详解React 条件渲染

function UserGreeting(props) {
 return <h1>欢迎回来!</h1>;
}

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

function Greeting(props) {
 const isLoggedIn = props.isLoggedIn;
 if (isLoggedIn) {
 return <UserGreeting />;
 }
 return <GuestGreeting />;
}
 
ReactDOM.render(
 // 尝试修改 isLoggedIn={true}:
 <Greeting isLoggedIn={false} />,
 document.getElementById('example')
);

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')
);

function Mailbox(props) {
 const unreadMessages = props.unreadMessages;
 return (
 <div>
  <h1>Hello!</h1>
  {unreadMessages.length > 0 &&
  <h2>
   您有 {unreadMessages.length} 条未读信息。
  </h2>
  }
 </div>
 );
}
 
const messages = ['React', 'Re: React', 'Re:Re: React'];
ReactDOM.render(
 <Mailbox unreadMessages={messages} />,
 document.getElementById('example')
);

condition ? true : false。

render() {
 const isLoggedIn = this.state.isLoggedIn;
 return (
 <div>
  {isLoggedIn ? (
  <LogoutButton onClick={this.handleLogoutClick} />
  ) : (
  <LoginButton onClick={this.handleLoginClick} />
  )}
 </div>
 );
}

function WarningBanner(props) {
 if (!props.warn) {
 return null;
 }
 
 return (
 <div className="warning">
  警告!
 </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 ? '隐藏' : '显示'}
  </button>
  </div>
 );
 }
}
 
ReactDOM.render(
 <Page />,
 document.getElementById('example')
);