getSupportFragmentManager().findFragmentById() returns null even though fragment exists

Question

I have a fragment_main.xml inside an activity_main.xml.

The fragment is added inside a Tabbed Activity created from an Android Studio template.

I want to do a change in the Fragment's view from within the Activity. That's why I created an Interface that allows the Fragment to call the Activity during the Fragment's onStart, when I know that the Fragment's view is available for modifications.

I did get this to work if I pass the actual Fragment instance using my interface callback. But I'm still frustrated because I don't understand why this function returns null. Does this have anything to do with reusing fragments in different screen orientations?

I've looked over similar questions here on SO reported with this function returning null. Somebody mentioned that there might be an issue with using import android.support.v4.app.Fragment; as opposed to just android.app.Fragment. I don't think this is my case because getsupportFragmentManager seems to return the correct type of Fragment (v4.app). I have no idea why the v4.app is there, I'm kinda new to all this.

Anyway here's the code, all of it, including imports, because they might be relevant.

package org.axonnsd.musicnexus;

import android.app.Activity;
import android.content.Context;
import android.media.MediaPlayer;
import android.os.Environment;
import android.os.Handler;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.os.Bundle;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewGroup;

import android.widget.MediaController;
import android.widget.TextView;

import java.io.FileInputStream;
import java.io.IOException;

public class MainActivity extends AppCompatActivity implements OnFragmentReadyListener {

    /**
     * The {@link android.support.v4.view.PagerAdapter} that will provide
     * fragments for each of the sections. We use a
     * {@link FragmentPagerAdapter} derivative, which will keep every
     * loaded fragment in memory. If this becomes too memory intensive, it
     * may be best to switch to a
     * {@link android.support.v4.app.FragmentStatePagerAdapter}.
     */
    private SectionsPagerAdapter mSectionsPagerAdapter;

    private static final String TAG = "MyTabbedApplication";

    private Handler handler = new Handler();

    /**
     * The {@link ViewPager} that will host the section contents.
     */
    private ViewPager mViewPager;

    @Override
    protected void onStop() {
        super.onStop();
        mediaController.hide();
        mediaPlayer.stop();
        mediaPlayer.release();
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        //the MediaController will hide after 3 seconds - tap the screen to make it appear again
        mediaController.show();
        return false;
    }


    public void onFragmentReady(PlaceholderFragment fragment)
    {
        this.getIntent().putExtra(AUDIO_FILE_NAME,AUDIO_FILE_NAME);
        audioFile = this.getIntent().getStringExtra(AUDIO_FILE_NAME);

        PlaceholderFragment fragment = (PlaceholderFragment)(getSupportFragmentManager().findFragmentById(R.id.main_audio_view));
                //FRAGMENT IS NULL!
        fragment.setNowPlayingText(audioFile);
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        // Create the adapter that will return a fragment for each of the three
        // primary sections of the activity.
        mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

        // Set up the ViewPager with the sections adapter.
        mViewPager = (ViewPager) findViewById(R.id.container);
        mViewPager.setAdapter(mSectionsPagerAdapter);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {
        /**
         * The fragment argument representing the section number for this
         * fragment.
         */
        private static final String ARG_SECTION_NUMBER = "section_number";
        private TextView _txtNowPlaying;
        private MainActivity _parentActivity;

        public PlaceholderFragment() {
        }

        /**
         * Returns a new instance of this fragment for the given section
         * number.
         */
        public static PlaceholderFragment newInstance(int sectionNumber) {
            PlaceholderFragment fragment = new PlaceholderFragment();
            Bundle args = new Bundle();
            args.putInt(ARG_SECTION_NUMBER, sectionNumber);
            fragment.setArguments(args);
            return fragment;
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                 Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container, false);
            TextView textView = (TextView) rootView.findViewById(R.id.section_label);
            _txtNowPlaying = (TextView)rootView.findViewById(R.id.now_playing_text);
            textView.setText(getString(R.string.section_format, getArguments().getInt(ARG_SECTION_NUMBER)));
            //notifyActivity(_parentActivity);
            return rootView;
        }

        @Override
        public void onAttach(Activity activity)
        {
            super.onAttach(activity);
            _parentActivity = (MainActivity)activity;
        }

        @Override
        public void onAttach(Context context)
        {
            super.onAttach(context);

            if (context instanceof Activity){
                _parentActivity = (MainActivity)context;
            }
        }

        @Override
        public void onStart()
        {
            super.onStart();
            notifyActivity(_parentActivity);
        }

        private void notifyActivity(MainActivity activity)
        {
            activity.onFragmentReady(this);
        }

        public void setNowPlayingText(String value)
        {
            _txtNowPlaying.setText(value);
        }
    }

    /**
     * A {@link FragmentPagerAdapter} that returns a fragment corresponding to
     * one of the sections/tabs/pages.
     */
    public class SectionsPagerAdapter extends FragmentPagerAdapter {

        public SectionsPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) {
            // getItem is called to instantiate the fragment for the given page.
            // Return a PlaceholderFragment (defined as a static inner class below).
            return PlaceholderFragment.newInstance(position + 1);
        }

        @Override
        public int getCount() {
            // Show 3 total pages.
            return 3;
        }

        @Override
        public CharSequence getPageTitle(int position) {
            switch (position) {
                case 0:
                    return "SECTION 1";
                case 1:
                    return "SECTION 2";
                case 2:
                    return "SECTION 3";
            }
            return null;
        }
    }
}

I'm pasting the XMLs too, even though they are fine (as I said, it works if I pass the actual fragment).

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/main_content"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:context="org.axonnsd.musicnexus.MainActivity">

    <android.support.design.widget.AppBarLayout
        android:id="@+id/appbar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:paddingTop="@dimen/appbar_padding_top"
        android:theme="@style/AppTheme.AppBarOverlay">

        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="?attr/colorPrimary"
            app:layout_scrollFlags="scroll|enterAlways"
            app:popupTheme="@style/AppTheme.PopupOverlay">

        </android.support.v7.widget.Toolbar>

    </android.support.design.widget.AppBarLayout>

    <android.support.v4.view.ViewPager
        android:id="@+id/container"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_behavior="@string/appbar_scrolling_view_behavior" />

    <android.support.design.widget.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="end|bottom"
        android:layout_margin="@dimen/fab_margin"
        app:srcCompat="@android:drawable/ic_dialog_email" />

</android.support.design.widget.CoordinatorLayout>

fragment_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:id="@+id/main_audio_view"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="org.axonnsd.musicnexus.MainActivity$PlaceholderFragment">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:text="Now playing:"
        android:textSize="25sp"
        android:textStyle="bold"
        />
    <TextView
        android:id="@+id/now_playing_text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dip"
        android:layout_marginLeft="10dip"
        android:layout_marginRight="10dip"
        android:layout_gravity="center"
        android:text="Now playing.."
        android:textSize="16sp"
        android:textStyle="italic"
        />

    <TextView
        android:id="@+id/section_label"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

</RelativeLayout>

score 2 · Accepted Answer · edited May 23 '17 at 12:34

2

First of all, you're using findFragmentById wrong, passing id of root layout of your fragment as a parameter, when it expects an id of a layout that contains your fragment (not layout inside a fragment!)

FragmentPagerAdapter is kinda tricky and getting current fragment is not straightforward. Check this answer: Getting the current Fragment instance in the viewpager

Replace your:

PlaceholderFragment fragment = (PlaceholderFragment)(getSupportFragmentManager().findFragmentById(R.id.main_audio_view));
            //FRAGMENT IS NULL!

with

PlaceholderFragment page = (PlaceholderFragment) 
getSupportFragmentManager().findFragmentByTag("android:switcher:" + 
R.id.container + ":" + mViewPager.getCurrentItem());

edited May 23 '17 at 12:34

Community

1
1

answered May 15 '17 at 15:07

Roman Kolomenskii

648
4
7

I see! Thank you :). This looks a bit... tricky, as you said. I guess it would be cleaner to just pass the instance itself as an argument via a function call? Any advice against doing that? Why would anybody use this way rather than pass an instance of the Fragment back to the Activity? – Axonn May 15 '17 at 21:10
1

It's okay to pass fragment instance for one time use, however saving that instance in a field is unreliable, because `FragmentManager` may recreate fragment on screen rotation or other `state` changes. The snippet I posted was more for the case when you need to update currently shown fragment, but you don't have an instance of it. – Roman Kolomenskii May 17 '17 at 12:59
Thank you :). Is the findFragmentByTag "stable" in terms of the naming convention ("android:switcher"...etc) being consistent across releases? Is there some better way to get/keep fragments? I am wondering why I can't get a view (TextView) inside the fragment by saying something like view_id.fragment_id.view_id Why can't I attach an ID to a fragment? – Axonn May 24 '17 at 19:24

score 0 · Answer 2 · answered May 15 '17 at 14:56

I would try the following, but I'm sure it's not the nicest solution.

I would pass the value of section_number instead of the PlaceholderFragment and get the fragment from the SectionPageAdapter. Something like this:

 public void onFragmentReady(int position)
 {
    this.getIntent().putExtra(AUDIO_FILE_NAME,AUDIO_FILE_NAME);
    audioFile = this.getIntent().getStringExtra(AUDIO_FILE_NAME);

    PlaceholderFragment fragment = (PlaceholderFragment)mSectionsPagerAdapter.getItem(position);

    fragment.setNowPlayingText(audioFile);
 }

getSupportFragmentManager().findFragmentById() returns null even though fragment exists

2 Answers2