React Native：useCallBack挂钩函数内部状态未定义

Question

状态在useCallBack挂钩中变得未定义，我认为它没有获得状态变量的作用域

const [selectedLocation, setSelectedLocation] = useState()
const selectLocationHandler = (event) => {
    setSelectedLocation({
        lat: event.nativeEvent.coordinate.latitude,
        lng: event.nativeEvent.coordinate.longitude
    })
    console.log('set location', selectedLocation)
}

const saveLocationPickerHandler = useCallback(() => {
    console.log('saveLocation', selectedLocation)
    if (!selectedLocation) {
        return;
    }
    props.navigation.navigate('DeliveryLocation', { pickedLocation: selectedLocation })
}, [])

设置位置Iam获取对象{“ lat”：37.775030512686214，“ lng”：-122.4273883345241，}

在控制台中未定义保存位置的地方

Answer 1

您需要提供selectedLocation作为依赖项。否则，状态更改时，回调将不会更新。

const saveLocationPickerHandler = useCallback(() => {
    console.log('saveLocation', selectedLocation)
    if (!selectedLocation) {
        return;
    }
    props.navigation.navigate('DeliveryLocation', { pickedLocation: selectedLocation })
}, [selectedLocation])

如果提供一个空数组作为依赖项，则useCallback函数将始终具有初始状态，并且从不更新（selectedLocation）。这与useEffect具有相同的行为