Do the initial callbacks of a fragment get called before or after Activity onStart()?

Question

https://github.com/xxv/android-lifecycle shows that fragment onAttach, onCreate, on CreateView, and onActivityCreated are invoked prior to activity onStart. However, I tried logging this:

   public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        Log.e("TRACE", "MainActivity onCreate");
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Fragment firstFragment = FirstFragment.newInstance();
        getSupportFragmentManager().beginTransaction().replace(R.id.content, firstFragment).commit();
    }

    @Override
    public void onStart() {
        Log.e("TRACE", "MainActivity onStart");
        super.onStart();
    }

FirstFragment looks like this:

public class FirstFragment extends Fragment {

    public FirstFragment() {
        // Required empty public constructor
    }

    public static FirstFragment newInstance() {
        FirstFragment fragment = new FirstFragment();
        return fragment;
    }

    @Override
    public void onAttach(Context context) {
        Log.d("TRACE", "FirstFragment onAttach");
        super.onAttach(context);
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        Log.d("TRACE", "FirstFragment onCreate");
        super.onCreate(savedInstanceState);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        Log.d("TRACE", "FirstFragment onCreateView");
        return inflater.inflate(R.layout.fragment_first, container, false);
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        Log.d("TRACE", "FirstFragment onActivityCreated");
        super.onActivityCreated(savedInstanceState);
    }

And it printed this:

1. MainActivity onCreate
2. MainActivity onStart
3. FirstFragment onAttach
4. FirstFragment onCreate
5. FirstFragment onCreateView
6. FirstFragment onActivityCreated
7. MainActivity onResume

Answer 1

It depends on how the Fragment is added to the Activity .

There are two common ways to add a fragment to an activity: via a <fragment> tag in XML or via a FragmentTransaction executed in Java.

If, instead of providing an R.id.content view and using a FragmentTransaction , you create your FirstFragment by using an activity layout like this one:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <fragment
        android:id="@+id/content"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        class="com.example.stackoverflow.FirstFragment"/>

</LinearLayout>

You'll see this in the logs:

 E/TRACE ( 6094): MainActivity onCreate D/TRACE ( 6094): FirstFragment onAttach D/TRACE ( 6094): FirstFragment onCreate D/TRACE ( 6094): FirstFragment onCreateView E/TRACE ( 6094): MainActivity onStart D/TRACE ( 6094): FirstFragment onActivityCreated E/TRACE ( 6094): MainActivity onResume 

As for why you're seeing the messages in the order you see them when using a FragmentTransaction , it comes down to the fact that the transaction is asynchronous ; there's no guarantee that it will be executed as soon as you call commit() . In your case, the system doesn't actually get around to executing it until after your activity's onStart() .