Question 242479 in Javascript Question posted in
A very good W3school tutorial can be found here.

Javascript – How to poll a promise in repeating syncroneous code?

Let’s say we have some Promise for we knew it resolves at some point. Now I have sync code (ag. no await possible) called repeatedly (eg. onAnimationFrame). How to use the result of the promise there, if it is fullfilled?

Like (JS pseudocode):

let promise=FetchSomething();
let updater = ()=>{
  drawSomething();
  if(promise IS FULLFILLED)  // how to know if fullfilled?
    drawThing(RESULT OF promise); // how to get the result here?
  requestAnimationFrame(updater);
}
updater();
Tags:

2

Answers


  1. 0

    The easiest way is to set a variable yourself:

    let promise = FetchSomething();
let done = false

/* variable will be set once promise settled */
promise.then(() => {
    done = true
})

let updater = () => {
  drawSomething();

  if (done) {  
      drawThing(/* RESULT OF promise */); 
  }

  requestAnimationFrame(updater);
}

updater();

    You can do the same with the result, i.e.:

    let promise = FetchSomething();
let done = false
let result = null

promise.then((value) => {
    done = true
    result = value
})

let updater = () => {
  drawSomething();

  if (done) {  
      drawThing(result); 
  }

  requestAnimationFrame(updater);
}

updater();
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  2. 0

    Perhaps this simple promise using your names will illustrate:

    // fake 
function FetchSomething(myfoo) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve(myfoo);
    }, 2000);
  });
}

function drawSomething(val) {
  console.log('draw', val);
}

function drawThing(val) {
  console.log('drawthing', val);
}

function woops(val) {
  console.log(val);
}

let handleFulfilled = (val) => {
  drawSomething(val);
  drawThing(val);
  requestAnimationFrame(updater);
}

let handleRejected = (val) => {
  drawSomething(val);
  woops(val);
  requestAnimationFrame(updater);
}
let goodfoo = "good foo";

function updateFun(v) {
  let fetchPromise = FetchSomething(v);

  fetchPromise.then(handleFulfilled, handleRejected);
  let t = new Date(Date.now()).getSeconds();
  setTimeout(function() {
    updateFun(goodfoo + t)
  }, 5000);
}
updateFun(goodfoo);
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