react-router-dom 源码分析&简单实现

今天药忘吃喽~ 2022-12-19 14:28 82阅读 0赞

源码地址：  
[https://github.com/ReactTraining/react-router][https_github.com_ReactTraining_react-router]

使用教程:  
[https://reactrouter.com/web/guides/quick-start][https_reactrouter.com_web_guides_quick-start]

## 基本使用 ##

import React from "react";
    
    import { 
      BrowserRouter as Router,
      Route,
      Link,
      Switch,
      withRouter
    } from "./my-react-router-dom/";
    
    import HomePage from "./pages/HomePage";
    import UserPage from "./pages/UserPage";
    import LoginPage from "./pages/LoginPage";
    import ProductPage from './pages/ProductPage';
    import _404Page from "./pages/_404Page";
    
    function App() { 
      return (
        <div className="App">
          <Router>
              <Link to="/">首页</Link>
              <Link to="/user">用户中心</Link>
              <Link to="/login">登录</Link>
              <Link to="/product/123">商品</Link>
    
              <Switch>
                <Route
                  path="/"
                  exact
                  component={ HomePage}
                />
                <Route
                  path="/product/:id"
                  render={ () => <Product />}
                />
                <Route path="/user" component={ UserPage} />
                <Route path="/login" component={ LoginPage} />
                <Route component={ _404Page} />
              </Switch>
            </Router>
        </div>
      );
    }
    
    export default App;

接下来本文将根据使用方式从外向内逐一介绍，并简单实现

本文的例子将只考虑 BrowserRouter，HashRouter 可以自己扩展或到源码处阅读，其实道理都差不多

--------------------

## 基本概念快速介绍 ##

1.  **Route 是怎么监听路由然后根据路由跳转后的地址实施不同组件的渲染？**  
      
    react-router-dom 使用了 React 的 ContextAPI 进行值的传递，其中 context 的来源如图所示  
    ![react-router-dom提供源解析][react-router-dom]  
      
    
2.  **history 对象哪来的？**  
      
    一般来说可以通过 `window` 对象来获取，但是在 `React` 中，引入了 `react-router-dom` 的时候就会引入一个 `history依赖`，所以我们只要使用这个依赖就可以了

--------------------

## BrowserRouter ##

**BrowserRouter 完成的事**

1.  通过 `history` 的 `createBrowserRouter` 创建 `history对象`
2.  通过参数形式传递给通用的 `<Router/>`

**Router**

1.  挂载时通过传入的 `history` 的 `listen()` 监听路由
2.  在 `componentWillUnmount` 中执行取消监听方法
3.  在返回的 `Provider` 中的 `value` 传入一个对象，键值对分别为 `history`、`location` 和 `根match`

注意：  
    熟悉 contextAPI 的同学都知道，`Provider` 所处的组件发生更新时，`Provider` 会对其 `value` 进行一次浅比较，当 `value` 传入的是引用数据类型的时候，更新组件就相当于给 `value` 赋值了一个新的引用数据，而引用数据类型比较的是地址，所以当这个 `Provider` 所处的组件发生更新时，`Provider` 及其后代组件都会发生更新，这样会造成性能浪费。。然而为什么这里传入对象是合适的呢？

回想一下我们平时使用 `<Router>` 的位置，通常都是放在最外层的组件，所以在这个情况下不用担心这种情况

`BrwoserRouter.js`

import React, { Component} from "react";
    import { createBrowserHistory} from "history";
    import Router from "./Router";
    
    export default class BrowserRouter extends Component { 
      constructor(props) { 
        super(props);
        this.history = createBrowserHistory();
      }
      render() { 
        return <Router history={ this.history} children={ this.props.children} />;
      }
    }

`Router`

import React, { Component} from "react";
    import { RouterContext} from "./RouterContext";
    
    export default class Router extends Component { 
      static computeRootMatch(pathname) { 
        return { path: "/", url: "/", params: { }, isExact: pathname === "/"};
      }
    
      constructor(props) { 
        super(props);
        this.state = { 
          location: props.history.location
        };
        // 监听路由
        this.unlisten = props.history.listen(location => { 
          this.setState({ location});
        });
      }
    
      componentWillUnmount() { 
        if (this.unlisten) { 
          this.unlisten();
        }
      }
    
      render() { 
        return (
          <RouterContext.Provider
            value={ { 
              history: this.props.history,
              location: this.state.location,
              match: Router.computeRootMatch(this.state.location.pathname)
            }}>
            { this.props.children}
          </RouterContext.Provider>
        );
      }
    }

这里将 `Router` 分离封装出来是因为方便 `BrowserRouter` 和 `HashRouter` 的使用

--------------------

## Link ##

本质是一个 `a标签`

点击时：

1.  阻止默认跳转
2.  通过 `BrowserRouter` 传递的 `context` 的 `history` 跳转  
    `context.history.push(to)`

import React, { useCallback} from "react";
    import { RouterContext} from "./RouterContext";
    
    export default function Link({ to, children, ...restProps}) { 
      const context = React.useContext(RouterContext);
      const handleClick = useCallback(event => { 
        event.preventDefault();
        context.history.push(to);
      }, [context.history, to])
    
      return (
        <a href={ to} { ...restProps} onClick={ handleClick}>
          { children}
        </a>
      );
    }

--------------------

## Switch ##

**Swtich 做的事：**

1.  得到 `根Router` 传递下来的 `context`，去其中的 `location`
2.  遍历 `children`，选出正确的 `element(Route)`

`Switch.js`

import React, { Component} from "react";
    import { RouterContext} from "./RouterContext";
    import matchPath from "./matchPath";
    
    // 独占路由
    // 渲染与该地址匹配的第一个子节点 <Route> 或者 <Redirect>。
    class Switch extends Component { 
      render() { 
        console.log('Switch rendered');
        return (
          <RouterContext.Consumer>
            { context => { 
              const location = this.props.location || context.location;
              let match; //标记匹配
              let element; //记录匹配的元素
    
              React.Children.forEach(this.props.children, child => { 
                if (match == null && React.isValidElement(child)) { 
                  element = child;
    
                  match = child.props.path
                    ? matchPath(location.pathname, child.props)
                    : context.match;
                }
              });
              
              return match
                ? React.cloneElement(element, { computedMatch: match})
                : null;
            }}
          </RouterContext.Consumer>
        );
      }
    }
    export default Switch;

注意：  
    路由更新时只会执行 `Consumer` 中的内容，不会重新触发 `Switch组件` 的 `render()`

--------------------

## Route ##

**Route 渲染方式 优先级从高到低：**

1.  children (func)
2.  component (component)
3.  render (func)

**Route 渲染逻辑**  
![Route渲染逻辑][Route]  
`Route.js`

import React, { Component} from "react";
    import { RouterContext} from "./RouterContext";
    import matchPath from "./matchPath";
    
    export default class Route extends Component { 
      render() { 
        return (
          <RouterContext.Consumer>
            { context => { 
              const { location} = context;
              const { path, children, component, render, computedMatch} = this.props;
              const match = computedMatch
                ? computedMatch
                : path
                ? matchPath(location.pathname, this.props)
                : context.match; 
    
              const props = { 
                ...context,
                match
              };
              //match children, component, render, null
              // 不匹配 children(function), null
              return (
                <RouterContext.Provider value={ props}>
                  { match
                    ? children
                      ? typeof children === "function"
                        ? children(props)
                        : children
                      : component
                      ? React.createElement(component, props)
                      : render
                      ? render(props)
                      : null
                    : typeof children === "function"
                    ? children(props)
                    : null}
                </RouterContext.Provider>
              );
            }}
          </RouterContext.Consumer>
        );
      }
    }

`matchPath.js`

大概知道这个文件的作用就行，第一次看可以不用深究

import pathToRegexp from "path-to-regexp";
    
    const cache = { };
    const cacheLimit = 10000;
    let cacheCount = 0;
    
    function compilePath(path, options) { 
      const cacheKey = `${ options.end}${ options.strict}${ options.sensitive}`;
      const pathCache = cache[cacheKey] || (cache[cacheKey] = { });
    
      if (pathCache[path]) return pathCache[path];
    
      const keys = [];
      const regexp = pathToRegexp(path, keys, options);
      const result = {  regexp, keys };
    
      if (cacheCount < cacheLimit) { 
        pathCache[path] = result;
        cacheCount++;
      }
    
      return result;
    }
    
    /** * Public API for matching a URL pathname to a path. */
    function matchPath(pathname, options = { }) { 
      if (typeof options === "string" || Array.isArray(options)) { 
        options = {  path: options };
      }
    
      const {  path, exact = false, strict = false, sensitive = false } = options;
    
      const paths = [].concat(path);
    
      return paths.reduce((matched, path) => { 
        if (!path && path !== "") return null;
        if (matched) return matched;
    
        const {  regexp, keys } = compilePath(path, { 
          end: exact,
          strict,
          sensitive
        });
        const match = regexp.exec(pathname);
    
        if (!match) return null;
    
        const [url, ...values] = match;
        const isExact = pathname === url;
    
        if (exact && !isExact) return null;
    
        return { 
          path, // the path used to match
          url: path === "/" && url === "" ? "/" : url, // the matched portion of the URL
          isExact, // whether or not we matched exactly
          params: keys.reduce((memo, key, index) => { 
            memo[key.name] = values[index];
            return memo;
          }, { })
        };
      }, null);
    }
    
    export default matchPath;

--------------------

## 其他组件 ##

### Redirect ###

import React, { Component} from "react";
    import { RouterContext} from "./RouterContext";
    
    export default class Redirect extends Component { 
      render() { 
        return (
          <RouterContext.Consumer>
            { context => { 
              const { history} = context;
              const { to, push = false} = this.props;
              return (
                <LifeCycle
                  onMount={ () => { 
                    push ? history.push(to) : history.replace(to);
                  }}
                />
              );
            }}
          </RouterContext.Consumer>
        );
      }
    }
    
    class LifeCycle extends Component { 
      componentDidMount() { 
        if (this.props.onMount) { 
          this.props.onMount.call(this, this);
        }
      }
      render() { 
        return null;
      }
    }

注意：  
    这里用到了一个工具人函数，借用了一下它的 `componentDidMount` 去做路由跳转

### Prompt ###

`Prompt.js`

import React from 'react';
    import { RouterContext} from './RouterContext';
    import LifeCycleFragment from './LifeCycleFragment';
    
    export default function Prompt({ message, when = true}) { 
      return (
        <div>
          <RouterContext.Consumer>
            { 
              context => { 
                if (!when) return null;
                let method = context.history.block;
             
                return (
                  <LifeCycleFragment
                    onMounted={ self => { 
                      self.release = method(message);
                    }}
                    onUnmount={ self => { 
                      self.release();
                    }}
                  />
                )
              }
            }
          </RouterContext.Consumer>
        </div>
      )
    }

`LifeCycleFragment.js`

import React from 'react';
    
    export default class LifeCycleFragment extends React.Component { 
      componentDidMount() { 
        if (this.props.onMounted) { 
          this.props.onMounted.call(this, this);
        }
      }
    
      // 在源码中这里还考虑了组件更新的情况
      // xxx
      
      componentWillUnmount() { 
        if (this.props.onUnmount) { 
          this.props.onUnmount.call(this, this);
        }
      }
      
      render () { 
        return null
      }
    }

## 钩子函数 ##

import { RouterContext} from "./RouterContext";
    import { useContext} from "react";
    
    export function useHistory() { 
      return useContext(RouterContext).history;
    }
    
    export function useLocation() { 
      return useContext(RouterContext).location;
    }
    export function useRouteMatch() { 
      return useContext(RouterContext).match;
    }
    export function useParams() { 
      const match = useContext(RouterContext).match;
      return match ? match.params : { };
    }

感谢你能看到这里，有任何疑问欢迎评论或私信~

希望更深一步研究请直接研究[源码][https_github.com_ReactTraining_react-router]

[https_github.com_ReactTraining_react-router]: https://github.com/ReactTraining/react-router
[https_reactrouter.com_web_guides_quick-start]: https://reactrouter.com/web/guides/quick-start
[react-router-dom]: /images/20221120/6f9387712b0048da85ea72d3a5f0bba9.png
[Route]: https://img-blog.csdnimg.cn/20201108193426481.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L1RNX0dvZHNwZWVk,size_16,color_FFFFFF,t_70#pic_center