Android :: Custom Changeable Handler When Service Finishes Downloading File

I have a Service that downloads a file from the internet. What I want is for the response (an InputStream in this case) to be handled by a custom handler that can be switched (like a strategy pattern) but I can't figure out how to do this. So basically the User of the API would be able to plug in different handlers for the response, some would parse XML, others might save files etc. I realise I could pass through the activity context and execute the method from this (given some interface) but I don't want to do this obviously, in case the Activity is closed in the meantime while the file is still downloaded.

UPDATE - I just had one idea it will work sort of but has problem if the DownloadRunnable gets changed between executions of the downloads.

The modified Runnable interface
interface DownloadRunnable {
void run(InputStream stream);
}

An enum

public enum ServiceHandler {.......................