javascript - react native : state is getting undefined inside useCallBack hook function

Question

状态在 useCallBack Hook 内变得未定义，我认为它没有获得状态变量的范围

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 })
}, [])

设置位置我正在获取对象{ “纬度”:37.775030512686214， “液化石油气”:-122.4273883345241， }

控制台中未定义保存位置

Answer 1

您需要提供 selectedLocation 作为依赖项。否则，如果状态发生变化，回调将不会更新。

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

如果您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永远不会更新(selectedLocation)。 这与 useEffect 具有相同的行为