React - 条件渲染

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

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

function UserGreeting(props) {
  return <h1>Welcome back!</h1>
}

function GuestGreeting(props) {
  return <h1>Please sign up.</h1>
}

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

ReactDOM.render(
  <LoginControl />,
  document.getElementById('root')
);

在 js 中，true && expression 返回 expression，而 false && expression 返回 false。

阻止组件渲染
可以让 render 返回 null，不进行任何渲染。

function WarningBanner(props) {
  if (!props.warn) {
    return null;
  }
  return (
    <div className="warning">
      warning!
    </div>
  );
}

class Page extends React.Component {
  constructor(props) {
    super(props);
    this.state = {showWarning: true};
    this.handleToggleClick = this.handleToggleClick.bind(this);
  }
  
  handleToggleClick() {
    this.setState(state => ({
      showWarning: !state.showWarning
    }));
  }
  render() {
    return (
      <div>
        <WarningBanner warn={this.state.showWarning} />
        <button onClick={this.handleToggleClick}>
          {this.state.showWarning ? 'Hide' : 'Show'}
        </button>
      </div>
    );
  }
}

ReactDOM.render(
  <Page />,
  document.getElementById('root')
);

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