React hook，更新后直接访问 state 值

Question

I know this has been asked a lot and in many different ways but I couldn't find something that works for me. The functionality I want in my code is this

const TestApp() => {
  const [count, setCount] = useState(null);
  const updateAndShowCount = () => {
    setCount(count + 1);
    // This console.log should be happen after setCount() has completed it's job
    console.log(count);
  }
}

The function will be called by a button onClick effect. What it currently does is to update count and print the value of count before the update.

I want the button press to update count and immediately "after" print the updated value.

This is a test case for achieving what I want, in my actual project the function will receive a value from an input field and then make a get request using that value (with axios). I am assuming that the answer to this simple test case will work in my case as well, and since this asynchronous behaviour is something I don't quite understand yet I thought it would be sensible to get a grasp of the general concept first.

From what I have gathered though, I think I should mention that useEffect() is already being used for an initial get request, so if your answer involves using that you should assume that I don't know how to add something to useEffect() while retaining it's current functionality.

The current useEffect() :

useEffect(() => {
  axios.get("mybackend/").then(res) => {
    setRequestData(res.data);
  });
}, []);

My actual question is in bold for clarity. Any info regarding the situation as a whole is welcome as well. Thanks for reading!

Answer 1

The solution follows naturally when adding count to your request (I guess it's some query parameter of some sort), because as you then use count inside the effect, it is a dependency of the effect, thus whenever the dependency changes, the effect will run again:

useEffect(() => {
  axios.get("mybackend?count=" + count).then(res) => {
   setRequestData(res.data);
  });
}, [count]);

Note that the effect is still missing a cleanup callback, which would cancel an ongoing request, so that when count changes again while the request is still ongoing, not two requests run at the same time concurrently.

Answer 2

i'am not sure that i understood your question completely, but i think the answer is: you should use the callback in your setState, like that:

setCount(count => count + 1)