Question

I’m developing an app that downloads some information from internet. At the beginning of the app, a dialog appears and asks the user if he wishes to download that information. If he does, it starts the downloading process (in a different thread).

This is the code:

Activity:

public void onCreate(Bundle savedInstanceState) 
{     
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    // Initialize preferences
    settings = PreferenceManager.getDefaultSharedPreferences(this);

    // Start Database
    db.open();
    db.close();

    /**Check for updates**/
    checkForDBUpdates(settings.getInt("db_version", 0));
}

private void checkForDBUpdates(int Version)
{
    // Check if Internet connection is available
    boolean online = isOnline();

    if (online) 
    {
        final UpdateNDGS update = new UpdateNDGS(this, Version);

        boolean update_status = update.check();

        if (update_status)
        {
            System.out.println("READY TO UPDATE!");

            Command updateCommand = new Command() 
            {
                public void execute() 
                {
                    update.getUpdate();
                    update.database();
                    //Download Complete, show list
                                    startNormasList();
                }
            };
            AlertDialog updateDialog = createUpdateDialog(this, updateCommand);
            updateDialog.show();
        }
        else
        {
            //Nothing to download, show list
            startNormasList();
        }
    }
    else
    {
        //offline, show list
        startNormasList();
    }
}

Everything is actually working, but if I wish to add functionalitites in the future it can get messy.

So my question is:
How can I improve this code?

Wouldn’t be better to add a listener that fires the event “show list” when download process is complete or never happened?

How can I do such thing? I’ve been reading a lot, but couldn’t find anything helpfull.

Answer 1

Having listeners are a good way to add extensibility to your code as well decoupling the events and the handlers. Here are the steps that you need to do to achieve that:

• Decide the interface that the listener should implement in the case of the event is triggered. This includes the parameter that you need to pass back to the listener in the case that “show list” event is triggered. Something like (I don’t know the parameters you need, so this is very generalized structure)

    public interface ShowListEventLisener {
        public void onShowList(Object... params);
    }

• In the class where the event would occur, add a method to register/add a listener for the event. Note that this could be set of listeners or just one. Following code illustrate adding one listener only

    public void setOnShowListListener(ShowListEventLisener showListListener) {
       this.showListListener = showListListener;
    }

This would allow you add flexibility in case you need different behavior for “show list” event

• Now you just need to trigger the event at appropriate time. You could achieve this similar to the following code from the code that does the download

   if( isDownloadFinished) {
       // call the listeners with the parameters that you need to pass back
       this.showListListener.onShowList(paramsThatYouNeed);
   }

Note that since you are executing download in different Thread, you might want to use Handler/AsyncTask before triggering the event. The example above is generalized, but should be enough for you to follow to build your own custom listener.