Nested Fragments working on pre API 17

Question

I'm very confused. Here it says:

nested fragments as shown in the screenshot are not possible without the support library.

and also here they said:

AFAIK, fragments cannot hold other fragments.

All this are confirmed by the Android Documentation here. So that means one thing:

You can't put Fragments into Fragments without Support v4 Library on pre API 17!

Ok, but when I have the following settings: build.gradle

apply plugin: 'com.android.application'


android {
    compileSdkVersion 16
    buildToolsVersion '19.1.0'

    defaultConfig {
        applicationId "package.supportfragments"
        minSdkVersion 14
        targetSdkVersion 16
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
}

MainActivity:

public class MainActivity extends Activity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    getFragmentManager().beginTransaction().add(R.id.frame_test, new TestFragment()).commit();
}

TestFragment:

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.fragment_test, container, false);

    getFragmentManager().beginTransaction().add(R.id.frame_test_2, new TestNestedFragment()).commit();

    return rootView;
}

TestNestedFragment:

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.fragment_test_nested, container, false);

    getFragmentManager().beginTransaction().add(R.id.frame_test_3, new TestNestedNestedFragment()).commit();

    return rootView;
}

The Layout looks all like that:

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent" >

   <!-- TODO: Update blank fragment layout -->
   <FrameLayout
       android:id="@+id/IDTOTHEFRAGMENT"
       android:layout_width="match_parent"
       android:layout_height="match_parent" />

</FrameLayout>

And it works like a charme... The question is.. WHY??!!

Answer 1

Your example doesn't nest the fragment. When calling getFragmentManager() from the fragment it returns the activities FragmentManager, so you are adding fragments to the activity.

To add nested fragments you need to use getChildFragmentManager() which was introduced in API 17 so you cant use it if you don't use the support fragment implementation.

Answer 2

Here the activity is having two fragments and you are adding both, thats correct. But since you add() another fragment inside a fragment that doesnt mean the whole thing is nested, You are using the getFragmentManager() in both senario which will always refer to the activity.

Nested Fragment senario:

Consider that FragmentA is having a viewpager with 3 swipable fragments and you are adding FragmentA to the activity. Here you can say the viewpager fragments are nested because you are having a fragment inside FragmentA and must use getChildFragmentManager() to add the 3 fragments to view pager.

Update:

I have gone though the android source code and there it is clearly mentioned

"Return the FragmentManager for interacting with fragments associated with this fragment's activity. Note that this will be non-null slightly before getActivity(), during the time from when the fragment is placed in a FragmentTransaction until it is committed and attached to its activity. If this Fragment is a child of another Fragment, the FragmentManager returned here will be the parent's getChildFragmentManager()".