SafeValueForKeyPath.swift

protocol SafeValueForKeyPath {}
extension SafeValueForKeyPath where Self: CFXManagedObject {
    func safeValueForKeyPath<Value>(_ keyPath: KeyPath<Self, Value?>) -> Value? {
        let keyPathStr = NSExpression(forKeyPath: keyPath).keyPath
        return managedObjectContext?.performAndWait {
            return value(forKeyPath: keyPathStr) as? Value
        }
    }
}