构造函数被调用两次React Component

Question

我的react组件的构造函数被调用了两次，但我不知道为什么。我正在使用react-redux来存储我的应用程序的语言。我有一个功能可根据浏览器的语言设置默认语言。LoginPage是第一个获得渲染的组件，因此我在其构造函数中调用了我的函数。基本上，它所做的是比较并调度一个动作。当我使用redux开发人员工具检查状态时，我看到它被调度了两次。我在构造函数中打印了伪数据，它也被打印了两次。

LoginPage.js

import React from 'react';
import {connect} from 'react-redux';
import {startLogin} from '../actions/auth';
import {setLanguage} from '../actions/lang';

export class LoginPage extends React.Component{

  constructor(props){
    super(props);
    this.setDefaultLanguage();
    console.log('i am constructor');
  }

  changeLanguage = (e) => {
    const lan = e.target.value;
    this.props.setLanguage(lan);
  };

  setDefaultLanguage = () => {
    const defaultLanguage = navigator.language || navigator.userLanguage || 'en-US';

    if(defaultLanguage == 'es'){
      this.props.setLanguage(defaultLanguage);
    }else{
      this.props.setLanguage('en');
    }
  }

  render(){
    return(
      <div className="box-layout">
        <div className="box-layout__box">
          <h1 className="box-layout__title">Expensify</h1>
          <p>It\'s time to get your expenses under control.</p>
          <button className="button" onClick={this.props.startLogin}>Log in with google</button>
          <select className="select" onChange={this.changeLanguage}>
            <option value="en">English</option>
            <option value="es">Español</option>
          </select>
        </div>
      </div>
    )
  };
}

const mapDispatchToProps = (dispatch) => ({
  startLogin: () => dispatch(startLogin()),
  setLanguage: (language) => dispatch(setLanguage(language))
});

export default connect(undefined, mapDispatchToProps)( LoginPage);

App.js

import React from 'react';
import ReactDom from 'react-dom';
import {Router, Route, Switch} from 'react-router-dom';
import createHistory from 'history/createBrowserHistory';
import ExpenseDashBoardPage from '../components/ExpenseDashBoardPage';
import AddExpensePage from '../components/AddExpensePage';
import EditExpensePage from '../components/EditExpensePage';
import NotFoundPage from '../components/NotFoundPage';
import LoginPage from '../components/LoginPage';
import PrivateRoute from './PrivateRoute';
import PublicRoute from './PublicRoute';

export const history = createHistory();

const AppRouter = () => (
  <Router history={history}>
    <div>
      <Switch>
        <PublicRoute path="/" component={LoginPage} exact={true} />
        <PrivateRoute path="/dashboard" component={ExpenseDashBoardPage} />
        <PrivateRoute path="/create" component={AddExpensePage} />
        <PrivateRoute path="/edit/:id" component={EditExpensePage} />
        <Route component={NotFoundPage} />
      </Switch>
    </div>

  </Router>
)

export default AppRouter;

Answer 1

我遇到了同样的问题，因为我在我的公共路由中重定向了用户。所以它渲染了组件两次。如果您正在使用

history.push('/');

这将渲染组件两次。

Answer 2

首先，您不应在构造函数中调用Redux操作或任何类型的AJAX。这些事情应该在componentDidMount()。

其次，我会要求商店提供语言作为道具的一部分。如果未定义，componentDidMount()请致电setDefaultLanguage()。

我至少会执行以下操作：

export class LoginPage extends React.Component {
  componentDidMount() {
    if (!this.props.lang) {
        this.setDefaultLanguage();
    }
  }

  changeLanguage = (e) => {
    const lan = e.target.value;
    this.props.setLanguage(lan);
  };

  setDefaultLanguage = () => {
    const defaultLanguage = navigator.language || navigator.userLanguage || 'en-US';

    if(defaultLanguage == 'es'){
      this.props.setLanguage(defaultLanguage);
    }else{
      this.props.setLanguage('en');
    }
  }

  render() {
    return(
      <div className="box-layout">
        <div className="box-layout__box">
          <h1 className="box-layout__title">Expensify</h1>
          <p>It\'s time to get your expenses under control.</p>
          <button className="button" onClick={this.props.startLogin}>Log in with google</button>
          <select className="select" onChange={this.changeLanguage}>
            <option value="en">English</option>
            <option value="es">Español</option>
          </select>
        </div>
      </div>
    )
  };
}

const mapStateToProps = state => ({
    // Assuming `steate.lang` is where you would set the language.
    lang: state.lang
});

const mapDispatchToProps = (dispatch) => ({
  startLogin: () => dispatch(startLogin()),
  setLanguage: (language) => dispatch(setLanguage(language))
});

const mergeProps = (stateProps, dispatchProps) => ({ ...stateProps, ...dispatchProps });

export default connect(mapStateToProps, mapDispatchToProps, mergeProps)(LoginPage);

Answer 3

如果你检查正在渲染的 React Dev Tools，你会看到类似的东西connect(App)，App所以你的 console.log 将被调用两次。这是因为 connect 是一个高阶组件。

如果您将this.setDefaultLanguage()（并且console.log您可以看到它的实际情况）移出构造函数并移入componentDidMount生命周期挂钩，那么它应该可以解决您的问题。