findFragmentById always returns null

问题:

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?

回答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)

回答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" />

回答3:

<?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" />

回答4:

I was using android.support.v4.app.Fragment in my layout while calling getFragmentManager() which actually searched for android.app.Fragment subclasses and I got null. So the fix was to call getSupportFragmentManager() instead.

In general make sure the package of a fragment you are subclassing and using in your layout is the same returned by the corresponding FragmentManager which performs search.

回答5:

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

回答6:

Or, you should have instead used :

(MyFragment) getFragmentManager().findFragmentById(R.id.fragment_container);

回答7:

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.

回答8:

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.