小编典典

如何从字符串值Swift中删除Optional

swift

我想使用不带可选扩展名的String值。我使用以下代码从firebase解析此数据:

Database.database().reference(withPath:
    "Locations").child("Cities").observe(.value, with: { (snapShot) in
        if snapShot.exists() {
            let array:NSArray = snapShot.children.allObjects as NSArray

            for child in array {
                let snap = child as! DataSnapshot

                let cityName = snap.key
                let cityNameString = "\(cityName)"
                if snap.value is NSDictionary {
                    let data:NSDictionary = snap.value as! NSDictionary
                    let lat = data.value(forKey: "lat")
                    let lng = data.value(forKey: "lng")
                    let radius = data.value(forKey: "radius")
                    let latstring = "\(lat)"
                    let lngstring = "\(lng)" 
                    let radiusstring = "\(radius)"

                    let city = CityObject(name: cityNameString , lat: latstring , lng: lngstring, radius: radiusstring)
                    print("Value is", laststring)  
                    self.selectCity(cityObject: city)
                }

            }
        }
    })

解析此数据后,我尝试打印例如latstring并得到以下结果:

值是可选的(52.523553)

我的CityObject如下所示:

class CityObject{
var name: String?
var lat: String?
var lng: String?
var radius: String?

init(name: String?, lat: String?, lng: String?, radius: String?){
    self.name = name
    self.lat = lat
    self.lng = lng
    self.radius = radius
}

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2020-07-07

共1个答案

小编典典

就像@GioR所说的那样,该值是Optional(52.523553),因为latstring的类型是隐式的:String?。这是由于let lat =
data.value(forKey:“ lat”)将返回字符串?隐式设置lat的类型。
有关值的文档,请参阅https://developer.apple.com/documentation/objectivec/nsobject/1412591-value(forKey
:)

Swift有多种处理nil的方法。这三个可能对您有帮助,nil合并运算符:

??

如果可选结果为nil,则此运算符将提供默认值:

let lat: String = data.value(forKey: "lat") ?? "the lat in the dictionary was nil!"

警卫声明

guard let lat: String = data.value(forKey: "lat") as? String else {
    //Oops, didn't get a string, leave the function!
}

Guard语句使您可以将可选变量转换为等效的非可选变量,如果恰好为nil,则可以退出该函数

如果让

if let lat: String = data.value(forKey: "lat") as? String {
    //Do something with the non-optional lat
}
//Carry on with the rest of the function

希望这可以帮助^^

2020-07-07