I have found a solution to the problem and have modified your classes so that this error no longer occurs.
The only major difference was that you should use a DialogFragment instead of a Dialog, that way you have access to call getChildFragmentManager() and receive the correct FragmentManager from the DialogFragment.
Even though you were using getChildFragmentManager() before, it was coming from MyFragment and the PagerDialog class was not a child fragment in MyFragment.
I have tested the code below, and it should be working fine now.
MyFragment
public class MyFragment extends Fragment {
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View v = inflater.inflate(R.layout.fragment_sandbox, container, false);
Button button = (Button) v.findViewById(R.id.button);
button.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View arg0) {
DialogFragment newFragment = PagerDialog.newInstance();
newFragment.show(getChildFragmentManager(), "dialog");
}
});
return v;
}
}
PagerDialog
public class PagerDialog extends DialogFragment {
public static PagerDialog newInstance() {
return new PagerDialog();
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View v = inflater.inflate(R.layout.dialog_fragment, container, false);
ViewPager mViewPager = (ViewPager) v.findViewById(R.id.view_pager);
/* Use childFragmentManager here provided from the PagerDialog */
MyAdapter mAdapter = new MyAdapter(getChildFragmentManager());
mViewPager.setAdapter(mAdapter);
return v;
}
private class MyAdapter extends FragmentStatePagerAdapter {
public MyAdapter(FragmentManager fm) {
super(fm);
}
@Override
public Fragment getItem(int index) {
return new DummyFragment();
}
@Override
public int getCount() {
return 2;
}
}
}
DummyFragment
public class DummyFragment extends Fragment {
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View v = inflater.inflate(R.layout.fragment_dummy_layout, container, false);
return v;
}
}
fragment_sandbox.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center">
<Button
android:id="@+id/button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Test Fragment Dialog Pager"
android:textSize="24sp"
android:padding="20dp" />
</LinearLayout>
dialog_fragment.xml
<?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:gravity="center"
android:orientation="vertical" >
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Fragment Dialog Title Text "
android:padding="10dp"
android:textColor="#333"
android:textSize="24sp"/>
<android.support.v4.view.ViewPager
android:id="@+id/view_pager"
android:layout_width="match_parent"
android:layout_height="200dp"/>
</LinearLayout>
fragment_dummy_layout.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Fragment Dummy Text"
android:textSize="24sp"
android:textColor="#ff0000"/>
</LinearLayout>
