有限制的promise。all（）？

Question

是否有一种方法/pattern来实现let res=promise.all([...p],limit)？

• promise-数组包含构造和返回promise
的函数
• res应在所有p
解析后解析
• 只有限制=3promise应并行运行
• 第n+1个promise应在n个完成后立即开始。以便始终有limit解析器并行运行。

Answer 1

我想出了创建n=limit链的想法，这些链并行运行，只要有promise就追加：

let promises = [];
for(let i=0; i<11; i++) promises[i] = () => {
  console.log('Construct:',i);
  return new Promise(resolve => {
    setTimeout(function() {
      console.log('Resolve:',i);
      resolve(i);
    }, Math.round(Math.random() * (2000 - 500) + 2000));
  });
}


function parallelLimit(promiseFactories, limit) {
  let result = [];
  let cnt = 0;

  function chain(promiseFactories) {
    if(!promiseFactories.length) return;
    let i = cnt++; // preserve order in result
    return promiseFactories.shift()().then((res) => {
      result[i] = res; // save result
      return chain(promiseFactories); // append next promise
    });
  }

  let arrChains = [];
  while(limit-- > 0 && promiseFactories.length > 0) {
    // create `limit` chains which run in parallel
    arrChains.push(chain(promiseFactories));
  }

  // return when all arrChains are finished
  return Promise.all(arrChains).then(() => result);
}


parallelLimit(promises, 4).then(console.log);