React Native：状态在 useCallBack 挂钩函数内变得未定义

Question

状态在 useCallBack 挂钩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 { "lat": 37.775030512686214, "lng": -122.4273883345241, }

其中 savelocation 在控制台中未定义

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 的行为相同