When do I need a base activity and base fragment?

Question

In lot of examples I see, all the activities and fragments extends from base activity and base fragments. 2 questions:

1. When should I use it?
2. What kind of code should go in it?

Answer 1

Usually I use a base Activity/Fragment when I need to do some work in some of life-cycle callbacks of all of my Activitys/Fragments.

For example if you use Butter Knife (very recommended), you need to call Butterknife.bind(Activity a) after calling setContentView. So it's better if you create a base activity and extend the setContentView method in it like this:

@Override
public void setContentView(int layoutResID) {
    super.setContentView(layoutResID);
    ButterKnife.bind(this);
}

In child activities when you call setContentView at the beginning of onCreate (after calling super.onCreate), ButterKnife.bind would be called automatically.

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Another use case is when you want to implement some helper methods. For example if you are calling startActivity multiple times in your activities, this would be a real headache:

startActivity(new Intent(this, NextActivity.class));

You can add a start method to your base activity like this:

protected void start(Class<? extends BaseActivity> activity) {
    startActivity(new Intent(this, activity));
}

and start the next activity like:

start(NextActivity.class);

Answer 2

Other activities can extend BaseActivity. If you define common elements in BaseActivities and all other activities extend BaseActivities, then all activities will have these common elements, for example: custom menu, custom bar, design layout or some query logic...etc.

Similar with BaseFragment. I usually log onCreate, onAtach, onPause events in BaseFragment. So I see these logs in all other Fragment that extend BaseFragment. Also, you can very easy and in one class turn-off these logs for all fragment. (useful before publishing realise)