Fragment lifecycle with respect to its Activity

What is Fragment?

A Fragment represents a behavior or a portion of user interface in an Activity.  We can combine multiple fragments in a single activity to build a multi-pane UI and reuse a fragment in multiple activities. 

The Fragment class has code that looks a lot like an Activity. It contains callback methods similar to an activity, such as onCreate(), onStart(), onPause(), andonStop(). In fact, if you're converting an existing Android application to use fragments, you might simply move code from your activity's callback methods into the respective callback methods of your fragment.

Fragment Lifecycle

Origin : http://developer.android.com/guide/components/fragments.html#Lifecycle

Fragment lifecycle with respect to its Activity

Fragment's lifecycle is tied to its owning activity and the normal flow of fragment in a activity.

1.    MainActivity onCreate

     Fragment onAttach

     Fragment onCreate

     Fragment onCreateView

     Fragment onViewCreated

     Fragment onActivityCreated

2.    MainActivity onStart

          Fragment onStart

3.    MainActivity onResume

          Fragment onResume

          Fragment onStop

4.    MainActivity onStop

          Fragment onDestroyView

          Fragment onDestroy

          Fragment onDetach

5.    MainActivity onDestroy

What happens? If we navigates to another activity from fragment and come back to main activity?

Here we have two scenarios, where fragment life cycle works.

1.    Activity is not destroyed when user leaves it.

2.    Activity got destroyed when user leaves it.

Activity Normal flow	Activity got destroyed
MainActivity onCreate           Fragment onAttach           Fragment onCreate           Fragment onCreateView           Fragment onViewCreated           Fragment onActivityCreated MainActivity onStart           Fragment onStart MainActivity onResume           Fragment onResume	MainActivity onCreate           Fragment onAttach           Fragment onCreate           Fragment onCreateView           Fragment onViewCreated           Fragment onActivityCreated MainActivity onStart           Fragment onStart MainActivity onResume           Fragment onResume
Fragment onStop MainActivity onStop	Fragment onStop MainActivity onStop           Fragment onDestroyView           Fragment onDestroy           Fragment onDetach MainActivity onDestroy
MainActivity onRestart MainActivity onStart           Fragment onStart MainActivity onResume           Fragment onResume	Fragment onAttach           Fragment onCreate MainActivity onCreate           Fragment onCreateView           Fragment onViewCreated           Fragment onActivityCreated MainActivity onStart           Fragment onStart MainActivity onResume           Fragment onResume

In the above differentiation, the 2^nd and 3rd row tells the flow of life cycle of fragment with respect to its activity.

By this flow change, we should be aware in writing our business logics.

Why, we should be aware?

Example: Assume, we have a custom action bar header, we are modifying the header depend upon the fragment selection in the fragment.

The custom header is called in the MainActivity Oncreate method as follows

         ActionBar bar = getActionBar();

        if (bar != null) {

bar.setDisplayOptions(ActionBar.DISPLAY_HOME_AS_UP | ActionBar.DISPLAY_SHOW_CUSTOM | ActionBar.DISPLAY_SHOW_HOME | ActionBar.DISPLAY_USE_LOGO);

          bar.setCustomView(R.layout.custom_header);

        }

        // enable ActionBar app icon to behave as action to toggle nav drawer

        bar.setDisplayHomeAsUpEnabled(true);

        bar.setHomeButtonEnabled(true);

In fragment, the custom header is updated as follows.

Create instance of the activity in fragment to update the action bar header.

onAttach(Activity)called once the fragment is associated with its activity. So, we can assign our variable inside the onAttach

public MainActivity activity;

@Override

public void onAttach(Activity activity) {

          Log.d("CHECK", " Fragment onAttach");

      super.onAttach(activity);

      this.activity = (MainActivity) activity;

}

activity.updateActionBarHeader(activity.getTitle().toString());

Usually, we will be setting headers in onCreate of activity or fragment, Since onCreate(Bundle)called to do initial creation of the fragment.

Application launches and works fine, when we placed the updateActionBarHeader in onCreate, but when application navigated from fragment and returns back, the application crashes if the activity is destroyed and the exception thrown as follows.

FATAL EXCEPTION: main
java.lang.RuntimeException: Unable to start activity ComponentInfo {PACKAGE_NAME/MainActivity}: java.lang.NullPointerException
          at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
          at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
          at android.app.ActivityThread.access$600(ActivityThread.java:130)
          at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
          at android.os.Handler.dispatchMessage(Handler.java:99)
          at android.os.Looper.loop(Looper.java:137)
          at android.app.ActivityThread.main(ActivityThread.java:4745)
          at java.lang.reflect.Method.invokeNative(Native Method)
          at java.lang.reflect.Method.invoke(Method.java:511)
          at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
          at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
          at MainActivity.updateActionBarHeader(MainActivity.java:232)
          at MainActivity$PlanetFragment.onCreate(MainActivity.java:276)
          at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:796)
          at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1035)
          at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1017)
          at android.app.FragmentManagerImpl.dispatchCreate(FragmentManager.java:1797)
          at android.app.Activity.onCreate(Activity.java:886)
          at MainActivity.onCreate(MainActivity.java:80)
          at android.app.Activity.performCreate(Activity.java:5008)
          at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
          at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
          ... 11 more

So, don't try to put the logic of updating action bar in onCreateView rather than onCreate of fragment and avoid the crash.

Thanks for reading :) 

Whether this post is helpful?

Have something to add to this post? If you have any other quick thoughts/hints that you think people will find useful? Share it in the comments.