EDIT (22 June 2020): as this question has some renewed interest, I realise there may be a few points of confusion. So I would like to highlight: the example in the question is intended as a toy example. It is not reflective of the problem. The problem which spurred this question, is in the use a third party library (over which there is limited control) that takes a callback as argument to a function. What is the correct way to provide that callback with the latest state. In react classes, this would be done through the use of this. In React hooks, due to the way state is encapsulated in the functions of React.useState(), if a callback gets the state through React.useState(), it will be stale (the value when the callback was setup). But if it sets the state, it will have access to the latest state through the passed argument. This means we can potentially get the latest state in such a callback with React hooks by setting the state to be the same as it was. This works, but is counter-intuitive.
——原问题继续如下——
我正在使用React钩子,并试图从回调中读取状态。每次回调访问它时,它都会回到默认值。
使用下面的代码。控制台会一直打印Count为:0无论我点击多少次。
function Card(title) {
const [count, setCount] = React.useState(0)
const [callbackSetup, setCallbackSetup] = React.useState(false)
function setupConsoleCallback(callback) {
console.log("Setting up callback")
setInterval(callback, 3000)
}
function clickHandler() {
setCount(count+1);
if (!callbackSetup) {
setupConsoleCallback(() => {console.log(`Count is: ${count}`)})
setCallbackSetup(true)
}
}
return (<div>
Active count {count} <br/>
<button onClick={clickHandler}>Increment</button>
</div>);
}
const el = document.querySelector("#root");
ReactDOM.render(<Card title='Example Component' />, el);
你可以在这里找到这个代码
我在回调中设置状态没有问题,只是在访问最新的状态。
如果要我猜的话,我认为任何状态的改变都会创建Card函数的一个新实例。这个回调是指向旧的那个。基于https://reactjs.org/docs/hooks-reference.html#functional-updates上的文档,我有一个想法,即在回调中调用setState,并将一个函数传递给setState,以查看是否可以从setState中访问当前状态。替换
setupConsoleCallback(() => {console.log(`Count is: ${count}`)})
with
setupConsoleCallback(() => {setCount(prevCount => {console.log(`Count is: ${prevCount}`); return prevCount})})
你可以在这里找到这个代码
这种方法也没有奏效。 编辑:实际上第二种方法是可行的。我刚才回试的时候打错了。这是正确的做法。我需要调用setState来访问之前的状态。即使我无意设置状态。
我觉得我对React类采取了类似的方法,但是。为了代码的一致性,我需要坚持使用React Effects。
如何从回调中访问最新的状态信息?