Custom changeable handler when a Service finishes downloading a 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 开发者_开发知识库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.

How might I do this?

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 {
    DOWNLOAD_HANDLER;

    private DownloadRunnable handler=null;

    /**Called from the Service*/
    public void doHandler(InputStream stream) {
        if(handler==null) throw new IllegalStateException("You must set the handler first: setDownloadHandler()");
        this.handler.run(stream);
    }

     /**Set before theservice runs*/
     public void setHandler(DownloadRunnable r) {
         this.handler = r;
     }
}

The client can set what happens for each enum

ServiceHandler.DOWNLOAD_HANDLER.setHandler(new DownloadRunnable() {

        @Override
        public void run(InputStream stream) {
            // The user can do anything they want with the stream
        }
    });

The Service runs this when the input stream is available

ServiceHandler.DOWNLOAD_HANDLER.doHandler(stream);

This kind of works but is not so elegant.

1. What if the user runs the same operation twice with different DownloadRunnables set (before the first is complete)

2. The user has to set the Handler first is a separate enum object (not so user friendly or intuitive).

3. Maybe I could create a Map of DownloadRunnables each with an ID (static field - auto increment) that gets passed back and the correct runnable can then get executed...but all this is getting more complex than I had hoped.

Step #1: Write an interface describing the strategy API

Step #2: Write implementations of the interface for each strategy pattern

Step #3: Have the Activity either pass in a strategy pattern instance or some identifier of the pattern to use (so the Service creates the object) when it requests the download

This seems too simple. I think I am missing the point of your question.