Concurrent on fiber and requestIdleCallback

Fiber Concurrent mode basic

To make the update as small chunks, for requestIdleCallback to use, React uses fiber nodes. Each fiber node can be a unitOfWork, and the workloop() function iterates through all unit of work with scheduling.

let nextUnitOfWork = null

function workLoop(deadline) {
  function enoughTime(){
    return deadline.timeRemaining() >= 1;
  }
  while (nextUnitOfWork && enoughTime()) {
    nextUnitOfWork = performUnitOfWork(
      nextUnitOfWork
    )
  }
  requestIdleCallback(workLoop)
}

requestIdleCallback(workLoop)

function performUnitOfWork(nextUnitOfWork) {
  // TODO add dom node
  // TODO create new fibers
  // TODO return nextUnitOfWork (next visited fiber)
}

Key Concept

window.requestIdleCallback:

Similar to window.requestAnimationFrame.

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

shim(just a shim, not polyfill) to explain the concept:

window.requestIdleCallback =
  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