将 nil 值转换为 NSNull 的字典扩展

Question

由于您不能将Dictionary<String, AnyObject?> 直接转换为NSDictionary，我想编写一个Dictionary 扩展来进行这种转换（用NSNull 代替任何nils）。

但为了做到这一点，我的Dictionary 扩展必须限制为值为Optional<T> where T:AnyObject 的字典。我该如何为此编写类型约束？

Answer 1

您不能在字典中添加 nil。因此，您必须将其强制转换为someType?，这样您才能拥有密钥。

借用 Q/A 的 OptionalConvertible 解决方案：

protocol OptionalConvertible {
    typealias WrappedValueType = AnyObject
    func toOptional() -> WrappedValueType?
}

extension Optional: OptionalConvertible {
    typealias WrappedValueType = Wrapped

    // just to cast `Optional<Wrapped>` to `Wrapped?`
    func toOptional() -> WrappedValueType? {
        return self
    }
}

---

extension Dictionary where Value: OptionalConvertible, Key: NSCopying {
    //dict: Dictionary<Key, Optional<Value>>
    func convertToNSDictionary() -> NSDictionary {

        let mutableDict : NSMutableDictionary = NSMutableDictionary()

        for key in self.keys {
            if let maybeValue = self[key] {
                if let value = maybeValue.toOptional() {
                    mutableDict[key] = value as? AnyObject
                } else {
                    mutableDict[key] = NSNull()
                }
            }
        }
        return mutableDict

    }
}

---

var optionalObject : UIView? = nil

var dict : [NSString:AnyObject?] = [:]
dict["alpha"] = 1
dict["beta"] = 2
dict["delta"] = optionalObject as AnyObject? // force a nil into the dict

dict // ["beta": {Some 2}, "alpha": {Some 1}, "delta": nil]

let nsdict = dict.convertToNSDictionary() // ["alpha": 1, "beta": 2, "delta": {NSObject}]

---

或者更实用的方法：

extension NSDictionary {

    static func fromDictionary<Key: Hashable, Value:AnyObject where Key: NSCopying>(dictionary:Dictionary<Key, Value>) -> NSDictionary {

        let mutableDict : NSMutableDictionary = NSMutableDictionary()

        for key in dictionary.keys {
            if let value = dictionary[key] {
                mutableDict[key] = value
            } else {
                mutableDict[key] = NSNull()
            }
        }
        return mutableDict
    }

    static func fromDictionary<Key: Hashable, Value:AnyObject where Key: NSCopying>(dict: Dictionary<Key, Optional<Value>>) -> NSDictionary {

        let mutableDict : NSMutableDictionary = NSMutableDictionary()

        for key in dict.keys {
            if let maybeValue = dict[key] {
                if let value = maybeValue {
                    mutableDict[key] = value
                } else {
                    mutableDict[key] = NSNull()
                }
            }
        }
        return mutableDict
    }
}