使用useRefHook的另一种功能组件：

const inputEl = useRef(null);

function handleFocus() {
  inputEl.current.select();
}

<input
  type="number"
  value={quantity}
  ref={inputEl}
  onChange={e => setQuantityHandler(e.target.value)}
  onFocus={handleFocus}
/>

谢谢，我很感激。我是这样做的：

var input = self.refs.value.getDOMNode();
            input.focus();
            input.setSelectionRange(0, input.value.length);

Functional component：

const handleFocus = (event) => event.target.select();
const Input = (props) => <input type="text" value="Some something" onFocus={handleFocus} />

---

ES6 class component：

class Input extends React.Component {
    handleFocus = (event) => event.target.select();

    render() {
        return (
            <input type="text" value="Some something" onFocus={this.handleFocus} />
        );
    }
}

---

React.createClass：

React.createClass({
    handleFocus: function(event) {
      event.target.select();
    },

    render: function() {
      return (
        <input type="text" value="Some something" onFocus={this.handleFocus} />
      );
    },
})