反应 set(a) 和 set(a => a) 的区别

Question

case1和case2有什么区别？

const [a, setA] = useState(0);

setA(a + 1);      //case 1
setA(a => a + 1); //case 2

Answer 1

• setA(a + 1);a将从当前附件的值更新。
• setA(a => a + 1);将从之前的状态值更新

尝试将其中任意一个的多个入队并查看差异。

const clickHandler1 = () => {
  // assume count equals some number n
  setCount(count + 1); // update queued, count === n, count = n + 1
  setCount(count + 1); // update queued, count === n, count = n + 1
  setCount(count + 1); // update queued, count === n, count = n + 1
  // when processed the count will be n + 1
};

const clickHandler2 = () => {
  // assume count equals some number n
  setCount((count) => count + 1); // update queued, count === n + 0, count = prevCount + 1
  setCount((count) => count + 1); // update queued, count === n + 1, count = prevCount + 1
  setCount((count) => count + 1); // update queued, count === n + 2, count = prevCount + 1
  // now when processed each call uses the result of the previous update
  // count will be n + 1 + 1 + 1, or n + 3
};

如果下一个状态取决于任何先前的状态，您应该使用功能状态更新。当包含按时间间隔或在数组循环中更新状态的回调时，功能状态更新也很方便。

请参阅此工作演示：