,
    //     BindOnce(&DoWorkAndReturn),
    //     BindOnce(&Callback));
    template <typename TaskReturnType, typename ReplyArgType>
    bool PostTaskAndReplyWithResult(const Location& from_here,
                                    OnceCallback<TaskReturnType()> task,
                                    OnceCallback<void(ReplyArgType)> reply) {
      DCHECK(task);
      DCHECK(reply);
      // std::unique_ptr used to avoid the need of a default constructor.
      auto* result = new std::unique_ptr<TaskReturnType>();
      return PostTaskAndReply(
          from_here,
          BindOnce(&internal::ReturnAsParamAdapter<TaskReturnType>,
                   std::move(task), result),
          BindOnce(&internal::ReplyAdapter<TaskReturnType, ReplyArgType>,
                   std::move(reply), Owned(result)));
    }
  
   protected:
    friend struct 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_
  