[swift-users] Swift 4 emulating Decoder behaviour

[Joanna Carter]

Greetings

I notice that, with Swift 4, I can decode an object like this :

	• let retrievedSpot = try decoder.decode(ParkingSpot.self, from: retrievedData)  

And that will return a ParkingSpot, as expected, into retrievedSpot.

However, I thought I would try the same technique with one of my pet projects…

I have a protocol  and an implementing struct :

	• public protocol PropertyProtocol  
	• {  
	•   static var propertyType: Any.Type { get }  
	•   
	•   var untypedValue: Any? { get }  
	• }  
	•   
	• public struct Property<valueT : DefaultValueProvider> : PropertyProtocol  
	• {  
	•   public static var propertyType: Any.Type  
	•   {  
	•     return valueT.self  
	•   }  
	•   
	•   public var untypedValue: Any?  
	•   {  
	•     return value  
	•   }  
	•   
	•   public var value = valueT()  
	• }  

Now, I want to be able to use a "factory" method to create an instance of Property<T>, bound to its parameter type. So, I followed the same principal as Decoder :

	• struct PropertyFactory  
	• {  
	•   static func createProperty<T>(_ type: T.Type) -> PropertyProtocol where T : DefaultValueProvider  
	•   {  
	•     return type.createProperty()  
	•   }  
	• }  

DefaultValueProvider is defined as follows and String is extended to conform to it :

	• public protocol DefaultValueProvider  
	• {  
	•   init()  
	• }  
	•   
	• extension String : DefaultValueProvider { }  

Now, this works fine if I pass a known type :

	• let pproperty = PropertyFactory.createProperty(String.self)  

But, if I hold the type to be passed in in an Any.Type or DefaultValueProvider.Type variable, then doing this :

	•     let type: Any.Type = String.self  
	•   
	•     if let propertyType = type as? DefaultValueProvider.Type  
	•     {  
	•       let p = PropertyFactory.createProperty(propertyType)  
	•      
	•       …  

Fails to compile with the message : Cannot invoke 'createProperty' with an argument list of type '(DefaultValueProvider.Type)'

Likewise Decoder.decode(…) will not accept storing the type in an Any.Type or Decodable.Type variable.

I find this odd and perplexing. Is this a known issue or has nobody yet realised that this could be useful ?

Joanna

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Joanna Carter
Carter Consulting