Using Self in Swift Class Extensions

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:

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.

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

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