Question

尽管f1现在应该包含一个FragmentA实例，为什么它仍然为空。

MainActivity.java

    public class MainActivity extends AppCompatActivity implements FragmentA.Communicator {


      FragmentManager manager;
      FragmentA f1;

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

        Fragment fragment = manager.findFragmentById(R.id.main_fragment_container);
            if (fragment == null) {
            fragment = new FragmentA();
            manager.beginTransaction()
                    .add(R.id.main_fragment_container, fragment)
                    .commit();
        }

        f1 = (FragmentA) manager.findFragmentById(R.id.main_fragment_container);
        f1.setCommunicator(this);

        }

        @Override
        public void changeText(String text) {
        FragmentB f2 = (FragmentB) manager.findFragmentById(R.id.details_fragment_container);

        if (f2 != null && f2.isVisible()) {
            f2.updateText(text);
        } else {
            Intent intent = new Intent(MainActivity.this, DetailsActivity.class);
            intent.putExtra("text", text);
            startActivity(intent);
        }

activity_main.XML

    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">

    <FrameLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/main_fragment_container" />

        </LinearLayout>

FragmentA.Java

    public class FragmentA extends Fragment {

    Communicator communicator;
    ListView listView;

    public FragmentA() {
        // Required empty public constructor
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment


        View view = inflater.inflate(R.layout.fragment_a, container, false);
        listView = view.findViewById(R.id.listview);
        return view;
    }

    public void setCommunicator(Communicator communicator) {
        this.communicator = communicator;
    }

     @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);

        final ArrayList<String> arrayList = new ArrayList<>();
        arrayList.add("first row");
        arrayList.add("second row");
        arrayList.add("third row");

        ArrayAdapter<String> adapter = new ArrayAdapter<>(getActivity(), 
        android.R.layout.simple_list_item_1,
                arrayList);
        listView.setAdapter(adapter);

        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                communicator.changeText(arrayList.get(position));
            }
        });
    }

    public interface Communicator {
        void changeText(String text);
    }

fragment_a.xml

    <FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".FragmentA">

 <ListView
     android:layout_width="wrap_content"
     android:layout_height="wrap_content"
     android:id="@+id/listview" />

     </FrameLayout>


       Log: `Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void 
       com.example.workout.FragmentA.setCommunicator(com.example.workout.FragmentA$Communicator)' on a 
        null object reference`

Answer 1

commit()是异步的。如果您希望片段可以通过commitNow()立即可用，则应使用findFragmentById。

当然，就您而言，您可以将fragment和f1变量组合在一起：

f1 = manager.findFragmentById(R.id.main_fragment_container);
if (f1 == null) {
    f1 = new FragmentA();
    manager.beginTransaction()
        .add(R.id.main_fragment_container, f1)
        .commit();
}
// f1 is now never null
f1.setCommunicator(this);

为什么即使片段中已经添加了片段，片段管理器仍返回null？

1 个答案: