Function ensureRootIsScheduled

docs/external/js/react-dom.development.js:21474–21538

(root)

  function ensureRootIsScheduled(root) {
    var lastExpiredTime = root.lastExpiredTime;

    if (lastExpiredTime !== NoWork) {
      // Special case: Expired work should flush synchronously.
      root.callbackExpirationTime = Sync;
      root.callbackPriority = ImmediatePriority;
      root.callbackNode = scheduleSyncCallback(performSyncWorkOnRoot.bind(null, root));
      return;
    }

    var expirationTime = getNextRootExpirationTimeToWorkOn(root);
    var existingCallbackNode = root.callbackNode;

    if (expirationTime === NoWork) {
      // There's nothing to work on.
      if (existingCallbackNode !== null) {
        root.callbackNode = null;
        root.callbackExpirationTime = NoWork;
        root.callbackPriority = NoPriority;
      }

      return;
    } // TODO: If this is an update, we already read the current time. Pass the
    // time as an argument.


    var currentTime = requestCurrentTimeForUpdate();
    var priorityLevel = inferPriorityFromExpirationTime(currentTime, expirationTime); // If there's an existing render task, confirm it has the correct priority and
    // expiration time. Otherwise, we'll cancel it and schedule a new one.

    if (existingCallbackNode !== null) {
      var existingCallbackPriority = root.callbackPriority;
      var existingCallbackExpirationTime = root.callbackExpirationTime;

      if ( // Callback must have the exact same expiration time.
      existingCallbackExpirationTime === expirationTime && // Callback must have greater or equal priority.
      existingCallbackPriority >= priorityLevel) {
        // Existing callback is sufficient.
        return;
      } // Need to schedule a new task.
      // TODO: Instead of scheduling a new task, we should be able to change the
      // priority of the existing one.


      cancelCallback(existingCallbackNode);
    }

    root.callbackExpirationTime = expirationTime;
    root.callbackPriority = priorityLevel;
    var callbackNode;

    if (expirationTime === Sync) {
      // Sync React callbacks are scheduled on a special internal queue
      callbackNode = scheduleSyncCallback(performSyncWorkOnRoot.bind(null, root));
    } else {
      callbackNode = scheduleCallback(priorityLevel, performConcurrentWorkOnRoot.bind(null, root), // Compute a task timeout based on the expiration time. This also affects
      // ordering because tasks are processed in timeout order.