Coroutine Scenarios

Generator

• 可以挂起返回很多值
• 当挂起的caller恢复运行时，使用value
• Lazy sequence
• 实现iterator/sentinel

// C++23实现该标准库
std::generator<uint64_t>
fibonacci_sequence(unsigned n) {
   if (n > 0) co_yield 0;
   if (n > 1) co_yield 1;
   uint64_t a = 0, b = 1;
   for (unsigned i = 2; i < n; ++i) {
       uint64_t s = a+b;
       co_yield s;
       a = b;
       b = s;
   }
}

Task

• 类似函数调用的感觉

• Caller co_await当task被创建

• task<T>::await_suspend()存放caller-handle(in its promise)同时开始callee

• The stored handle is called the continuation

• task<T>::final_suspend()resumes the continuation

Asynchronous I/O

• await_suspend()stores the coroutine_handle<> and registers a callback with the OS

• The callback will put results in the Awaiter and resume()the coroutine

• await_resume()provides the results to the coroutine

char data[1024];
while(1) {
    size_t n = co_await inSocket.async_read(buffer(data));
    co_await async_write(outSocket, buffer(data, n));
}

Scheduler

• act as an Awaitable
• 所有的actors 定期co_await

struct Scheduler {
  std::list<std::coroutine_handle<>> _tasks{};

  bool schedule() {
    auto task = _tasks.front();
    _tasks.pop_front();

    if (not task.done()) {
      task.resume();
    }
    return not _tasks.empty();
  }
  auto suspend() {
    struct awaiter : std::suspend_always {
      Scheduler &_sched;
      explicit awaiter(Scheduler &sched) : _sched{sched} {}
      bool await_ready() { return false; }
      void await_suspend(std::coroutine_handle<> hdl) const noexcept {
        _sched._tasks.push_back(hdl);
      }
      void await_resume() {}
    };
    return awaiter{*this};
  }
};