Thanks for the reply. I think that is not possible. I only ever get access to the Data object when the completion callback is called.
In the AGSLoadable Protocol there is a property called
public var loadStatus: AGSLoadStatus { get }
which is stated to be KVO compliant. It should be easy to add another kvo compliant property that has the download progress. How can I file an enhancement request to the API? Having a download of possible hundreds of MB of data triggered by a user without any indication how long the download will take is out of the question.