Android - How to avoid duplicate code between activities

Question

This is a bit of a general question, but I will give a specific example for you.

I have a bunch of activities in an App. In all of the activities, there is a Facebook button. When you click the button it takes you to a specific Facebook page. I wish for the button to behave exactly the same way on every page.

Right now, in every single Activity, I create an onClickListener() for the Facebook button and make the intent and start the activity. It's the same code in every single Activity.

What is the best way to write this code once and include it in multiple activities? Is there anyway to include other .java files?

One solution that I know would work, is to make a base CustomActivity class that extends Activity and then have all activities extend CustomActivity. Then put my onClickListener() code in CustomActivity. I'm new to Java though and I wasn't sure if that was the best approach or not. Some of my Activities already extend other custom activity classes as is, so extending things that extend more things might get kinda messy, I dunno.

UPDATE

Playing the devil's advocate here: Lets say I went with the inheritance route and I create some CustomActivity that I want my Activities to extend. CustomActivity would contain a bunch of general code that I need to use for all Activities, including but not limited to the Facebook button functionality. What happens when there is an Activity that I need to use generic code from the CustomActivity but there is no Facebook button in that specific Activity?

Answer 1

A common base class is perhaps the best approach. (It doesn't work quite so well if some of your activities extend Activity and some extend Activity subclasses (such as ListActivity).

An alternate approach is to create a separate class that implements the logic of your click listener. This doesn't eliminate all duplicate code — each activity still needs to instantiate and register a listener — but the logic for what to do will only need to be written once in the listener class.

In either alternative, you might consider assigning the android:onClick attribute to the button. That way you don't need to register a click listener; you just need to implement the target method in each activity. This is particularly useful with the base class approach.

UPDATE

Suppose you go the inheritance route and you want an activity with no Facebook button. If you are using the android:onClick technique, then you don't have to do anything different in your code — since no button will invoke your onClick method, the method will just sit there doing nothing. If you are installing an OnClickListener in code, then you just need to test that the button exists (i.e., that findViewById() did not return null) before registering the listener.

Answer 2

Generally a common base class is NOT the best approach (although it's certainly valid).

This took me (and every OO programmer who "gets" OO that I know of) a while to really grok, but you should use inheritance as sparingly as you possibly can. Every time you do it you should ask yourself if there is REALLY no other way to do this.

One way to find out is to be very strict with the "is-a" test--if you call your base activity a "Facebook Activity", could you really say that each child "is" a Facebook activity? Probably not. Also if you decided to add in Twitter to some of the pages (but not others), how do you do this?

Not that inheritance is completely out! A great solution might be to extend a control to launch your facebook activity and call it a facebook button--have it encapsulate all the stuff you need to do to connect to facebook. Now you can add this to any page you want by simply dragging it on (I'm pretty sure android tools let you add new components to the pallet). It's not "Free" like extending your activity class, but in the long run it will cost you a lot less stress.

You probably won't believe me now, we all need to learn from our own experience, just keep this in mind and use it to evaluate your code as you evolve it over time.

--edit, comment response--

You can encapsulate any facebook activity you think you will use a lot in it's own class--get it to a bare minimum so you can just add it to any class in a one-liner.

At some point, however, you may decide that it's STILL too much boilerplate, I totally understand. At that point you COULD use an abstract base activity like you suggest, but I wouldn't hard-code it to handle facebook explicitly, instead I'd have it support behaviors such as facebook (and maybe others), and turn-on these behaviors as desired. You could then tell it NOT to add the facebook behavior to a given screen if you like, or add in Twitter to some of them.

You can make this boilerplate minimum, for instance if you want "Standard" functionality, you shouldn't have to do anything special, if you wish to disable facebook you might start your constructor with:

super(DISABLE_FACEBOOK_BEHAVIOR);

and if you want one that also enables Twitter you could use:

super(DISABLE_FACEBOOK_BEHAVIOR, ENABLE_TWITTER_BEHAVIOR);

with a constructor like AbstractAction(BehaviorEnum... behaviors).

This is more flexible and you actually can say that each if your activities IS-A "behavior supporting activity" with a clear conscience.

It is, of course, a perfectly good approach to be less flexible at first and refactor into a pattern like this later when you need to, just be on the look-out for your inheritance model causing problems so you don't let it mess you up for too long before you fix it.

Answer 3

Well, extending things is the principle of OOP, so I don't think this is a problem to have more than one level of subclasses. The solution you thought about is in my opinion the best.

Answer 4

Absolutely. Use inheritance to gain some reusability as you should with OOP. You'll find, as you progress, that there are gonna be more and more things you'd like to reuse in your activities -- things more complex than an onClickListener for a FB button -- so it's a great idea to start building a nice, reusable "super" activity that you can inherit from.