如何在确保前一个函数完成的情况下以固定间隔调用函数

Question

我有一个基于 Promise 的函数，我想以固定的时间间隔调用它，可以说每 60 秒一次。但我也想确保只有在先前调用的函数完全执行时才调用该函数，并且我想无限期地继续执行此操作

function fun(){
    return new Promise((resolve,reject)=>{
        //Some database queries which may or may not complete in 60 seconds
        resolve("done")
    })
}

setInterval(()=>{
    fun().then(d=>{
        console.log(d)
    })
},60000)

上面的代码不会检查之前调用的函数是否完成。但我想确保

Answer 1

class AsyncQueue {
  constructor() {
    this.queue = null;
  }

  push(task) {
    // If we have task in query, append new at the end, if not execute immediately
    // Every task appended to existing queue will be executed immediately after previous one is finished
    return this.queue = this.queue ? this.queue.then(() => task()) : task();
  }
}

const task = (id) => () => new Promise((resolve) => {
  console.log('Task', id, 'started');
  // Random time betwen 500-1300ms
  const time = Math.round(Math.random() * 800 + 500);
  setTimeout(() => {
    console.log("Finished task '" + id + "' after " + time + "ms");
    resolve();
  }, time);
});
const queue = new AsyncQueue();
let id = 0;
// This will push new task to queue every 1s
setInterval(() => {
  console.log("Pushing new task", ++id);
  queue.push(task(id));
}, 1000);

当然我们可以不使用类来实现

let queue;
function push(task) {
  return queue = queue ? queue.then(() => task()) : task();
}
// or
const push = (task) => queue = queue ? queue.then(() => task()) : task();