Refactor ensureUpdateQueues to avoid returning a tuple

[zombieJ]

Refactor ensureUpdateQueues and insertUpdate & addTopLevelUpdate.
Use queue & alternateQueue instead of queue1 & queue2.

[acdlite]

Hi! Thanks for the PR.

What I meant by that TODO was that I wanted to avoid allocating an array. Now you're allocating an object instead, which is no better.

The way I figured we could solve this is by storing the return values in a pair of module-level variables which the caller can read from.

[acdlite]

Also let's please keep the naming of queue1 and queue2

[zombieJ]

ok. Got it～

[zombieJ]

hi @acdlite, update the code. Please check.

[acdlite]

@zombieJ Hey sorry for the delay.

So I'm realizing now that I should have been more specific. That's my bad.

I would prefer not to have to do these checks twice: https://github.com/zombieJ/react/blob/4690e50c7d1d51761e5d128b9c446f0f2c1493e8/src/renderers/shared/fiber/ReactFiberUpdateQueue.js#L210-L213

I also don't want to allocate an object or array in order to return two queues. So what I think we should do is keep the body of ensureUpdateQueues mostly the same, but instead of returning a tuple at the end, mutate a module-level variable that both ensureUpdateQueues and insertUpdate have access to:

let _queue1;
let _queue2;

function ensureUpdateQueues(fiber) {
  // ...Same body as before...

  // At the end, instead of returning an array, assign to _queue1 and queue2.
  _queue1 = queue1;
  // Return null if there is no alternate queue, or if its queue is the same.		
  _queue2 = queue2 !== queue1 ? queue2 : null;
}

function insertUpdate(fiber, update) {
  // ...
  ensureUpdateQueue(fiber);
  const queue1 = _queue1;
  const queue2 = _queue2;
}

Hope this makes sense!

[zombieJ]

@acdlite ,
Updated. Please check~

[acdlite]

@zombieJ Thanks for your work on this, and your patience in getting it landed!