findFragmentById return null

Question

When I call findFragmentById() with the id of my fragment, it returns null.

activity_main.xml

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

        <fragment android:name="com.madduck.test.app.fragment.MainFragment"
                  android:id="@+id/main_fragment"
                  android:layout_width="match_parent"
                  android:layout_height="match_parent" />

        <fragment android:name="com.madduck.test.app.fragment.LoginFragment"
                  android:id="@+id/login_fragment"
                  android:layout_width="match_parent"
                  android:layout_height="match_parent" />
</LinearLayout>

MainActivity.java

private static final int LOGIN = 0;
private static final int MAIN = 1;
private static final int FRAGMENT_COUNT = MAIN +1;
private Fragment[] fragments = new Fragment[FRAGMENT_COUNT]

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

    FragmentManager fm = getSupportFragmentManager();
    fragments[LOGIN] = fm.findFragmentById(R.id.login_fragment);
    fragments[MAIN] = fm.findFragmentById(R.id.main_fragment);

    FragmentTransaction transaction = fm.beginTransaction();
    for (Fragment f : fragments) {
        if (f != null)
            transaction.hide(f);
        else
            Log.e(TAG, "???");
    }

    transaction.commit();
}

The thing is that when I call fm.findFragmentById(R.id.login_fragment); I get a null but when I call fm.findFragmentById(R.id.main_fragment); I get the fragment.

Answer 1

just found out my mistake.

In my MainActivity.java i was importing android.support.v4.app.Fragment; and in my LoginFragment.java i was importing android.app.Fragment;. I changed it to the same thing and fm.findFragmentById(R.id.login_fragment) now return the right fragment.

Answer 2

The answer Kar0t is perfectly fine but this may help somebody. On my case I had a fragment inside of a fragment and I was getting the wrong FragmentManager. I just had to call:

getChildFragmentManager()

and then just find the Fragment as usual:

fm.findFragmentById(R.id.fragment)

Answer 3

Not really related to the specific question but related to receiving null on findFragmentById, if you call findFragmentById immediately after commit it will return null or the last fragment(before commit), the reason because commit do async request.

From docs:

Schedules a commit of this transaction. The commit does not happen immediately; it will be scheduled as work on the main thread to be done the next time that thread is ready.

If you need to findFragmentById immediately, for example change status bar text color after adding fragment, call executePendingTransactions() after commit()

getSupportFragmentManager().executePendingTransactions();
//call findFragmentById 

Answer 4

Kotlin

I also had problem because I was in a child fragment. I could use:

    supportFragmentManager.fragments.forEach {
        it.childFragmentManager.fragments.forEach { fragment ->
            if (fragment is HomeFragment) {
                //do something
            } 
        }

but one coworker optimized it with :

     if( findNavController(R.id.nav_host_fragment).currentDestination?.id == R.id.homeFragment) {
       // do something
   }

Answer 5

I encountered this problem because my project has two layout with the same file name. eg. In your case, I have two activity_main.xml in different modules.

So somehow it get compiled and when you run

fm.findFragmentById(R.id.login_fragment)

then it is looking for the fragment in the other activity_main.xml, which is null.

If this is the case, just check your file names of layouts, make sure they are unique.

Hope this help anyone with the same issue as mine. :)

Answer 6

It is android life everyday something DEPRICATED, we are looking to samples and manual but they already not working)

My issue was in mix "import android.app.Fragment;" with modern "androidx.fragment.app.Fragment". Below sample for androidx

import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentTransaction;

            Fragment myFrag = getSupportFragmentManager().findFragmentById(R.id.frgmContainer);
            if(myFrag==null)
                return;
            ((TextView) myFrag.getView().findViewById(R.id.textView))
                    .setText("Access to Fragment 2 from Activity");