Overriding AnyObject to a more specific type

🎤 General Discussion Swift
1

Hi there,

I’ve hit a bit of a wall on trying to figure out how best to solve the following situation:

I’ve got a protocol that requires the following method for all objects that implement it:

protocol OperationDataProvider { func provideData() -> OperationResultType? }

I’m trying to declare several “types” that all behave in a similar fashion. I’ve created the following protocol in hopes that each type that implements it can override the “AnyObject” defined within the protocol:

protocol OperationResultType { var result: AnyObject? { get } var error: NSError? { get } }

I’m then trying to create other types that can specify the type of “result”:

TestOperationResult<T>: OperationResultType { var result: T? var error: NSError? }

I’m using a generic to try and infer the type based on the type when the TestOperationResult is created.

Unfortunately, I’m running into an issue as it doesn’t appear that I can override result to be anything other than AnyObject

Is it possible to achieve what I’m attempting to do?

Any help and/or direction would be very much appreciated.

Thank you!
Frank

2

I would always use OperationResultType there instead of AnyObject. Essentially it is still a generic but one guaranteed to conform to the protocol, which is what you want.

3

If each conformer to OperationResultType should be able to specify its type of result, then in principle you would need an associated type.

protocol OperationResultType {
  associatedtype Result : AnyObject
  var result : Result? { get }
  var error: NSError? { get } 
}

However, it’s not quite that simple, because OperationDataProvider.provideData() can’t now return an OperationResultType any more than it could return an Equatable or an Optional - it has ‘Self’ requirements - and you need to know what the associated type is to make use of it. (e.g. instead of provideData you might need a provideIntData to return an OperationResultType whose result was of type Int). You can make generic types, but you can’t use them to hide what the type is.