Using Self in Swift Class Extensions

Posted 10 Aug 2017

It might be tempting to use Self as a parameter type when extending classes but Swift only allows it in a protocol or as the result of a class method invocation.

In fact, this is a semantically correct restriction for non-final classes in most cases, except when we want to use Self as an argument of the closure, think about completion handlers for example. In that case Self is used just as an another method call result which is absolutely valid option.

My favorite example here is the continuation helper for the Operation class:

• Using continuation operation instead of completion block gives control over its execution context.

• I tend to define result accessors in my Operation subclasses and I usually want to access them in a continuation.

• Accessing operation from the continuation does not introduce a retain cycle because any operation already owns its dependencies.

So we need to pass a block receiving operation that has just finished execution as a parameter. That is not allowed in a class extension.

Hm, but we can use Self that way in a protocol, right… Protocols to the rescue! The plan is to define a dummy protocol, conform our class to it and extend the protocol instead of the class.

public protocol BlockContinuable { }

extension Operation: BlockContinuable { }

public extension BlockContinuable where Self: Operation {
    public func `continue`(on queue: OperationQueue = .main, with block: @escaping (Self) -> Void) {
        let continuation = BlockOperation { block(self) }
        continuation.addDependency(self)
        queue.addOperation(continuation)
    }
}

Now we can use our continue(on:with:) extension and access operation result in a type-safe manner:

public class SampleOperation: Operation {
    public var result: String?
    public override func main() {
        result = "Hello, Continuation!"
    }
}

let operation = SampleOperation()

operation.continue {
    print($0.result ?? "operation failed")
}

OperationQueue.main.addOperation(operation)