React 源碼閱讀-6_037

React 源碼閱讀-6

lazy

React.lazy 函數能讓你像渲染常規組件一樣處理動態引入（的組件）。

React.lazy 接受一個函數，這個函數需要動態調用 import()。它必須返回一個 Promise，該 Promise 需要 resolve 一個 defalut export 的 React 組件。

React.lazy並不適合SSR

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

import {REACT_LAZY_TYPE} from 'shared/ReactSymbols';
import warning from 'shared/warning';

export function lazy<T, R>(ctor: () => Thenable<T, R>): LazyComponent<T> {
  let lazyType = {
    $$typeof: REACT_LAZY_TYPE,
    _ctor: ctor,
    // React uses these fields to store the result.
    _status: -1,
    _result: null,
  };

  if (__DEV__) {
    // In production, this would just set it on the object.
    let defaultProps;
    let propTypes;
    Object.defineProperties(lazyType, {
      defaultProps: {
        configurable: true,
        get() {
          return defaultProps;
        },
        set(newDefaultProps) {
          warning(
            false,
            'React.lazy(...): It is not supported to assign `defaultProps` to ' +
              'a lazy component import. Either specify them where the component ' +
              'is defined, or create a wrapping component around it.',
          );
          defaultProps = newDefaultProps;
          // Match production behavior more closely:
          Object.defineProperty(lazyType, 'defaultProps', {
            enumerable: true,
          });
        },
      },
      propTypes: {
        configurable: true,
        get() {
          return propTypes;
        },
        set(newPropTypes) {
          warning(
            false,
            'React.lazy(...): It is not supported to assign `propTypes` to ' +
              'a lazy component import. Either specify them where the component ' +
              'is defined, or create a wrapping component around it.',
          );
          propTypes = newPropTypes;
          // Match production behavior more closely:
          Object.defineProperty(lazyType, 'propTypes', {
            enumerable: true,
          });
        },
      },
    });
  }

  return lazyType;
}

使用前

import OtherComponent from './OtherComponent';

使用後

const OtherComponent = React.lazy(() => import('./OtherComponent'));

在組件首次渲染時，自動導入包含 OtherComponent 組件的包。

路由分割代碼

import { BrowserRouter as Router, Route, Switch } from 'react-router-dom';
import React, { Suspense, lazy } from 'react';

const Home = lazy(() => import('./routes/Home'));
const About = lazy(() => import('./routes/About'));

const App = () => (
  <Router>
    <Suspense fallback={<div>Loading...</div>}>
      <Switch>
        <Route exact path="/" component={Home}/>
        <Route path="/about" component={About}/>
      </Switch>
    </Suspense>
  </Router>
);

https://juejin.im/post/5c7d4a...
https://zh-hans.reactjs.org/d...