A EventLoop represents the runner for running asynchronous computation. One of implementations is SingleThreadEventLoop. In the case of SingleThreadEventLoop, it manages its tasks using queues.
The client can use SingleThreadEventLoop APIs (submit(), schedule()) to request to run asynchronous task. Internally SingleThreadEventLoop manages several queues. One is ScheduldTaskQueue, when the client schedule the task to be run after some amount of time, SingleThreadEventLoop uses ScheduldTaskQueue to manage scheduled tasks. When the scheduled task is the time to be run, SingleThreadEventLoop place the task to the RunnableQueue, the second queue it manages. The tasks on the RunnableQueue now can be dequeue using takeTask(). The responsibility to run those retrieved tasks is on the client side.
A Promise is a placeholder for the result of asynchronous computation. One that runs asynchronous computation can return the Promise to the client for the result of request. On the client side, it can wait until Promise to be set result or the error.
A PromiseListener can be registered to the Promise, once it register, it can receive the Promise when it update its state.
