React 深度学习：Fiber-ReactFiberLazyComponent

ReactLazyComponent

包： packages/shared/ReactLazyComponent.js

/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @flow
 */

export type Thenable<T, R> = {
  then(resolve: (T) => mixed, reject: (mixed) => mixed): R,
};

export type LazyComponent<T> = {
  $$typeof: Symbol | number,
  _ctor: () => Thenable<{default: T}, mixed>,
  _status: 0 | 1 | 2,
  _result: any,
};

type ResolvedLazyComponent<T> = {
  $$typeof: Symbol | number,
  _ctor: () => Thenable<{default: T}, mixed>,
  _status: 1,
  _result: any,
};

export const Pending = 0;
export const Resolved = 1;
export const Rejected = 2;

export function refineResolvedLazyComponent<T>(
  lazyComponent: LazyComponent<T>,
): ResolvedLazyComponent<T> | null {
  return lazyComponent._status === Resolved ? lazyComponent._result : null;
}
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ReactFiberLazyComponent

包：packages/react-reconciler/src/ReactFiberLazyComponent.js

/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @flow
 */

import type {LazyComponent, Thenable} from 'shared/ReactLazyComponent';

import {Resolved, Rejected, Pending} from 'shared/ReactLazyComponent';
import warning from 'shared/warning';

export function resolveDefaultProps(Component: any, baseProps: Object): Object {
  if (Component && Component.defaultProps) {
    // Resolve default props. Taken from ReactElement
    const props = Object.assign({}, baseProps);
    const defaultProps = Component.defaultProps;
    for (let propName in defaultProps) {
      if (props[propName] === undefined) {
        props[propName] = defaultProps[propName];
      }
    }
    return props;
  }
  return baseProps;
}

export function readLazyComponentType<T>(lazyComponent: LazyComponent<T>): T {
  const status = lazyComponent._status;
  const result = lazyComponent._result;
  switch (status) {
    case Resolved: {
      const Component: T = result;
      return Component;
    }
    case Rejected: {
      const error: mixed = result;
      throw error;
    }
    case Pending: {
      const thenable: Thenable<T, mixed> = result;
      throw thenable;
    }
    default: {
      lazyComponent._status = Pending;
      const ctor = lazyComponent._ctor;
      const thenable = ctor();
      thenable.then(
        moduleObject => {
          if (lazyComponent._status === Pending) {
            const defaultExport = moduleObject.default;
            if (__DEV__) {
              if (defaultExport === undefined) {
                warning(
                  false,
                  'lazy: Expected the result of a dynamic import() call. ' +
                    'Instead received: %s\n\nYour code should look like: \n  ' +
                    "const MyComponent = lazy(() => import('./MyComponent'))",
                  moduleObject,
                );
              }
            }
            lazyComponent._status = Resolved;
            lazyComponent._result = defaultExport;
          }
        },
        error => {
          if (lazyComponent._status === Pending) {
            lazyComponent._status = Rejected;
            lazyComponent._result = error;
          }
        },
      );
      // Handle synchronous thenables.
      switch (lazyComponent._status) {
        case Resolved:
          return lazyComponent._result;
        case Rejected:
          throw lazyComponent._result;
      }
      lazyComponent._result = thenable;
      throw thenable;
    }
  }
}复制代码

从上面的代码中，可以看到 React 内部解析 Component 组件的默认 props 的方式：

• 使用 Object.assign 将传入的 props 合并到一个新对象中
• 遍历 Component.defaultProps 属性，拷贝 props 中不存的属性到新对象中
• 返回新对象