// Copyright 2016 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_SCHEDULER_TASK_SCHEDULER_H_
#define BASE_TASK_SCHEDULER_TASK_SCHEDULER_H_

#include <memory>
#include <vector>

#include "base/base_export.h"
#include "base/callback_forward.h"
#include "base/memory/ref_counted.h"
#include "base/sequenced_task_runner.h"
#include "base/single_thread_task_runner.h"
#include "base/task_runner.h"
#include "base/task_scheduler/task_traits.h"

namespace tracked_objects {
class Location;
}

namespace base {

class HistogramBase;
class SchedulerWorkerPoolParams;

// Interface for a task scheduler and static methods to manage the instance used
// by the post_task.h API. Note: all base/task_scheduler users should go through
// post_task.h instead of TaskScheduler except for the one callsite per process
// which manages the process' instance.
class BASE_EXPORT TaskScheduler {
 public:
  // Returns the index of the worker pool in which a task with |traits| should
  // run. This should be coded in a future-proof way: new traits should
  // gracefully map to a default pool.
  using WorkerPoolIndexForTraitsCallback =
      Callback<size_t(const TaskTraits& traits)>;

  virtual ~TaskScheduler() = default;

  // Posts |task| with specific |traits|.
  // For one off tasks that don't require a TaskRunner.
  virtual void PostTaskWithTraits(const tracked_objects::Location& from_here,
                                  const TaskTraits& traits,
                                  const Closure& task) = 0;

  // Returns a TaskRunner whose PostTask invocations result in scheduling tasks
  // using |traits|. Tasks may run in any order and in parallel.
  virtual scoped_refptr<TaskRunner> CreateTaskRunnerWithTraits(
      const TaskTraits& traits) = 0;

  // Returns a SequencedTaskRunner whose PostTask invocations result in
  // scheduling tasks using |traits|. Tasks run one at a time in posting order.
  virtual scoped_refptr<SequencedTaskRunner>
  CreateSequencedTaskRunnerWithTraits(const TaskTraits& traits) = 0;

  // Returns a SingleThreadTaskRunner whose PostTask invocations result in
  // scheduling tasks using |traits|. Tasks run on a single thread in posting
  // order.
  virtual scoped_refptr<SingleThreadTaskRunner>
  CreateSingleThreadTaskRunnerWithTraits(const TaskTraits& traits) = 0;

  // Returns a vector of all histograms available in this task scheduler.
  virtual std::vector<const HistogramBase*> GetHistograms() const = 0;

  // Synchronously shuts down the scheduler. Once this is called, only tasks
  // posted with the BLOCK_SHUTDOWN behavior will be run. When this returns:
  // - All SKIP_ON_SHUTDOWN tasks that were already running have completed their
  //   execution.
  // - All posted BLOCK_SHUTDOWN tasks have completed their execution.
  // - CONTINUE_ON_SHUTDOWN tasks might still be running.
  // Note that an implementation can keep threads and other resources alive to
  // support running CONTINUE_ON_SHUTDOWN after this returns. This can only be
  // called once.
  virtual void Shutdown() = 0;

  // Waits until there are no pending undelayed tasks. May be called in tests
  // to validate that a condition is met after all undelayed tasks have run.
  //
  // Does not wait for delayed tasks. Waits for undelayed tasks posted from
  // other threads during the call. Returns immediately when shutdown completes.
  virtual void FlushForTesting() = 0;

  // CreateAndSetDefaultTaskScheduler() and SetInstance() register a
  // TaskScheduler to handle tasks posted through the post_task.h API for this
  // process. The registered TaskScheduler will only be deleted when a new
  // TaskScheduler is registered and is leaked on shutdown. The methods must
  // not be called when TaskRunners created by the previous TaskScheduler are
  // still alive. The methods are not thread-safe; proper synchronization is
  // required to use the post_task.h API after registering a new TaskScheduler.

  // Creates and sets a default task scheduler. CHECKs on failure.
  // |worker_pool_params_vector| describes the worker pools to create.
  // |worker_pool_index_for_traits_callback| returns the index in |worker_pools|
  // of the worker pool in which a task with given traits should run.
  static void CreateAndSetDefaultTaskScheduler(
      const std::vector<SchedulerWorkerPoolParams>& worker_pool_params_vector,
      const WorkerPoolIndexForTraitsCallback&
          worker_pool_index_for_traits_callback);

  // Registers |task_scheduler| to handle tasks posted through the post_task.h
  // API for this process.
  static void SetInstance(std::unique_ptr<TaskScheduler> task_scheduler);

  // Retrieve the TaskScheduler set via CreateAndSetDefaultTaskScheduler() or
  // SetInstance(). This should be used very rarely; most users of TaskScheduler
  // should use the post_task.h API.
  static TaskScheduler* GetInstance();
};

}  // namespace base

#endif  // BASE_TASK_SCHEDULER_TASK_SCHEDULER_H_