React DOM 同步刷新

import { disableLegacyMode } from 'shared/ReactFeatureFlags';
import { DiscreteEventPriority } from 'react-reconciler/src/ReactEventPriorities';

import ReactSharedInternals from 'shared/ReactSharedInternals';

import ReactDOMSharedInternals from 'shared/ReactDOMSharedInternals';

function flushSyncImpl<R>(fn: () => R): R;
function flushSyncImpl(): void;
function flushSyncImpl<R>(fn?: () => R): R | undefined {
  // 上一次过渡
  const previousTransition = ReactSharedInternals.T;
  //   上一次更新优先级
  const previousUpdatePriority =
    ReactDOMSharedInternals.p; /* ReactDOMCurrentUpdatePriority */

  try {
    ReactSharedInternals.T = null;
    ReactDOMSharedInternals.p /* ReactDOMCurrentUpdatePriority */ =
      DiscreteEventPriority;
    if (fn) {
      return fn();
    } else {
      return undefined;
    }
  } finally {
    ReactSharedInternals.T = previousTransition;
    ReactDOMSharedInternals.p /* ReactDOMCurrentUpdatePriority */ =
      previousUpdatePriority;
    const wasInRender =
      ReactDOMSharedInternals.d /* ReactDOMCurrentDispatcher */
        .f(); /* flushSyncWork */
    if (__DEV__) {
      if (wasInRender) {
        console.error(
          'flushSync was called from inside a lifecycle method. React cannot ' +
            'flush when React is already rendering. Consider moving this call to ' +
            'a scheduler task or micro task.',
        );
      }
    }
  }
}

function flushSyncErrorInBuildsThatSupportLegacyMode<R>(fn: () => R): R;
function flushSyncErrorInBuildsThatSupportLegacyMode(): void;
function flushSyncErrorInBuildsThatSupportLegacyMode() {
  throw new Error(
    'Expected this build of React to not support legacy mode but it does. This is a bug in React.',
  );
}

export const flushSync: typeof flushSyncImpl = disableLegacyMode
  ? flushSyncImpl
  : flushSyncErrorInBuildsThatSupportLegacyMode;