Can I use generics to cast NSNumber to T in swift? - swift

Question

What I want
func safeGet<T>() -> T {
let value = magic()
if let typedValue = value as? T {
return typedValue
}
}
The reason this doesn't work is the fact that you can't do <NSNumber> as Int in swift
What do I put in the <what do I put here?> placeholder?
func safeGet<T>() -> T {
let value = magic()
if let typedValue = value as? NSNumber {
return <what do I put here?>
} else if let typedValue = value as? T {
return typedValue
}
}

Answer 1

In the end I did this to be able to get typed values from the dictionary in a way that throws errors. It's used like this
let name:String = dictionary.safeGet("name")
or
let name = dictionary.safeGet("name") as String`
The source code:
import Foundation
extension Dictionary {
func safeGet<T>(key:Key) throws -> T {
if let value = self[key] as? AnyObject {
if let typedValue = value as? T {
return typedValue
}
let typedValue: T? = parseNumber(value)
if typedValue != nil {
return typedValue!
}
let typeData = Mirror(reflecting: value)
throw generateNSError(
domain: "DictionaryError.WrongType",
message: "Could not convert `\(key)` to `\(T.self)`, it was `\(typeData.subjectType)` and had the value `\(value)`"
)
} else {
throw generateNSError(
domain: "DictionaryError.MissingValue",
message: "`\(key)` was not in dictionary. The dictionary was:\n\(self.description)"
)
}
}
private func parseNumber<T>(value: AnyObject) -> T? {
if Int8.self == T.self {
if let numericValue = value as? NSNumber {
return numericValue.charValue as? T
} else if let stringValue = value as? String {
return Int8(stringValue) as? T
}
} else if Int16.self == T.self {
if let numericValue = value as? NSNumber {
return numericValue.shortValue as? T
} else if let stringValue = value as? String {
return Int16(stringValue) as? T
}
} else if Int32.self == T.self {
if let numericValue = value as? NSNumber {
return numericValue.longValue as? T
} else if let stringValue = value as? String {
return Int32(stringValue) as? T
}
} else if Int64.self == T.self {
if let numericValue = value as? NSNumber {
return numericValue.longLongValue as? T
} else if let stringValue = value as? String {
return Int64(stringValue) as? T
}
} else if UInt8.self == T.self {
if let numericValue = value as? NSNumber {
return numericValue.unsignedCharValue as? T
} else if let stringValue = value as? String {
return UInt8(stringValue) as? T
}
} else if UInt16.self == T.self {
if let numericValue = value as? NSNumber {
return numericValue.unsignedShortValue as? T
} else if let stringValue = value as? String {
return UInt16(stringValue) as? T
}
} else if UInt32.self == T.self {
if let numericValue = value as? NSNumber {
return numericValue.unsignedIntValue as? T
} else if let stringValue = value as? String {
return UInt32(stringValue) as? T
}
} else if UInt64.self == T.self {
if let numericValue = value as? NSNumber {
return numericValue.unsignedLongLongValue as? T
} else if let stringValue = value as? String {
return UInt64(stringValue) as? T
}
} else if Double.self == T.self {
if let numericValue = value as? NSNumber {
return numericValue.doubleValue as? T
} else if let stringValue = value as? String {
return Double(stringValue) as? T
}
} else if Float.self == T.self {
if let numericValue = value as? NSNumber {
return numericValue.floatValue as? T
} else if let stringValue = value as? String {
return Float(stringValue) as? T
}
} else if String.self == T.self {
if let numericValue = value as? NSNumber {
return numericValue.stringValue as? T
} else if let stringValue = value as? String {
return stringValue as? T
}
}
return nil
}
private func generateNSError(domain domain: String, message: String) -> NSError {
return NSError(
domain: domain,
code: -1,
userInfo: [
NSLocalizedDescriptionKey: message
])
}
}