源码

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.
            //将当前location逐个与switch下的route上的path进行匹配，遇到一个route匹配成功，后面的都不会执行if中的逻辑
          React.Children.forEach(this.props.children, child => {
             //如果已经匹配成功了match != null，之后if中的逻辑不会执行
            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;
            }
          });
          //渲染匹配到的route
          return match
            ? React.cloneElement(element, { location, computedMatch: match })
            : null;
        }}
      </RouterContext.Consumer>
    );
  }
}