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OnTheNatureOfFragmentsAndTheActivityLifeCycle

This app demonstrates relationship between Activity, ViewPager, FragmentPagerAdapter and Fragments in the activity and fragment life-cycle.

I've written an article on the lifecycle of fragments using the basic ViewPager as an example. You can read the article before or after playingwith this app.

This app is the default Tabbed Activity template with SwipeViews navigation with a lot of log messages and a little UI added on.

Roles:

  • Activity - This is more or less the app.
  • ViewPager - Facilitates the swiping between pages
  • FragmentPagerAdapter (PagerAdapter) - Manages the views (fragments) and supplies them to the pager.
  • FragmentManager - Manages the fragment life-cycle.
  • Fragment - The view for a page.

Things to Try

Prep

Download and run this app in in Android Studio.

What ViewPager is doing

Don't rotate the screen to avoid confusion.

First just swipe between pages 1 and 2. Nothing insteresting appears to be happening.

But now swipe to page 3; you will notice it has been created many times.

The pager needs the adjacent views ready to show on swiping so they are created off screen in preparation.

You will be able to see this happening in the debug log in android studio.

The identifier saved in the constructor remains newInstance-number all though this. That means the same object instance created using the newInstance() method is re-used, with onCreated called many times. The view is destroyed to save memory, but the object remains with all its fields intact.

  • An fragment instance may have onCreateView() be called multiple times.

Now Rotate

Screen rotation is more interesting. Rotate the screen.

A new instance of the fragment is created using the default constructor. All field vars are lost. The bundle is the only thing that persists.

  • Fields in the fragment instance are lost on rotation so the instance cannot be relied upon to persist data.
  • Writing code that requires onCreateView() to know the difference between a re-used and new isntance would be complicated.

Other Things

These kinds of changes can also be witnessed by navigating away from the app and loading many other large apps to use the RAM.

Read my article to understand what is going on.