javascript - 使用嵌套字段/对象时，使用 React setState 钩子(Hook)更新后状态消失

Question

在下面的示例中，当我更新某个字段时，状态对象中的另一个字段消失/取消设置，并且我收到有关输入变得不受控制的警告。下面的例子:

const {useState} = React;

const App = () => {
  const [user, setUser] = useState({ 
    firstName: 'Mary',
    lastName: 'Poppins',
  });
  
  return (
    <div>
      First Name: <input value={user.firstName} onChange={e => { 
        setUser({firstName: e.target.value});
      }} />
      <br />
      Last Name: <input value={user.lastName} onChange={e => { 
        setUser({lastName: e.target.value});
      }} />
      <br />
      {JSON.stringify(user, null, 2)}
    </div>
  );
};

ReactDOM.render(<App />, document.querySelector('#app'));

<script src="https://unpkg.com/react@16.7.0-alpha.0/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@16.7.0-alpha.0/umd/react-dom.development.js"></script>
<div id="app"></div>

Answer 1

正如 React 文档所述:

However, unlike this.setState in a class, updating a state variable always replaces it instead of merging it.

因此，使用setUser不会对对象进行浅层合并，而是用整个对象替换状态，这会导致另一个字段消失。在执行 setUser 时，您可以使用 Object.assign 或 spread (...) 运算符将对象合并在一起。

const {useState} = React;

const App = () => {
  const [user, setUser] = useState({ 
    firstName: 'Mary',
    lastName: 'Poppins',
  });
  
  return (
    <div>
      First Name: <input value={user.firstName} onChange={e => { 
        setUser(Object.assign({}, user, {firstName: e.target.value}));
      }} />
      <br />
      Last Name: <input value={user.lastName} onChange={e => { 
        setUser(Object.assign({}, user, {lastName: e.target.value}));
      }} />
      <br />
      {JSON.stringify(user, null, 2)}
    </div>
  );
};

ReactDOM.render(<App />, document.querySelector('#app'));

<script src="https://unpkg.com/react@16.7.0-alpha.0/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@16.7.0-alpha.0/umd/react-dom.development.js"></script>
<div id="app"></div>

或者，创建一个名为 useMergeState 的自定义 Hook 帮助您像通常的 setState 一样自动合并。