// Copyright 2017 The Fuchsia 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 LIB_ASYNC_CPP_TASK_H_
#define LIB_ASYNC_CPP_TASK_H_

#include <lib/async/task.h>
#include <lib/fit/function.h>
#include <lib/zx/time.h>

#include <utility>

namespace async {

// Posts a task to invoke |handler| with a deadline of now.
//
// The handler will not run if the dispatcher shuts down before it comes due.
//
// Returns |ZX_OK| if the task was successfully posted.
// Returns |ZX_ERR_BAD_STATE| if the dispatcher is shutting down.
// Returns |ZX_ERR_NOT_SUPPORTED| if not supported by the dispatcher.
zx_status_t PostTask(async_dispatcher_t* dispatcher, fit::closure handler);

// Posts a task to invoke |handler| with a deadline expressed as a |delay| from now.
//
// The handler will not run if the dispatcher shuts down before it comes due.
//
// Returns |ZX_OK| if the task was successfully posted.
// Returns |ZX_ERR_BAD_STATE| if the dispatcher is shutting down.
// Returns |ZX_ERR_NOT_SUPPORTED| if not supported by the dispatcher.
zx_status_t PostDelayedTask(async_dispatcher_t* dispatcher, fit::closure handler,
                            zx::duration delay);

// Posts a task to invoke |handler| with the specified |deadline|.
//
// The handler will not run if the dispatcher shuts down before it comes due.
//
// Returns |ZX_OK| if the task was successfully posted.
// Returns |ZX_ERR_BAD_STATE| if the dispatcher is shutting down.
// Returns |ZX_ERR_NOT_SUPPORTED| if not supported by the dispatcher.
zx_status_t PostTaskForTime(async_dispatcher_t* dispatcher, fit::closure handler,
                            zx::time deadline);

// Holds context for a task and its handler, with RAII semantics.
// Automatically cancels the task when it goes out of scope.
//
// After successfully posting the task, the client is responsible for retaining
// the structure in memory (and unmodified) until the task's handler runs, the task
// is successfully canceled, or the dispatcher shuts down.  Thereafter, the task
// may be posted again or destroyed.
//
// This class must only be used with single-threaded asynchronous dispatchers
// and must only be accessed on the dispatch thread since it lacks internal
// synchronization of its state.
//
// Concrete implementations: |async::Task|, |async::TaskMethod|,
//   |async::TaskClosure|, |async::TaskClosureMethod|.
// Please do not create subclasses of TaskBase outside of this library.
class TaskBase {
 protected:
  explicit TaskBase(async_task_handler_t* handler);
  ~TaskBase();

  TaskBase(const TaskBase&) = delete;
  TaskBase(TaskBase&&) = delete;
  TaskBase& operator=(const TaskBase&) = delete;
  TaskBase& operator=(TaskBase&&) = delete;

 public:
  // Returns true if the task has been posted and has not yet executed or been canceled.
  bool is_pending() const { return dispatcher_ != nullptr; }

  // The last deadline with which the task was posted, or |zx::time::infinite()|
  // if it has never been posted.
  zx::time last_deadline() const { return zx::time(task_.deadline); }

  // Posts a task to invoke the handler with a deadline of now.
  //
  // Returns |ZX_OK| if the task was successfully posted.
  // Returns |ZX_ERR_BAD_STATE| if the dispatcher is shutting down or if the
  // task is already pending.
  // Returns |ZX_ERR_ALREADY_EXISTS| if the task is already pending.
  // Returns |ZX_ERR_NOT_SUPPORTED| if not supported by the dispatcher.
  zx_status_t Post(async_dispatcher_t* dispatcher);

  // Posts a task to invoke the handler with a deadline expressed as a |delay| from now.
  //
  // Returns |ZX_OK| if the task was successfully posted.
  // Returns |ZX_ERR_BAD_STATE| if the dispatcher is shutting down or if the
  // task is already pending.
  // Returns |ZX_ERR_ALREADY_EXISTS| if the task is already pending.
  // Returns |ZX_ERR_NOT_SUPPORTED| if not supported by the dispatcher.
  zx_status_t PostDelayed(async_dispatcher_t* dispatcher, zx::duration delay);

  // Posts a task to invoke the handler with the specified |deadline|.
  //
  // The |deadline| must be expressed in the time base used by the asynchronous
  // dispatcher (usually |ZX_CLOCK_MONOTONIC| except in unit tests).
  // See |async_now()| for details.
  //
  // Returns |ZX_OK| if the task was successfully posted.
  // Returns |ZX_ERR_BAD_STATE| if the dispatcher is shutting down or if the
  // task is already pending.
  // Returns |ZX_ERR_ALREADY_EXISTS| if the task is already pending.
  // Returns |ZX_ERR_NOT_SUPPORTED| if not supported by the dispatcher.
  zx_status_t PostForTime(async_dispatcher_t* dispatcher, zx::time deadline);

  // Cancels the task.
  //
  // If successful, the task's handler will not run.
  //
  // Returns |ZX_OK| if the task was pending and it has been successfully
  // canceled; its handler will not run again and can be released immediately.
  // Returns |ZX_ERR_NOT_FOUND| if task was not pending either because its
  // handler already ran, or the task had not been posted.
  // Returns |ZX_ERR_NOT_SUPPORTED| if not supported by the dispatcher.
  zx_status_t Cancel();

 protected:
  template <typename T>
  static T* Dispatch(async_task_t* task) {
    static_assert(offsetof(TaskBase, task_) == 0, "");
    auto self = reinterpret_cast<TaskBase*>(task);
    self->dispatcher_ = nullptr;
    return static_cast<T*>(self);
  }

 private:
  async_task_t task_;
  async_dispatcher_t* dispatcher_ = nullptr;
};

// A task whose handler is bound to a |async::Task::Handler| function.
//
// Prefer using |async::TaskMethod| instead for binding to a fixed class member
// function since it is more efficient to dispatch.
class Task final : public TaskBase {
 public:
  // Handles execution of a posted task.
  //
  // The |status| is |ZX_OK| if the task's deadline elapsed and the task should run.
  // The |status| is |ZX_ERR_CANCELED| if the dispatcher was shut down before
  // the task's handler ran or the task was canceled.
  using Handler =
      fit::function<void(async_dispatcher_t* dispatcher, async::Task* task, zx_status_t status)>;

  explicit Task(Handler handler = nullptr);
  ~Task();

  void set_handler(Handler handler) { handler_ = std::move(handler); }
  bool has_handler() const { return !!handler_; }

 private:
  static void CallHandler(async_dispatcher_t* dispatcher, async_task_t* task, zx_status_t status);

  Handler handler_;
};

// A task whose handler is bound to a fixed class member function.
//
// Usage:
//
// class Foo {
//     void Handle(async_dispatcher_t* dispatcher, async::TaskBase* task, zx_status_t status) { ...
//     } async::TaskMethod<Foo, &Foo::Handle> task_{this};
// };
template <class Class, void (Class::*method)(async_dispatcher_t* dispatcher, async::TaskBase* task,
                                             zx_status_t status)>
class TaskMethod final : public TaskBase {
 public:
  explicit TaskMethod(Class* instance) : TaskBase(&TaskMethod::CallHandler), instance_(instance) {}
  ~TaskMethod() = default;

 private:
  static void CallHandler(async_dispatcher_t* dispatcher, async_task_t* task, zx_status_t status) {
    auto self = Dispatch<TaskMethod>(task);
    (self->instance_->*method)(dispatcher, self, status);
  }

  Class* const instance_;
};

// A task whose handler is bound to a |fit::closure| function with no arguments.
// The closure is not invoked when errors occur since it doesn't have a |zx_status_t|
// argument.
//
// Prefer using |async::TaskClosureMethod| instead for binding to a fixed class member
// function since it is more efficient to dispatch.
class TaskClosure final : public TaskBase {
 public:
  explicit TaskClosure(fit::closure handler = nullptr);
  ~TaskClosure();

  void set_handler(fit::closure handler) { handler_ = std::move(handler); }
  bool has_handler() const { return !!handler_; }

 private:
  static void CallHandler(async_dispatcher_t* dispatcher, async_task_t* task, zx_status_t status);

  fit::closure handler_;
};

// A task whose handler is bound to a fixed class member function with no arguments.
// The closure is not invoked when errors occur since it doesn't have a |zx_status_t|
// argument.
//
// Usage:
//
// class Foo {
//     void Handle() { ... }
//     async::TaskClosureMethod<Foo, &Foo::Handle> trap_{this};
// };
template <class Class, void (Class::*method)()>
class TaskClosureMethod final : public TaskBase {
 public:
  explicit TaskClosureMethod(Class* instance)
      : TaskBase(&TaskClosureMethod::CallHandler), instance_(instance) {}
  ~TaskClosureMethod() = default;

 private:
  static void CallHandler(async_dispatcher_t* dispatcher, async_task_t* task, zx_status_t status) {
    auto self = Dispatch<TaskClosureMethod>(task);  // must do this if status is not ok
    if (status == ZX_OK) {
      (self->instance_->*method)();
    }
  }

  Class* const instance_;
};

}  // namespace async

#endif  // LIB_ASYNC_CPP_TASK_H_