Dictionary extension to convert nil values to NSNull - swift

Question

Since you can't cast a Dictionary<String, AnyObject?> into a NSDictionary directly, I want to write a Dictionary extension that will do that conversion (substituting NSNull for any nils).
But in order to do that, my Dictionary extension must be constrained to dicts whose values are Optional<T> where T:AnyObject. How do I write the type constraint for that?

Answer 1

You can't add nil to a dict. So you have to force it in there with a cast to someType? just so you have a key.
Borrowing from this Q/A for the OptionalConvertible solution:
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}]
Or a more practical approach :
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
}
}