Type safe NSValueTransformer

usage.swift

func stringToInt(value: String) -> Int? {
    return value.toInt()
}

func intToString(value: Int) -> String? {
    return "\(value)"
}

NSValueTransformer.setValueTransformerWithName("StringToInt", transform: stringToInt, reverseTransform: intToString)

ValueTransformer.swift

import Foundation

private class BasicValueTransformer: ValueTransformer {
    let transform: (AnyObject?) -> (AnyObject?)

    init(transform: @escaping (AnyObject?) -> (AnyObject?)) {
        self.transform = transform
    }

    // MARK: NSValueTransformer

    override class func allowsReverseTransformation() -> Bool {
        return false
    }

    override class func transformedValueClass() -> AnyClass {
        return NSObject.self
    }

    func transformedValue(value: AnyObject?) -> AnyObject? {
        return transform(value)
    }
}

private class ReversibleValueTransformer: BasicValueTransformer {
    let reverseTransform: (AnyObject?) -> (AnyObject?)

    init(transform: @escaping (AnyObject?) -> (AnyObject?), reverseTransform: @escaping (AnyObject?) -> (AnyObject?)) {
        self.reverseTransform = reverseTransform
        super.init(transform: transform)
    }

    // MARK: NSValueTransformer

    override class func allowsReverseTransformation() -> Bool {
        return true
    }

    func reverseTransformedValue(value: AnyObject?) -> AnyObject? {
        return reverseTransform(value)
    }
}

public extension ValueTransformer {
    /**
     Registers a value transformer with a given name and transform function.

     :param: name The name of the transformer.
     :param: transform The function that performs the transformation.
     */
    class func setValueTransformerWithName<T, U>(name: String, transform: @escaping (T) -> (U?)) {
        let transformer = BasicValueTransformer { value in
            return (value as? T).flatMap {
                transform($0) as? AnyObject
            }
        }

        self.setValueTransformer(transformer, forName: NSValueTransformerName(name))
    }

    /**
     Registers a reversible value transformer with a given name and transform functions.

     :param: name The name of the transformer.
     :param: transform The function that performs the forward transformation.
     :param: reverseTransform The function that performs the reverse transformation.
     */
    class func setValueTransformerWithName<T, U>(name: String, transform: @escaping (T) -> (U?), reverseTransform: @escaping (U) -> (T?)) {
        let transformer = ReversibleValueTransformer(transform: { value in
            return (value as? T).flatMap {
                transform($0) as? AnyObject
            }
        }, reverseTransform: { value in
            return (value as? U).flatMap {
                reverseTransform($0) as? AnyObject
            }
        })

        self.setValueTransformer(transformer, forName: NSValueTransformerName(name))
    }
}