Value transformer that returns true for strings that are non-empty

IsNotNilOrEmptyValueTransformer.swift

import Foundation

/// Given a `String?`, return an `NSNumber` Boolean value that is true if the value is non-nil and not empty.
///
/// This is useful for binding the `enabled` property of a button to a string value that is bound to a text field.

final class IsNotNilOrEmptyValueTransformer: NSValueTransformer {
    
    override class func transformedValueClass() -> AnyClass {
        return NSNumber.self
    }
    
    override class func allowsReverseTransformation() -> Bool {
        return false
    }
    
    override func transformedValue(value: AnyObject?) -> AnyObject? {
        guard let stringValue = value as? String else { return NSNumber(bool: false) }
        return NSNumber(bool: !stringValue.isEmpty)
    }
    
    class func registerValueTransformer() {
        let instance = IsNotNilOrEmptyValueTransformer()
        NSValueTransformer.setValueTransformer(instance, forName: "IsNotNilOrEmptyValueTransformer")
    }
}