Function scheduleCallback

packages/react-reconciler/src/ReactFiberRootScheduler.js:626–639

(
  priorityLevel: PriorityLevel,
  callback: RenderTaskFn,
)

const fakeActCallbackNode = {};

function scheduleCallback(
  priorityLevel: PriorityLevel,
  callback: RenderTaskFn,
) {
  if (__DEV__ && ReactSharedInternals.actQueue !== null) {
    // Special case: We're inside an `act` scope (a testing utility).
    // Instead of scheduling work in the host environment, add it to a
    // fake internal queue that's managed by the `act` implementation.
    ReactSharedInternals.actQueue.push(callback);
    return fakeActCallbackNode;
  } else {
    return Scheduler_scheduleCallback(priorityLevel, callback);
  }
}

function cancelCallback(callbackNode: mixed) {
  if (__DEV__ && callbackNode === fakeActCallbackNode) {