FindfragmentById在MainActivity中返回null，托管3个片段下的ViewPager

Question

我试图根据MainActivity当前托管的片段为我的FloatingActionButton编写不同的功能。然而，出于我的onClick()方法，getSupportFragmentManager().findFragmentById()返回null。

我还没有看到使用ViewPager实现的任何示例，我很好奇我是否需要采用另一种方法。

 MainActivity
    import android.content.Intent;
    import android.os.Bundle;
    import android.support.design.widget.FloatingActionButton;
    import android.support.v4.app.Fragment;
    import android.support.v4.app.FragmentActivity;
    import android.support.v4.view.ViewPager;
    import android.util.Log;
    import android.view.View;

    public class MainActivity extends FragmentActivity {
    private Adapter mAdapter;
    private ViewPager mViewPager;
    private static FloatingActionButton bButton;
    private static String UID;
    private Intent intent;

    public static String getUID(){
        return UID;
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mAdapter = new Adapter(getSupportFragmentManager());
        mViewPager = (ViewPager)findViewById(R.id.vPager);
        mViewPager.setAdapter(mAdapter);
        intent = getIntent();
        UID = intent.getStringExtra("uid");
        bButton = (FloatingActionButton)findViewById(R.id.bButton);
        bButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Fragment fragment = getSupportFragmentManager().findFragmentById(R.id.fragContainer);
                if(fragment == null){
                    Log.e("TAG","FRAGMENT IS NULL!!");
                }
                else{
                    Log.e("TAG","FRAGMENT IS NOT NULL!!");
                }
              }
           });
      }
     }

NewsFeedFragment

    import android.os.Bundle;
    import android.support.v4.app.FragmentManager;
    import android.support.v4.app.ListFragment;
    import android.util.Log;
    import android.view.LayoutInflater;
    import android.view.View;
    import android.view.ViewGroup;
    import android.widget.AdapterView;
    import android.widget.ArrayAdapter;
    import android.widget.ListView;
    import android.widget.TextView;
    import android.widget.Toast;
    import com.firebase.client.Firebase;
    import com.firebase.client.FirebaseError;
    import com.mycompany.neighbors.R;
    import com.mycompany.neighbors.SinglePost;
    import java.util.ArrayList;
    /**
     * Created by joshua on 5/25/2016.
     */
    public class NewsFeedFragment extends ListFragment implements AdapterView.OnItemClickListener{
        private ListView lv;
        private TextView tvUserName;
        private TextView tvStatus;
        private ArrayList<SinglePost> posts = new ArrayList<>();
        private static final String POSTS_PATH = "MY_PATH";
        private Firebase postsRef;
    //    private static final String FRAGMENT_POST = "post";

        public void postFragment(){
            Log.d("TAG", "Doing something else");
            PostFragment postFragment = new PostFragment();
            FragmentManager fragmentManager = getFragmentManager();
            fragmentManager.beginTransaction()
                    .replace(R.id.fragContainer,postFragment)
                    .addToBackStack(null)
                    .commit();
        }
        @Override
        public void onViewCreated(View v, Bundle s){
            lv =  getListView();
            lv.setOnItemClickListener(this);
        }
        @Override
        public void onCreate(Bundle savedInstance){
            super.onCreate(savedInstance);
        }
        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup parent, Bundle savedInstanceState){
            View v = inflater.inflate(R.layout.fragment_post_feed_item,parent,false);//changed
            tvUserName = (TextView)v.findViewById(R.id.tvUN);
            tvStatus = (TextView)v.findViewById(R.id.tvStatus);
            postsRef = new Firebase(POSTS_PATH);
            postsRef.addChildEventListener(new com.firebase.client.ChildEventListener() {
                @Override
                public void onChildAdded(com.firebase.client.DataSnapshot dataSnapshot, String s) {
                    SinglePost post = dataSnapshot.getValue(SinglePost.class);
                    post.setKey(dataSnapshot.getKey());
                    posts.add(0, post);
                    if(posts.size() > 0) {
                        PostAdapter adapter = new PostAdapter(posts);
                        setListAdapter(adapter);
                    }else{
                        Toast toast = Toast.makeText(getActivity(),"No data", Toast.LENGTH_SHORT);
                        toast.show();
                    }
                }
                @Override
                public void onChildChanged(com.firebase.client.DataSnapshot dataSnapshot, String s) {
                }
                @Override
                public void onChildRemoved(com.firebase.client.DataSnapshot dataSnapshot) {
                }
                @Override
                public void onChildMoved(com.firebase.client.DataSnapshot dataSnapshot, String s) {
                }
                @Override
                public void onCancelled(FirebaseError firebaseError) {
                }
            });

            return v;
        }
        @Override
        public void onListItemClick(ListView l, View v, int position, long id){
            SinglePost p = ((PostAdapter) getListAdapter()).getItem(position);
        }
        @Override
        public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
        }
        private class PostAdapter extends ArrayAdapter<SinglePost>{
           public PostAdapter(ArrayList<SinglePost> singlePost){
               super(getActivity(),android.R.layout.simple_list_item_1,singlePost);
           }
            @Override
            public View getView(int position, View convertView, ViewGroup parent){
                if(convertView == null){
                    convertView = getActivity().getLayoutInflater().inflate(R.layout.fragment_post_feed_item,null);
                }
                SinglePost p = getItem(position);
                TextView tvUserName = (TextView)convertView.findViewById(R.id.tvUN);
                tvUserName.setText(p.getUserName());
                TextView tvStatus = (TextView)convertView.findViewById(R.id.tvStatus);
                tvStatus.setText(p.getStatus());

                return convertView;
            }
        }
    }
I have 2 other fragments but i'll post one as an example. Here is my adapter for the viewpager.
Adapter
    import android.support.v4.app.Fragment;
    import android.support.v4.app.FragmentManager;
    import android.support.v4.app.FragmentPagerAdapter;
    import com.mycompany.neighbors.Fragments.MapFragment;
    import com.mycompany.neighbors.Fragments.NewsFeedFragment;
    import com.mycompany.neighbors.Fragments.ProfileFragment;
    /**
     * Created by joshua on 5/25/2016.
     */
    public class Adapter extends FragmentPagerAdapter {

            private String Fragment[] = {"Posts" , "Map" , "Profile"};
            public Adapter(FragmentManager fm){
                super (fm);
            }

            @Override
            public Fragment getItem(int position) {
                switch(position){
                    case 0:
                        return new NewsFeedFragment();
                    case 1:
                        return new MapFragment();
                    case 2:
                        return new ProfileFragment();
                    default:
                        return null;
                }
            }
            @Override
            public int getCount(){return Fragment.length;}
            @Override
            public CharSequence getPageTitle(int position) {
                return Fragment[position];
            }
        }

Answer 1

请检查下面的链接，对您的问题有很好的解释：

http://tamsler.blogspot.in/2011/11/android-viewpager-and-fragments-part-part-ii.html

或者您可以尝试以下代码段：

1.在查看Pager中添加片段或查看Pager适配器：

MyFragment myFragment = MyFragment.newInstance();
mPageReferenceMap.put(index, "Some Tag");
getSupportFragmentManager().beginTransaction().add(myFragment,"Some Tag").commit();

2.要获取当前可见页面的标签，然后致电：

int index = mViewPager.getCurrentItem();
String tag = mPageReferenceMap.get(index);

3.然后获取片段页：

Fragment myFragment = getSupportFragmentManager().findFragmentByTag(tag);