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React中的条件渲染和Javascrip中的一样, 使用Javascrip运算符if或者条件运算符去创建元素来表现当前的状态, 然后让React根据它们来更新UI.

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) {
    return <UserGreeting />;
  }
  return <GuestGreeting />;
}
 
ReactDOM.render(
  // Try changing to isLoggedIn={true}:
  <Greeting isLoggedIn={false} />,
  document.getElementById('root')
);

与运算符&&

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')
);

如果条件是true, &&右侧的元素就会被渲染, 如果是false, React会忽略并跳过它.

三目运算符

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

阻止组件渲染

可以上render方法直接返回 null, 而不进行任何渲染.

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')
);

在组件的render方法中返回 null 并不会影响组件的生命周期.