export function updateContainer(
  element: ReactNodeList,
  container: OpaqueRoot,
  parentComponent: ?React$Component<any, any>,
  callback: ?Function,
): ExpirationTime {
// createFiberRoot中创建的fiber对象
  const current = container.current;
  const currentTime = requestCurrentTime();
  if (__DEV__) {
    // $FlowExpectedError - jest isn't a global, and isn't recognized outside of tests
    if ('undefined' !== typeof jest) {
      warnIfNotScopedWithMatchingAct(current);
    }
  }
  const suspenseConfig = requestCurrentSuspenseConfig();
  // 获取任务到期时间
  const expirationTime = computeExpirationForFiber(
    currentTime,
    current,
    suspenseConfig,
  );
  return updateContainerAtExpirationTime(
    element,
    container,
    parentComponent,
    expirationTime,
    suspenseConfig,
    callback,
  );
}

computeExpirationForFiber

export function computeExpirationForFiber(
  currentTime: ExpirationTime,
  fiber: Fiber,
  suspenseConfig: null | SuspenseConfig,
): ExpirationTime {
  const mode = fiber.mode;
  if ((mode & BatchedMode) === NoMode) {
    return Sync;
  }

  const priorityLevel = getCurrentPriorityLevel();
  if ((mode & ConcurrentMode) === NoMode) {
    return priorityLevel === ImmediatePriority ? Sync : Batched;
  }

  if ((executionContext & RenderContext) !== NoContext) {
    // Use whatever time we're already rendering
    return renderExpirationTime;
  }

  let expirationTime;
  if (suspenseConfig !== null) {
    // Compute an expiration time based on the Suspense timeout.
    expirationTime = computeSuspenseExpiration(
      currentTime,
      suspenseConfig.timeoutMs | 0 || LOW_PRIORITY_EXPIRATION,
    );
  } else {
    // Compute an expiration time based on the Scheduler priority.
    switch (priorityLevel) {
      case ImmediatePriority:
        expirationTime = Sync;
        break;
      case UserBlockingPriority:
        // TODO: Rename this to computeUserBlockingExpiration
        expirationTime = computeInteractiveExpiration(currentTime);
        break;
      case NormalPriority:
      case LowPriority: // TODO: Handle LowPriority
        // TODO: Rename this to... something better.
        expirationTime = computeAsyncExpiration(currentTime);
        break;
      case IdlePriority:
        expirationTime = Never;
        break;
      default:
        invariant(false, 'Expected a valid priority level');
    }
  }

  // If we're in the middle of rendering a tree, do not update at the same
  // expiration time that is already rendering.
  // TODO: We shouldn't have to do this if the update is on a different root.
  // Refactor computeExpirationForFiber + scheduleUpdate so we have access to
  // the root when we check for this condition.
  if (workInProgressRoot !== null && expirationTime === renderExpirationTime) {
    // This is a trick to move this update into a separate batch
    expirationTime -= 1;
  }

  return expirationTime;
}

updateContainerAtExpirationTime

// 根据渲染优先级更新dom
export function updateContainerAtExpirationTime(
  element: ReactNodeList,
  container: OpaqueRoot,
  parentComponent: ?React$Component<any, any>,
  expirationTime: ExpirationTime,
  suspenseConfig: null | SuspenseConfig,
  callback: ?Function,
) {
  // TODO: If this is a nested container, this won't be the root.
  const current = container.current;

  if (__DEV__) {
    if (ReactFiberInstrumentation.debugTool) {
      if (current.alternate === null) {
        ReactFiberInstrumentation.debugTool.onMountContainer(container);
      } else if (element === null) {
        ReactFiberInstrumentation.debugTool.onUnmountContainer(container);
      } else {
        ReactFiberInstrumentation.debugTool.onUpdateContainer(container);
      }
    }
  }
// 获得上下文对象
  const context = getContextForSubtree(parentComponent);
  if (container.context === null) {
    container.context = context;
  } else {
    container.pendingContext = context;
  }

  return scheduleRootUpdate(
    current,
    element,
    expirationTime,
    suspenseConfig,
    callback,
  );
}

getContextForSubtree

// 获得上下文对象
function getContextForSubtree(
  parentComponent: ?React$Component<any, any>,
): Object {
  if (!parentComponent) {
    return emptyContextObject;
  }

  const fiber = getInstance(parentComponent);
  const parentContext = findCurrentUnmaskedContext(fiber);

  if (fiber.tag === ClassComponent) {
    const Component = fiber.type;
    if (isLegacyContextProvider(Component)) {
      return processChildContext(fiber, Component, parentContext);
    }
  }

  return parentContext;
}

updateContainer的源码很简单，通过computeExpirationForFiber获得计算优先级，然后丢给updateContainerAtExpirationTime，这里updateContainerAtExpirationTime其实相当于什么都没做，通过getContextForSubtree（这里getContextForSubtree因为一开始parentComponent是不存在的，于是返回一个空对象。注意，这个空对象可以重复使用，不用每次返回一个新的空对象，这是一个很好的优化）获得上下文对象，然后分配给container.context或container.pendingContext，最后一起丢给scheduleRootUpdate。