Why fragments, and when to use fragments instead of activities?

In Android API 11+, Google has released a new class called Fragment.

In the videos, Google suggests that whenever possible (link1, link2), we should use fragments instead of activities, but they didn't explain exactly why.

What's the purpose of fragments and some possible uses of them (other than some UI examples that can be easily be achieved by simple views/layouts)?

My question is about fragments:

1. What are the purposes of using a fragment?
2. What are the advantages and disadvantages of using fragments compared to using activities/views/layouts?

Bonus questions:

3. Can you give some really interesting uses for fragments? Things that Google didn't mention in their videos?
4. What's the best way to communicate between fragments and the activities that contain them?
5. What are the most important things to remember when you use fragments? Any tips and warnings from your experience?

#1 & #2 what are the purposes of using a fragment & what are the advantages and disadvantages of using fragments compared to using activities/views/layouts?

Fragments are Android's solution to creating reusable user interfaces. You can achieve some of the same things using activities and layouts (for example by using includes). However; fragments are wired in to the Android API, from HoneyComb, and up. Let me elaborate;

• The ActionBar. If you want tabs up there to navigate your app, you quickly see that ActionBar.TabListener interface gives you a FragmentTransaction as an input argument to the onTabSelected method. You could probably ignore this, and do something else and clever, but you'd be working against the API, not with it.

• The FragmentManager handles «back» for you in a very clever way. Back does not mean back to the last activity, like for regular activities. It means back to the previous fragment state.

• You can use the cool ViewPager with a FragmentPagerAdapter to create swipe interfaces. The FragmentPagerAdapter code is much cleaner than a regular adapter, and it controls instantiations of the individual fragments.

• Your life will be a lot easier if you use Fragments when you try to create applications for both phones and tablets. Since the fragments are so tied in with the Honeycomb+ APIs, you will want to use them on phones as well to reuse code. That's where the compatibility library comes in handy.

• You even could and should use fragments for apps meant for phones only. If you have portability in mind. I use ActionBarSherlock and the compatibility libraries to create "ICS looking" apps, that look the same all the way back to version 1.6. You get the latest features like the ActionBar, with tabs, overflow, split action bar, viewpager etc.

Bonus 2

The best way to communicate between fragments are intents. When you press something in a Fragment you would typically call StartActivity() with data on it. The intent is passed on to all fragments of the activity you launch.

Not sure what video(s) you are referring to, but I doubt they are saying you should use fragments instead of activities, because they are not directly interchangeable. There is actually a fairly detailed entry in the Dev Guide, consider reading it for details.

In short, fragments live inside activities, and each activity can host many fragments. Like activities, they have a specific lifecycle, unlike activities, they are not top-level application components. Advantages of fragments include code reuse and modularity (e.g., using the same list view in many activities), including the ability to build multi-pane interfaces (mostly useful on tablets). The main disadvantage is (some) added complexity. You can generally achieve the same thing with (custom) views in a non-standard and less robust way.

A Fragment is a piece of an application's user interface or behavior that can be placed in an Activity which enable a more modular activity design. It will not be wrong if we say a fragment is a kind of subactivity.

Following are important points about a fragment:

1. A fragment has its own layout and its own behavior with its own lifecycle callbacks.

2. You can add or remove fragments in an activity while the activity is running.

3. You can combine multiple fragments in a single activity to build a multi-pane UI.

4. A fragment can be used in multiple activities.

5. The fragment life cycle is closely related to the lifecycle of its host activity.

6. When the activity is paused, all the fragments available in the acivity will also be stopped.

7. A fragment can implement a behavior that has no user interface component.

8. Fragments were added to the Android API in Android 3 (Honeycomb) with API version 11.

For more details, please visit the official site, Fragments.