Android Interview Questions Part 3 - Fragment

39. What are intent Filters?

Specifies the type of intent that the activity/service can respond to.

40. Describe fragments:

Fragment is a UI entity attached to Activity. Fragments can be reused by attaching in different activities. Activity can have multiple fragments attached to it. Fragment must be attached to an activity and its lifecycle will depend on its host activity.

41. Describe fragment lifecycle

• onAttach() : The fragment instance is associated with an activity instance.The fragment and the activity is not fully initialized. Typically you get in this method a reference to the activity which uses the fragment for further initialization work.
• onCreate() : The system calls this method when creating the fragment. You should initialize essential components of the fragment that you want to retain when the fragment is paused or stopped, then resumed.
• onCreateView() : The system calls this callback when it’s time for the fragment to draw its user interface for the first time. To draw a UI for your fragment, you must return a View component from this method that is the root of your fragment’s layout. You can return null if the fragment does not provide a UI.
• onActivityCreated() : The onActivityCreated() is called after the onCreateView() method when the host activity is created. Activity and fragment instance have been created as well as the view hierarchy of the activity. At this point, view can be accessed with the findViewById() method. example. In this method you can instantiate objects which require a Context object
• onStart() : The onStart() method is called once the fragment gets visible.
• onResume() : Fragment becomes active.
• onPause() : The system calls this method as the first indication that the user is leaving the fragment. This is usually where you should commit any changes that should be persisted beyond the current user session.
• onStop() : Fragment going to be stopped by calling onStop()
• onDestroyView() : Fragment view will destroy after call this method
• onDestroy() :called to do final clean up of the fragment’s state but Not guaranteed to be called by the Android platform.

42. What is the difference between fragments & activities. Explain the relationship between the two.

An Activity is an application component that provides a screen, with which users can interact in order to do something whereas a Fragment represents a behavior or a portion of user interface in an Activity (with its own lifecycle and input events, and which can be added or removed at will).

43. When should you use a fragment rather than an activity?

• When there are ui components that are going to be used across multiple activities.
• When there are multiple views that can be displayed side by side (viewPager tabs)
• When you have data that needs to be persisted across Activity restarts (such as retained fragments)

44. Difference between adding/replacing fragment in backstack?

• replace removes the existing fragment and adds a new fragment. This means when you press back button the fragment that got replaced will be created with its onCreateView being invoked.
• add retains the existing fragments and adds a new fragment that means existing fragment will be active and they wont be in ‘paused’ state hence when a back button is pressed onCreateView is not called for the existing fragment(the fragment which was there before new fragment was added).
• In terms of fragment’s life cycle events onPause, onResume, onCreateView and other life cycle events will be invoked in case of replace but they wont be invoked in case of add.

45. Why is it recommended to use only the default constructor to create a Fragment?

The reason why you should be passing parameters through bundle is because when the system restores a fragment (e.g on config change), it will automatically restore your bundle. This way you are guaranteed to restore the state of the fragment correctly to the same state the fragment was initialised with.

46. You’re replacing one Fragment with another — how do you ensure that the user can return to the previous Fragment, by pressing the Back button?

We need to save each Fragment transaction to the backstack, by calling addToBackStack() before you commit() that transaction

47. Callbacks invoked during addition of a fragment to back stack and while popping back from back stack:

addOnBackStackChangedListener is called when fragment is added or removed from the backstack. Checkout this link for reference.

48. What are retained fragments?

By default, Fragments are destroyed and recreated along with their parent Activity’s when a configuration change occurs. Calling setRetainInstance(true) allows us to bypass this destroy-and-recreate cycle, signaling the system to retain the current instance of the fragment when the activity is recreated.

49. Difference between FragmentPagerAdapter vs FragmentStatePagerAdapter?

• FragmentPagerAdapter: the fragment of each page the user visits will be stored in memory, although the view will be destroyed. So when the page is visible again, the view will be recreated but the fragment instance is not recreated. This can result in a significant amount of memory being used. FragmentPagerAdapter should be used when we need to store the whole fragment in memory. FragmentPagerAdapter calls detach(Fragment) on the transaction instead of remove(Fragment).
• FragmentStatePagerAdapter: the fragment instance is destroyed when it is not visible to the User, except the saved state of the fragment. This results in using only a small amount of Memory and can be useful for handling larger data sets. Should be used when we have to use dynamic fragments, like fragments with widgets, as their data could be stored in the savedInstanceState.Also it won’t affect the performance even if there are large number of fragments.