使用 React 钩子设置状态后如何从组件调用事件？

Question

I have two input s whose values reflect the state of a counter.

The first input sets the state immediately as its value changes. This input can represent other parts of the application changing state.

The second input part of a component I made named LazyInput . LazyInput doesn't update state immediately , only when it's blurred, which I think gives a better usability. This input can represent the user input, like a timer to be set or the position of something on the screen.

However, I would like the LazyInput not to behave "lazily" and actually update the counter immediately only when the value is changed via the ↑ ↓ keys .

The App code (which includes the normal input ) is the following:

 const {Fragment, useState, useEffect} = React; function LazyInput({ name, value, onComplete }) { const initialValue = value; const [lazyValue, setLazyValue] = useState(value); // Sync to state changed by the regular input useEffect(() => { setLazyValue(value); }, [value]); const handleKeyDown = e => { const { key, target } = e; switch (key) { case "ArrowUp": { setLazyValue(parseFloat(lazyValue) + 1); onComplete(e); break; } case "ArrowDown": { setLazyValue(parseFloat(lazyValue) - 1); onComplete(e); break; } case "Escape": { setLazyValue(initialValue); target.blur(); break; } case "Enter": { target.blur(); break; } default: return; } }; return ( <input name={name} value={lazyValue} onChange={e => setLazyValue(e.target.value)} onKeyDown={e => handleKeyDown(e)} onBlur={e => onComplete(e)} /> ); } function App() { const [counter, setCounter] = useState(0); function handleCounterChange(e) { setCounter(parseFloat(e.target.value)); } return ( <Fragment> <div>Counter: {counter}</div> <LazyInput name="counter" value={counter} onComplete={e => handleCounterChange(e)} /> <input value={counter} type="number" onChange={e => handleCounterChange(e)} /> </Fragment> ); } ReactDOM.render(<App />, document.getElementById("app"));

 <script crossorigin src="https://unpkg.com/react@16/umd/react.development.js"></script> <script crossorigin src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script> <div id="app"></div>

The problem is: pressing the ↑ ↓ keys fires the onComplete(e) function before the setLazyValue() – I suppose because it's asynchronous – losing sync with the state.

For the same reason, pressing Esc blurs the input before resetting its value to initialValue .

I know the setState() class callback has been substituted by the useEffect() hook , but:

1. How do I call the onComplete(e) function – like any other event – from useEffect() ?
2. How do I do that only when the ↑ ↓ keys are pressed, but not for all other keys?

Thanks in advance for you help, here's an interactive sandbox .

Answer 1

I would recommend passing counter and setCounter down to LazyInput so they can be accessed directly. useEffect can be given a dependency array where it will only fire if one of its dependencies changes. However, useEffect is not right for this situation. You could assign counter as a dependency of the useEffect but it would fire every time the value of counter is changed. useEffect can only watch variables, you can't pass values to it. I suggest reading this article for learning about how useEffect works and when to use it.

https://overreacted.io/a-complete-guide-to-useeffect/

My solution passes counter and setCounter to LazyInput . If you want LazyInput to retain its own state, you can use a useEffect to watch lazyValue . When lazyValue changes, the counter should be updated.

Here is my solution:

 const {Fragment, useState, useEffect} = React; function LazyInput({ name, counter, setCounter }) { const [initialValue] = useState(counter); const handleKeyDown = e => { const { key, target } = e; switch (key) { case "ArrowUp": { setCounter(counter + 1); break; } case "ArrowDown": { setCounter(counter - 1); break; } case "Escape": { setCounter(initialValue); target.blur(); break; } case "Enter": { target.blur(); break; } default: return; } }; return ( <input name={name} value={counter} onChange={e => setCounter(e.target.value)} onKeyDown={e => handleKeyDown(e)} /> ); } function App() { const [counter, setCounter] = useState(0); return ( <Fragment> <div>Counter: {counter}</div> <LazyInput name="counter" counter={counter} setCounter={setCounter} /> <input value={counter} type="number" onChange={e => setCounter(parseFloat(e.target.value))} /> </Fragment> ); } ReactDOM.render(<App />, document.getElementById("app"));

 <script crossorigin src="https://unpkg.com/react@16/umd/react.development.js"></script> <script crossorigin src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script> <div id="app"></div>