findFragmentByTag() returns null after perform a F

问题:

My Android app consists three fragments: A, B and C. They're loaded in the two containers defined in the MainActivity layout.

When the app is started, it shows the fragmentA loaded in the left_container and the fragmentC in the right_container.

If you press the button in the fragmentA, a FragmentTransaction changes FragmentC by FragmentB.

At the moment everything OK. But the trouble appears when I try to get a reference to the loaded fragmentB using findFragmentByTag(), because it returns null. I've used the method replace in the FragmentTransaction and I've finished it with commit(), but there isn't way to call FragmentB method. My code:

MainActivity.java:

 public class MainActivity extends Activity{
 static String fragmentTag = "FRAGMENTB_TAG";

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


    //Adds the left container's fragment
    getFragmentManager().beginTransaction().add(R.id.left_container, new FragmentA()).commit(); //Adds the fragment A to the left container

    //Adds the right container's fragment
    getFragmentManager().beginTransaction().add(R.id.right_container, new FragmentC()).commit(); //Adds the Fragment C to the right container
 }

 /**
 * Called when the button "Activate Fragment B" is pressed
 */
public void buttonListener(View v){

        FragmentTransaction ft = getFragmentManager().beginTransaction();
        ft.replace(R.id.right_container, new FragmentB(),fragmentTag); //Replaces the Fragment C previously in the right_container with a new Fragment B
        ft.commit(); //Finishes the transaction


        //!!HERE THE APP CRASHES (java.lang.NullPointerException = findFragmentByTag returns null
        ((FragmentB) getFragmentManager().findFragmentByTag(fragmentTag)).testView();


    }
}

FragmentB.java:

public class FragmentB extends Fragment {

public View onCreateView(LayoutInflater inflater,
        ViewGroup container,
        Bundle savedInstanceState) {
    return inflater.inflate(R.layout.fragment_b, container,false);

}


/**
 * Gets a reference to the text_fragment_b TextView and calls its method setText(), changing "It doesn't work" text by "It works!"
 */
public void testView(){
    TextView tv = (TextView)getView().findViewById(R.id.text_fragment_b);
    tv.setText("It works!");
}

}

activity_main.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:orientation="horizontal" >

<FrameLayout android:id="@+id/left_container" android:layout_width="0px" android:layout_weight="50" android:layout_height="match_parent"/>    
<FrameLayout android:id="@+id/right_container" android:layout_width="0px" android:layout_weight="50" android:layout_height="match_parent"/>

</LinearLayout>

fragment_b.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:orientation="vertical" 
android:layout_margin="5sp">

<TextView 
    android:id="@+id/text_fragment_b"
    android:text="It doesn't works!"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"/>

</LinearLayout>

Please help me! I'm a beginner in Android development!

回答1:

I've fixed it! I called getSupportFragmentManager().executePendingTransactions() after doing the transaction and it worked! After calling that method I can get the fragment using both findFragmentById() and findFragmentByTag() methods.

回答2:

if you use setRetainInstance(true) than you can't use findFragmentByTag() in onCreate from the Activity. Do it at onResume

see the documentation: setRetainInstance

回答3:

I'll start by apologising since I'm still very new myself...

I think the problem may be in the declaration of the fragmentTag static String not properly getting access from the class's instances, just change that line to:

private final static String FRAGMENT_TAG = "FRAGMENTB_TAG"; // using uppercase since it's a constant

Also, I would be more explicit when declaring instances, for example:

public void buttonListener(View v){

    FragmentTransaction ft = getFragmentManager().beginTransaction();
    ft.replace(R.id.right_container, new FragmentB(), FRAGMENT_TAG);
    ft.commit();

    FragmentB fragB = (FragmentB) getFragmentManager().findFragmentByTag(FRAGMENT_TAG);
    fragB.testView();
}

I hope you get this sorted, as I seen this question posted earlier and was surprised that it hadn't got any activity yet.

Also, here are a couple of links to the android documentation on replace:

Android Training - Replace

Android Reference - Replace

回答4:

I had the same problem and realized that there is a really simple way to fix this. When using a tag please do make sure to add the

fragmentTransaction.addToBackStack(null); 

method so that your Fragment is resumed instead of destroyed as mentioned in the developer guides.

If you don't call addToBackStack() when you perform a transaction that removes a fragment, then that fragment is destroyed when the transaction is committed and the user cannot navigate back to it. Whereas, if you do call addToBackStack() when removing a fragment, then the fragment is stopped and is later resumed if the user navigates back.

You can find this at the end of this section.

Every time I tried to reference back to my created Fragment, it turns out it had already been destroyed so I lost about 30 minutes trying to figure out why my Fragment was not being found through a simple findFragmentByTag(); call.

Hope this helps!

回答5:

Be sure you are adding or replacing the fragment in the proper way

Next statement will add the fragment but it will return null when using getFragmentManager().findFragmentByTag(tag):

transaction.add(R.id.mainContent, fragment);


This way it will work;

transaction.add(R.id.mainContent, fragment, tag);

回答6:

We are also seeing this problem but the cause is slightly different. The suggested solution by https://stackoverflow.com/a/21170693/1035008 doesn't work for us.

void updateFragment(Fragment newFragment) {
    FragmentTransaction ft = getFragmentManager().beginTransaction();

    // We have these 2 lines extra
    Fragment current = getChildFragmentManager().findFragmentByTag(fragmentTag);           
    if (current != null) { ft.remove(current); }

    ft.replace(R.id.right_container, newFragment, fragmentTag); //Replaces the Fragment C previously in the right_container with a new Fragment B
    ft.commit(); //Finishes the transaction

    //!!HERE THE APP CRASHES (java.lang.NullPointerException = findFragmentByTag returns null
    ((FragmentB) getFragmentManager().findFragmentByTag(fragmentTag)).testView();
}

And after reading the documentation about replace:

Replace an existing fragment that was added to a container. This is essentially the same as calling remove(Fragment) for all currently added fragments that were added with the same containerViewId and then add(int, Fragment, String) with the same arguments given here.

I realize that the remove call was not necessary since it is done by replace automatically. So after delete ft.remove(current), it works fine.