Notes-03-15-Concurrent-with-requestIdleCallback

Concurrent Basic

let nextUnitOfWork = null

function workLoop(deadline) {
  let shouldYield = false
  while (nextUnitOfWork && !shouldYield) {
    nextUnitOfWork = performUnitOfWork(
      nextUnitOfWork
    )
    shouldYield = deadline.timeRemaining() < 1
  }
  requestIdleCallback(workLoop)
}

requestIdleCallback(workLoop)

function performUnitOfWork(nextUnitOfWork) {
  // return nextUnitOfWork
}

Key Concept

window.requestIdleCallback:

Similar to window.requestAnimationFrame.

Pass a deadline object to the callback, which contains a timeRemaining() to calculate

shim(just a shim, not polyfill):

window.cancelIdleCallback =
  window.requestIdleCallback ||
  function (cb) {
    const start = new Date();
    
    return setTimeout(function (){
      cb({
        didTimeout: false,
        timeRemaining: function(){
          const delta = new Date() - start;
          return Math.max(0, 50 - delta);
        }
      });
    }, 1);
  }

window.cancelIdleCallback =
  window.cancelIdleCallback ||
  function (id) {
    clearTimeout(id);
  }

Key:

1. Use setTimeout to put callback to the end of main loop/thread, as a macro task, so that it is able to check the timeRemain after main loop

2. Return timeRemain ONLY when it's less than 50 ms, quoted as "When trying to maintain a responsive application, all responses to user interactions should be kept under 100ms. Should the user interact the 50ms window should, in most cases, allow for the idle callback to complete, and for the browser to respond to the user’s interactions."

3. Function returns id of setTimeout, as a common pattern in JS event loop task handling, for program to call cancelIdleCallback upon, this is used to just trying mocking the js schedule ability

4. Quote: "Ideally the work you do should be in small chunks (microtasks) that have relatively predictable characteristics." (nextUnitOfWork)

Ref:

Using requestIdleCallback  |  Web  |  Google DevelopersGoogle Developers

https://pomb.us/build-your-own-react/

https://developer.mozilla.org/en-US/docs/Web/API/Window/requestIdleCallback