Make dynamically created promises execute in sequence

问题

How can I dynamically create a series of promises and have them execute in sequence?

pseudocode
 for x=0 to maxValue
   promiseArray.push(createNewPromise(x))

 executeAllPromisesSequentially(promiseArray)

where

executeAllPromisesSequentially is functionally equivalent to

promise1()
.then(promise2)
.then(promise3)
etc
...

回答1:

There are some patterns displayed on my gist

Promise Iteration with Reduce

let tasks = [ /* ... */ ]
let promise = tasks.reduce((prev, task) => {
  return prev.then(() => {
    return task();
  });
}, Promise.resolve());
promise.then(() => {
//All tasks completed
});

Sequential Iteration Pattern

let tasks = [ /* ... */ ]
let promise = Promise.resolve();
tasks.forEach(task => {
  promise = promise.then(() => {
    return task();
  });
});
promise.then(() => {
//All tasks completed
});

Sequential Iteration Example

function spiderLinks(currentUrl, body, nesting) {
  let promise = Promise.resolve();
  if(nesting === 0) {
    return promise;
  }
  const links = utilities.getPageLinks(currentUrl, body);
  links.forEach(link => {
    promise = promise.then(() => spider(link, nesting - 1));
  });
  return promise;
}

回答2:

Just build up a chain as jaromandaX said. However you need to make sure that you use let inside the loop to closure the x:

  let chain = Promise.resolve();
  const promises = [];

  for(let x = 0; x < maxValue; x++)
    promises.push(chain = chain.then(() => createNewPromise(x)));

回答3:

Reducing or loop / recursion chaining comes to mind as a common practice however if you would like to keep and access to the intermediate resolutions here i have another approach by using an invention of Haskell's scanl function in JS.

scanl is similar to JS .reduce() but like .map() always returns a same size array holding the interim values. So the scanl function would look something like;

var scanl = (xs, f, acc) => xs.map((a => e => a = f(a,e))(acc));

So if you do;

scanl([1,2,3,4], (a,e) => a + e, 0) // -> [1,3,6,10]

So having scanl at hand now we may attempt to sequence promises by keeping the intermetiate resolutions in a resulting array.

var scanl = (xs, f, acc) => xs.map((a => e => a = f(a,e))(acc)),
    proms = Array(5).fill().map((_,i) => new Promise((v,x) => setTimeout(v,100+Math.random()*1900,`res ${i+1}`)));
  
proms = scanl(proms, (a,p,t) => a.then(v => (t = v, p))
                                 .then(v => `${t} and ${v}`)
                                 .then(s => (console.log(`did stg with ${s}`),s)), Promise.resolve("init 0"));
Promise.all(proms)
       .then(vs => vs.forEach(v => console.log(v)));

.as-console-wrapper {
max-height : 100% !important
}

Of course above function is just makeshift. I use an unused t argument as a temporary variable defined in the callbacks context.

来源：https://stackoverflow.com/questions/49685980/make-dynamically-created-promises-execute-in-sequence

标签

javascript

node.js

promise

es6-promise