React 条件渲染

元素变量
与运算符 &&
三目运算符
阻止组件渲染

React笔记

元素变量

<body>
<div id="example"></div>
<script type="text/babel">
class LoginControl extends React.Component {
  constructor(props) {
    super(props);
    this.handleLoginClick = this.handleLoginClick.bind(this);
    this.handleLogoutClick = this.handleLogoutClick.bind(this);
    this.state = {isLoggedIn: false};
  }

  handleLoginClick() {
    this.setState({isLoggedIn: true});
  }

  handleLogoutClick() {
    this.setState({isLoggedIn: false});
  }

  render() {
    const isLoggedIn = this.state.isLoggedIn;
    let button;

    if (isLoggedIn) {
      button = <LogoutButton onClick={this.handleLogoutClick} />;
    } else {
      button = <LoginButton onClick={this.handleLoginClick} />;
    }

    return (
      <div>
        <Greeting isLoggedIn={isLoggedIn} />
        {button}
      </div>
    );
  }
}

function UserGreeting(props) {
  return <h1>给你一拳</h1>;
}

function GuestGreeting(props) {
  return <h1>请先注册</h1>;
}

function Greeting(props) {
  const isLoggedIn = props.isLoggedIn;
  if (isLoggedIn) {
    return <UserGreeting />;
  }
  return <GuestGreeting />;
}

function LoginButton(props) {
  return (
    <button onClick={props.onClick}>
      注册
    </button>
  );
}

function LogoutButton(props) {
  return (
    <button onClick={props.onClick}>
      退出
    </button>
  );
}

ReactDOM.render(
  <LoginControl />,
  document.getElementById('example')
);
</script>
</body>

查看效果

与运算符 &&

true && expression 总是返回expression 而 false && expression 总是返回 false 条件是 true, && 右侧的元素就会被渲染是 false会忽略

<body>
<div id="example"></div>
<script type="text/babel">
function Mailbox(props) {
  const unreadMessages = props.unreadMessages;
  return (
    <div>
      <h1>我叫史迪奇</h1>
      {unreadMessages.length > 0 &&
        <h2>
          您浪费了{unreadMessages.length} 秒
        </h2>
      }
    </div>
  );
}

const messages = ['React', 'Re: React', 'Re:Re: React'];
ReactDOM.render(
  <Mailbox unreadMessages={messages} />,
  document.getElementById('example')
);
</script>
</body>

查看效果

三目运算符

condition ? true : false

阻止组件渲染

根据属性 warn 的值条件渲染 warn 的值是 false则组件不会渲染

<style>
button {
  height: 40px;
  width: 200px;
}
.warning {
  background-color: red;
  text-align: center;
  width: 100%;
  padding: 10px;

  font-size: 14pt;
  color: white;
}
</style>
</head>
<body>
<div id="example"></div>

<script type="text/babel">
function WarningBanner(props) {
  if (!props.warn) {
    return null;
  }

  return (
    <div className="warning">
      给你一拳
    </div>
  );
}

class Page extends React.Component {
  constructor(props) {
    super(props);
    this.state = {showWarning: true}
    this.handleToggleClick = this.handleToggleClick.bind(this);
  }

  handleToggleClick() {
    this.setState(prevState => ({
      showWarning: !prevState.showWarning
    }));
  }
  
  render() {
    return (
      <div>
        <WarningBanner warn={this.state.showWarning} />
        <button onClick={this.handleToggleClick}>
          {this.state.showWarning ? '刷新' : '点击'}
        </button>
      </div>
    );
  }
}

ReactDOM.render(
  <Page />,
  document.getElementById('example')
);
</script>
</body>

查看效果

作者: 我叫史迪奇
本文来自于: https://sdq3.link/React-conditional-rendering.html博客内容遵循署名-非商业性使用-相同方式共享 4.0 国际 (CC BY-NC-SA 4.0) 协议