从源码对react-router v5进行原理分析[重编制](二)

前言

此篇文章默认读者已经掌握react-router的api, 或是对其有所了解;

在看这篇文章之前, 需要先对react-router和react-router-dom有一个简单的了解;

首先来看官方对两者的描述

The core of React Router (react-router)

DOM bindings for React Router (react-router-dom)

react-router是React Router的核心, 实现了路由的核心功能;

react-router-dom是React Router的DOM绑定, 提供了浏览器环境下的功能, 比如<Link>, <BrowserRouter>等组件;

可以理解为:

react-router-dom基于react-router, 安装依赖的时候只需要安装react-router-dom就好了;

react-router结构分析

根据官方文档, 使用react-router-dom进行路由管理, 首先我们需要选择一个路由模式:

• BrowserRouter: History模式
• HashRouter: Hash模式
• MemoryRouter: 在没有url的情况下, 使用Memory记住路由, 常见在React Native中使用, 这里不进行讨论

以下都以create-react-app为例, 选择History模式, 也就是在最外层使用<BrowserRouter>组件:

index.tsx

import React from 'react';
import ReactDOM from 'react-dom';
import {BrowserRouter as Router} from 'react-router-dom';
import App from './App';

ReactDOM.render(
  <Router>
    <App />
  </Router>,
  document.getElementById('root')
);

然后在被<BrowserHistory>组件包裹的组件中可以使用<Route>进行路由划分:

App.tsx

import React from 'react';
import {Route} from 'react-router-dom';

const Page1.React.FC = props => {
  return <div>Page1</div>;
};

const Page2.React.FC = props => {
  return <div>Page2</div>;
};

function App() {
  return (
    <div className="App">
      <Route path="/page1" component={Page1}></Route>
      <Route path="/page2" component={Page2}></Route>
    </div>
  );
}

export default App;

以上就是react-router的大概结构, 下面将对`react-router-dom的组件进行源码分析;

BrowserHistory

<BrowserHistory>和<HashHistory>的代码结构和逻辑相似, 这里只对<BrowserHistory>作分析;

以下是<BrowserHistory>核心代码逻辑分析:

定义<BrowserHistory>的prop-types

import PropTypes from "prop-types";

class BrowserRouter extends React.Component {
  // 此处代码略去
}

BrowserRouter.propTypes = {
  basename: PropTypes.string,
  children: PropTypes.node,
  forceRefresh: PropTypes.bool,
  getUserConfirmation: PropTypes.func,
  keyLength: PropTypes.number
};

<BrowserHistory>的核心逻辑

使用history的createBrowserHistory方法, 将props作为参数, 创建一个history实例, 并将history传入Router组件中:

import { Router } from "react-router";
import { createBrowserHistory as createHistory } from "history";

class BrowserRouter extends React.Component {
  history = createHistory(this.props);

    render() {
    return <Router history={this.history} children={this.props.children} />;
  }
}

从源码中可以看出, <BrowserHistory>是一个注入了history的<Router>组件;

Router

react-router-dom中的<Router>实际上就是react-router的Router, 此处直接对react-router的<Router>进行源码分析:

定义<Router>的prop-types

import PropTypes from "prop-types";

Router.propTypes = {
  children: PropTypes.node,
  history: PropTypes.object.isRequired,
  staticContext: PropTypes.object
};

此处的staticContext是<staticRouter>中传入<Router>的属性, 这里不做分析;

<Router>的路由渲染逻辑

<Router>的构造函数中,声明this.state.location, 使用history的监听函数对history.location进行监听, 并将history.listen的返回值赋值给this.unlisten:

this.state = {
  location: props.history.location
};

this._isMounted = false;
this._pendingLocation = null;

this.unlisten = props.history.listen(location => {
  if (this._isMounted) {
    this.setState({ location });
  } else {
    this._pendingLocation = location;
  }
});

之所以在构造函数中就对history.location进行监听, 而不是在componentDidMount中进行监听, 官方是这么解释的:

This is a bit of a hack. We have to start listening for location changes here in the constructor in case there are any <Redirect>s on the initial render. If there are, they will replace/push when they mount and since cDM fires in children before parents, we may get a new location before the <Router> is mounted.

大概意思就是, 因为子组件会比父组件更早渲染完成, 以及<Redirect>的存在, 若是在<Router>的componentDidMount生命周期中对history.location进行监听, 则有可能在监听事件注册之前, history.location已经由于<Redirect>发生了多次改变, 因此我们需要在<Router>的constructor中就注册监听事件;

接下来, 在componentWillUnmount生命周期中进行移除监听函数操作:

componentWillUnmount() {
  if (this.unlisten) {
    this.unlisten();
    this._isMounted = false;
    this._pendingLocation = null;
  }
}

react-router中使用context进行组件通信

在<Router>中, 使用<RouterContext.Provider>进行路由数据传递(history,location, match以及staticContext), 使用<HistoryContext.Provider>进行history数据传递, 子组件(<Route>或是<Redirect>等)可以通过<RouterContext.Consumer>以及<HistoryContext.Consumer>对上层数据进行接收; HistoryContext和RouterContext都是使用mini-create-react-context创建的context, 而mini-create-react-context工具库定义如下:

(A smaller) Polyfill for the React context API

mini-create-react-context是React context API的Polyfill, 因此可以直接将mini-create-react-context当成React的context;

import React from "react";
import HistoryContext from "./HistoryContext.js";
import RouterContext from "./RouterContext.js";

class Router extends React.Component {
  static computeRootMatch(pathname) {
    return { path: "/", url: "/", params: {}, isExact: pathname === "/" };
  }

  render() {
    return (
      <RouterContext.Provider
        value={{
          history: this.props.history,
          location: this.state.location,
          match: Router.computeRootMatch(this.state.location.pathname),
          staticContext: this.props.staticContext
        }}
        >
        <HistoryContext.Provider
          children={this.props.children || null}
          value={this.props.history}
        />
      </RouterContext.Provider>
    );
  }
}

Switch

<Switch> is unique in that it renders a route exclusively

即使有多个路由组件成功匹配, Switch也只展示一个路由

<Switch>必须作为<Router>的子组件进行使用, 若是脱离<Router>, 则会报错:

"You should not use <Switch> outside a <Router>"

定义<Switch>中传入的prop-types

import PropTypes from "prop-types";

Switch.propTypes = {
  children: PropTypes.node,
  location: PropTypes.object
};

<Switch>的渲染逻辑

<Switch>使用<RouterContext.Consumer>进行路由数据接收; <Switch>对路由组件进行顺序匹配, 使用React.Children.forEach对<Switch>的子组件进行遍历, 每次遍历逻辑如下:

使用React.isValidElement判断子组件是否为有效的element:

• 有效: 则进入下个步骤;
• 无效: 结束此轮循环, 进行下一轮循环;

声明path:

const path = child.props.path || child.props.from;

注: <Route>使用path进行路由地址声明, <Redirect>使用from进行重定向来源地址声明;

接着判断path是否存在:

• 存在path: 表示子组件存在路由映射关系, 使用matchPath对path进行匹配, 判断路由组件的路径与当前location.pathname是否匹配:

  • 若是匹配, 则对子组件进行渲染, 并将matchPath返回的值作为computedMatch传递到子组件中, 并且不再对其他组件进行渲染;
  • 若是不匹配, 则直接进行下次循环; 注意: location可以是外部传入的props.location, 默认为context.location;
• 不存在path: 表示子组件不存在路由映射关系, 直接渲染该子组件, 并将context.match作为computedMatch传入子组件中;

matchPath是react-router的一个公共api, 源码中注释对matchPath的介绍如下:

Public API for matching a URL pathname to a path.

主要用于匹配路由, 匹配成功则返回一个match对象, 若是匹配失败, 则返回null;

import React from 'react';
import RouterContext from "./RouterContext.js";
import matchPath from "./matchPath.js";

/**
 * The public API for rendering the first <Route> that matches.
 */
class Switch extends React.Component {
  render() {
    return (
      <RouterContext.Consumer>
        {context => {
          invariant(context, "You should not use <Switch> outside a <Router>");

          const location = this.props.location || context.location;

          let element, match;

          // We use React.Children.forEach instead of React.Children.toArray().find()
          // here because toArray adds keys to all child elements and we do not want
          // to trigger an unmount/remount for two <Route>s that render the same
          // component at different URLs.
          React.Children.forEach(this.props.children, child => {
            if (match == null && React.isValidElement(child)) {
              element = child;

              const path = child.props.path || child.props.from;

              match = path
                ? matchPath(location.pathname, { ...child.props, path })
                : context.match;
            }
          });

          return match
            ? React.cloneElement(element, { location, computedMatch: match })
            : null;
        }}
      </RouterContext.Consumer>
    );
  }
}

Route

The Route component is perhaps the most important component in React Router to understand and learn to use well. Its most basic responsibility is to render some UI when its path matches the current URL

<Route>可能是react-router中最重要的组件, 它最基本的职责是在其路径与当前URL匹配时呈现对应的UI组件;

与其他非<Router>组件一样, 若是不被<RouterContext.Provider>包裹, 则会报错:

"You should not use <Switch> outside a <Router>"

定义<Route>的prop-types:

import PropTypes from "prop-types";

Route.propTypes = {
  children: PropTypes.oneOfType([PropTypes.func, PropTypes.node]),
  component: (props, propName) => {
    if (props[propName] && !isValidElementType(props[propName])) {
      return new Error(
        `Invalid prop 'component' supplied to 'Route': the prop is not a valid React component`
      );
    }
  },
  exact: PropTypes.bool,
  location: PropTypes.object,
  path: PropTypes.oneOfType([
    PropTypes.string,
    PropTypes.arrayOf(PropTypes.string)
  ]),
  render: PropTypes.func,
  sensitive: PropTypes.bool,
  strict: PropTypes.bool
};

<Route>的渲染逻辑

与其它路由组件一样, 使用<RouterContext.Consumer>接收全局路由信息; <Route>的逻辑比较简单, 主要判断path与当前路由是否匹配, 若是匹配则进行渲染对应路由组件, 若是不匹配则不进行渲染, 核心代码如下:

const match = this.props.computedMatch
  ? this.props.computedMatch // <Switch> already computed the match for us
  : this.props.path
  ? matchPath(location.pathname, this.props)
  : context.match;

...

<RouterContext.Provider value={props}>
  {
    props.match
    ? children
      ? typeof children === "function"
          ? __DEV__
              ? evalChildrenDev(children, props, this.props.path)
                : children(props)
            : children
        : component
          ? React.createElement(component, props)
            : render
              ? render(props)
                : null
    : typeof children === "function"
      ? __DEV__
          ? evalChildrenDev(children, props, this.props.path)
            : children(props)
        : null
  }
</RouterContext.Provider>

注: 根据上面代码, 不论props.match是否为true, 当<Route>的children为函数时都会进行渲染;

总结

本篇文章对react-router的部分核心组件进行源码解读; react-router使用<Context.Provider>向路由树传递路由信息, <Route>等组件通过<Context.Consumer>接收路由信息, 匹配路径并渲染路由组件, 以及与上篇文章讲到的history的紧密配合, 才让react-router如此优秀; 下一篇文章将对剩余组件以及react-router的hooks进行源码解读!