findFragmentById always returns null

Question

I'm defining an ID for my fragment in the xml layout:

<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:id="@+id/test_fragment" ... 

Then I add this fragment in the activity's onCreate method:

MyFragment myFragment = new MyFragment(); fragmentTransaction.add(R.id.fragment_container, myFragment); fragmentTransaction.commit(); 

This is all working fine. Replacing fragments and is also working.

Later I'm trying to retrieve this fragment by its ID in one of the activity's methods:

MyFragment myFragment = (MyFragment) getFragmentManager().findFragmentById(R.id.test_fragment); 

Doing so leads to myFragment being null. Always.

When I try to do the same with tags instead of IDs I can retrieve the fragment by its tag without any problems:

MyFragment myFragment = new MyFragment(); fragmentTransaction.add(R.id.fragment_container, myFragment, "testfragment"); fragmentTransaction.commit(); 

...

MyFragment myFragment = (MyFragment) getFragmentManager().findFragmentByTag("testfragment"); 

Why can't findFragmentById find the fragment, but findFragmentByTag does so? Am I missing something?

Answer 1

R.id.test_fragment is not the ID of your fragment but the id of your LinearLayout

Calling add(int containerViewId, Fragment fragment) will add a fragment without a tag. So or you use add(int containerViewId, Fragment fragment, String tag) and you get back your fragment using your tag (as an ID)

Answer 2

Use the <FrameLayout> tag as a container in your layout file. Later to replace the fragments from your activity dynamically, you can use the ID of the <FrameLayout> container to replace the fragments in your activity.

<FrameLayout             android:id="@+id/test_fragment"             android:layout_width="match_parent"             android:layout_height="match_parent" />