findFragmentById always returns null

Question

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

Answer 1

The reasons mentioned above are valid but another possible reason is that you are trying to search for the fragment while being in a fragment this also results in fragmentManager.findFragmentById to return null. We should use childFragmentManager.findFragmentById to find the fragment inside a fragment. According to the official documentation.

public final androidx.fragment.app.FragmentManager getChildFragmentManager()

Return a private FragmentManager for placing and managing Fragments inside of this Fragment.

Answer 2

R.id.test_fragment is your LinearLayout ID not your Fragment.

You can define and id on a fragment when it is inflated from an xml like in this sample http://developer.android.com/guide/topics/fundamentals/fragments.html#Adding

Answer 3

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" />

Answer 4

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

it should be a fragment not a LinearLayout

<fragment android:name="com.example.yourfragment"
            android:id="@+id/test_fragment"
            android:layout_width="match_parent"
            android:layout_height="match_parent" />

Answer 5

FragmentB fragmentB = 
(FragmentB) getSupportFragmentManager().findFragmentById(R.id.containerb);

if(fragmentB != null)
{
   fragmentB.showuserResponse(msg);
}

Use container id as fragment id. And then check for null reference.

Answer 6

fragmentTransaction.add(R.id.fragment_container, myFragment);

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

Please notice the difference.

You should pass R.id.fragment_container as the argument to findFragmentById, as you pass it to the add function, instead of R.id.test_fragment

By the way , according to the inner implementation of the two functions, it should be right that the id can be that of its container view.