条件渲染

React 中的条件渲染和 JS 一样,使用 if 或者条件运算符来创建元素表现当前的状态

if 条件

function UserGreeting(props) {
  return <h1>Welcome back!</h1>;
}
function GuestGreeting(props) {
  return <h1>Please sign up</h1>;
}

在创建一个 Greeting,他会依据用户是否登录来决定显示上面哪个组件

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

ReactDOM.render(<Greeting flag={false} />, document.getElementById("root"));

实际效果

function Greeting(props) {
  const isLoggedIn = props.isLoggedIn;
  if (isLoggedIn) {
    return <UserGreeting />;
  }
  return <GuestGreeting />;
}
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 = null;
    if (isLoggedIn) {
      button = <LogoutButton onClick={this.handleLogoutClick} />;
    } else {
      button = <LoginButton onClick={this.handleLoginClick} />;
    }

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

ReactDOM.render(<LoginControl />, document.getElementById("example"));

与&&逻辑运算符

function Mailbox(props) {
  const unreadMessages = props.unreadMessages;
  return (
    <div>
      <h1>Hello!</h1>
      {unreadMessages.length > 0 && (
        <h2>You have {unreadMessages.length} unread messages.</h2>
      )}
    </div>
  );
}

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

三目运算符

render() {
  const isLoggedIn = this.state.isLoggedIn;
  return (
    <div>
      The user is <b>{isLoggedIn ? 'currently' : 'not'}</b> logged in.
    </div>
  );
}

当你通过表达式不希望返回 null,从而不进行渲染

function WarningBanner(props) {
  if (!props.warn) {
    return null;
  }

  return <div className="warning">Warning!</div>;
}

总结

1. class 外面可以写方法传进来,props 来渲染

2. 利用 if 判断,表达式等等