v4.app.Fragment when is it created and destroyed?

Question

How exactly v4.app.Fragment works? I have viewPager with 7 Fragments. I was sure that when moving between fragments, depending on the situation each fragment can be automatically destroyed and created. However it seems not to be true.

I decided to play a bit with debugging and set some flag for my Fragment class object, something like

class MyClass extends Fragment {
public boolean myFlag=false;

When I set ex. true somewhere in the code which is run only once (ex. true is set after button click), it seems that this value is true until the app ends. Thus it suggest the object is kept all the time in memory.

However when user moves between Fragments onCreateView and onViewStateRestored methods seems to be called.

And now I feel quite confused. If Fragments are not destroyed, why those methods are executed by Android?

Do I have the guarantee that my Fragment object will be kept always with all fields (when user only moves beteween Fragments and doesn't leave the app)?

If not how should I save and restore its state? public void onSaveInstanceState(Bundle savedInstanceState) method seems to be only run when user left the app, not when user moves between Fragments.

Have you seen any good tutorial concerning ViewPager and Fragments?

Answer 1

And now I feel quite confused. If Fragments are not destroyed, why those methods are executed by Android?

If you look at this diagram, Fragments do not need to be destroyed to call onCreateView().

Do I have the guarantee that my Fragment object will be kept always with all fields (when user only moves beteween Fragments and doesn't leave the app)?

This depends on what you are doing. If your case is dealing with ViewPager it really boils down to what PagerAdapter you use and your ViewPager configuration.

If not how should I save and restore its state? public void onSaveInstanceState(Bundle savedInstanceState) method seems to be only run when user left the app, not when user moves between Fragments.

It depends under what conditions you are wanting to restore state. For example, for orientation changes you can call setRetainInstance(true) on the Fragment and the Fragment's state will be preserved when there is a configuration change given your Fragment is not on the backstack.

Have you seen any good tutorial concerning ViewPager and Fragments?

The Android Guide has a good tutorial.

Answer 2

You can re-initialize all the values of a specific fragment in it's onCreate(...) method. So that when ever it is navigated to, it will have the default values that you setup in your onCreate(...) method.

Here are some useful links related to the Implementation of Fragments in a ViewPager:
1. http://thepseudocoder.wordpress.com/2011/10/13/android-tabs-viewpager-swipe-able-tabs-ftw/
2. http://androidtrainningcenter.blogspot.co.il/2012/10/viewpager-example-in-android.html
3. http://manishkpr.webheavens.com/android-viewpager-example/
4. http://thepseudocoder.wordpress.com/2011/10/05/android-page-swiping-using-viewpager/

I hope this helps.