React のソースコードを読んでみる

import { useState } from 'react';
import { createRoot } from 'react-dom/client';

function Counter() {
  const [count, setCount] = useState(0);
  return (
    <>
      <h1>{count}</h1>
      <button onClick={() => setCount(count + 1)}>
        Increment
      </button>
    </>
  );
}

const root = createRoot(document.getElementById('root'));
root.render(<Counter />);

export function createRoot(
  container: Element | Document | DocumentFragment,
  options?: CreateRootOptions,
): RootType {
  if (__DEV__) {
    Internals.usingClientEntryPoint = true;
  }
  try {
    return createRootImpl(container, options);
  } finally {
    if (__DEV__) {
      Internals.usingClientEntryPoint = false;
    }
  }
}

export function createRoot(
  container: Element | Document | DocumentFragment,
  options?: CreateRootOptions,
): RootType {
  if (!isValidContainer(container)) {
    throw new Error('createRoot(...): Target container is not a DOM element.');
  }

  warnIfReactDOMContainerInDEV(container);

  let isStrictMode = false;
  let concurrentUpdatesByDefaultOverride = false;
  let identifierPrefix = '';
  let onRecoverableError = defaultOnRecoverableError;
  let transitionCallbacks = null;

  if (options !== null && options !== undefined) {
    if (__DEV__) {
      if ((options: any).hydrate) {
        console.warn(
          'hydrate through createRoot is deprecated. Use ReactDOMClient.hydrateRoot(container, <App />) instead.',
        );
      } else {
        if (
          typeof options === 'object' &&
          options !== null &&
          (options: any).$$typeof === REACT_ELEMENT_TYPE
        ) {
          console.error(
            'You passed a JSX element to createRoot. You probably meant to ' +
              'call root.render instead. ' +
              'Example usage:\n\n' +
              '  let root = createRoot(domContainer);\n' +
              '  root.render(<App />);',
          );
        }
      }
    }
    if (options.unstable_strictMode === true) {
      isStrictMode = true;
    }
    if (
      allowConcurrentByDefault &&
      options.unstable_concurrentUpdatesByDefault === true
    ) {
      concurrentUpdatesByDefaultOverride = true;
    }
    if (options.identifierPrefix !== undefined) {
      identifierPrefix = options.identifierPrefix;
    }
    if (options.onRecoverableError !== undefined) {
      onRecoverableError = options.onRecoverableError;
    }
    if (options.transitionCallbacks !== undefined) {
      transitionCallbacks = options.transitionCallbacks;
    }
  }

  const root = createContainer(
    container,
    ConcurrentRoot,
    null,
    isStrictMode,
    concurrentUpdatesByDefaultOverride,
    identifierPrefix,
    onRecoverableError,
    transitionCallbacks,
  );
  markContainerAsRoot(root.current, container);

  const rootContainerElement: Document | Element | DocumentFragment =
    container.nodeType === COMMENT_NODE
      ? (container.parentNode: any)
      : container;
  listenToAllSupportedEvents(rootContainerElement);

  return new ReactDOMRoot(root);
}

const root = createContainer(...)
return new ReactDomRoot(root);

export function createContainer(
  containerInfo: Container,
  tag: RootTag,
  hydrationCallbacks: null | SuspenseHydrationCallbacks,
  isStrictMode: boolean,
  concurrentUpdatesByDefaultOverride: null | boolean,
  identifierPrefix: string,
  onRecoverableError: (error: mixed) => void,
  transitionCallbacks: null | TransitionTracingCallbacks,
): OpaqueRoot {
  const hydrate = false;
  const initialChildren = null;
  return createFiberRoot(
    containerInfo,
    tag,
    hydrate,
    initialChildren,
    hydrationCallbacks,
    isStrictMode,
    concurrentUpdatesByDefaultOverride,
    identifierPrefix,
    onRecoverableError,
    transitionCallbacks,
  );
}

export function createFiberRoot(
  containerInfo: any,
  tag: RootTag,
  hydrate: boolean,
  initialChildren: ReactNodeList,
  hydrationCallbacks: null | SuspenseHydrationCallbacks,
  isStrictMode: boolean,
  concurrentUpdatesByDefaultOverride: null | boolean,
  // TODO: We have several of these arguments that are conceptually part of the
  // host config, but because they are passed in at runtime, we have to thread
  // them through the root constructor. Perhaps we should put them all into a
  // single type, like a DynamicHostConfig that is defined by the renderer.
  identifierPrefix: string,
  onRecoverableError: null | ((error: mixed) => void),
  transitionCallbacks: null | TransitionTracingCallbacks,
): FiberRoot {
  const root: FiberRoot = (new FiberRootNode(
    containerInfo,
    tag,
    hydrate,
    identifierPrefix,
    onRecoverableError,
  ): any);
  if (enableSuspenseCallback) {
    root.hydrationCallbacks = hydrationCallbacks;
  }

  if (enableTransitionTracing) {
    root.transitionCallbacks = transitionCallbacks;
  }

  // Cyclic construction. This cheats the type system right now because
  // stateNode is any.
  const uninitializedFiber = createHostRootFiber(
    tag,
    isStrictMode,
    concurrentUpdatesByDefaultOverride,
  );
  root.current = uninitializedFiber;
  uninitializedFiber.stateNode = root;

  if (enableCache) {
    const initialCache = createCache();
    retainCache(initialCache);

    // The pooledCache is a fresh cache instance that is used temporarily
    // for newly mounted boundaries during a render. In general, the
    // pooledCache is always cleared from the root at the end of a render:
    // it is either released when render commits, or moved to an Offscreen
    // component if rendering suspends. Because the lifetime of the pooled
    // cache is distinct from the main memoizedState.cache, it must be
    // retained separately.
    root.pooledCache = initialCache;
    retainCache(initialCache);
    const initialState: RootState = {
      element: initialChildren,
      isDehydrated: hydrate,
      cache: initialCache,
    };
    uninitializedFiber.memoizedState = initialState;
  } else {
    const initialState: RootState = {
      element: initialChildren,
      isDehydrated: hydrate,
      cache: (null: any), // not enabled yet
    };
    uninitializedFiber.memoizedState = initialState;
  }

  initializeUpdateQueue(uninitializedFiber);

  return root;
}

- createRoot
  - return new ReactDomRoot(createContainer)
- createContainer
  - return createFiberRoot(...)
- createFiberRoot
  - return new FiberRootNode(...)

const root = createRoot(document.getElementById('root'));
root.render(<App />);

const root = createRoot(document.getElementById('root'));
root.render(<App />);

export type RootType = {
  render(children: ReactNodeList): void,
  unmount(): void,
  _internalRoot: FiberRoot | null,
  ...
};

ReactDOMHydrationRoot.prototype.render = ReactDOMRoot.prototype.render = function(
  children: ReactNodeList,
): void {
  const root = this._internalRoot;
  if (root === null) {
    throw new Error('Cannot update an unmounted root.');
  }

  if (__DEV__) {
    if (typeof arguments[1] === 'function') {
      console.error(
        'render(...): does not support the second callback argument. ' +
          'To execute a side effect after rendering, declare it in a component body with useEffect().',
      );
    } else if (isValidContainer(arguments[1])) {
      console.error(
        'You passed a container to the second argument of root.render(...). ' +
          "You don't need to pass it again since you already passed it to create the root.",
      );
    } else if (typeof arguments[1] !== 'undefined') {
      console.error(
        'You passed a second argument to root.render(...) but it only accepts ' +
          'one argument.',
      );
    }

    const container = root.containerInfo;

    if (container.nodeType !== COMMENT_NODE) {
      const hostInstance = findHostInstanceWithNoPortals(root.current);
      if (hostInstance) {
        if (hostInstance.parentNode !== container) {
          console.error(
            'render(...): It looks like the React-rendered content of the ' +
              'root container was removed without using React. This is not ' +
              'supported and will cause errors. Instead, call ' +
              "root.unmount() to empty a root's container.",
          );
        }
      }
    }
  }
  updateContainer(children, root, null, null);
};