如何在方法instantiateItem上實例化一個片段？

Question

我對Android編程非常陌生，我試圖在我的SlidingTabsBasicFragment類上做出instantiateItem，以返回我的ConversationsFragment的片段。

在我的理解中，我的類ConversationsFragment返回一個片段，它將被實例化。但是，第二頁沒有顯示任何內容。

我使用SlidingTabsBasic樣品，使這個：

主要活動的onCreate：

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    Log.v("Logs", "first"); 

    if (savedInstanceState == null) { 
     FragmentTransaction transaction = getSupportFragmentManager().beginTransaction(); 
     SlidingTabsBasicFragment fragment = new SlidingTabsBasicFragment(); 
     transaction.replace(R.id.sample_content_fragment, fragment); 
     //ConversationsFragment fragment = new ConversationsFragment(); 
     //transaction.replace(R.id.sample_content_fragment, new ConversationsFragment()); 
     transaction.commit(); 
    } 
} 

SlidingTabsBasicFragment：

package com.example.android.slidingtabsbasic; 

import com.example.android.common.logger.Log; 
import com.example.android.common.view.SlidingTabLayout; 

import android.content.Context; 
import android.os.Bundle; 
import android.support.v4.app.Fragment; 
import android.support.v4.app.FragmentManager; 
import android.support.v4.app.FragmentPagerAdapter; 
import android.support.v4.app.FragmentTransaction; 
import android.support.v4.view.PagerAdapter; 
import android.support.v4.view.ViewPager; 
import android.util.SparseArray; 
import android.view.LayoutInflater; 
import android.view.View; 
import android.view.ViewGroup; 
import android.widget.AbsListView; 
import android.widget.AdapterView; 
import android.widget.ListAdapter; 
import android.widget.TextView; 


/** 
* A basic sample which shows how to use {@link com.example.android.common.view.SlidingTabLayout} 
* to display a custom {@link ViewPager} title strip which gives continuous feedback to the user 
* when scrolling. 
*/ 
public class SlidingTabsBasicFragment extends Fragment { 

    static final String LOG_TAG = "SlidingTabsBasicFragment"; 

    /** 
    * A custom {@link ViewPager} title strip which looks much like Tabs present in Android v4.0 and 
    * above, but is designed to give continuous feedback to the user when scrolling. 
    */ 
    private SlidingTabLayout mSlidingTabLayout; 

    /** 
    * A {@link ViewPager} which will be used in conjunction with the {@link SlidingTabLayout} above. 
    */ 
    private ViewPager mViewPager; 

    /** 
    * Inflates the {@link View} which will be displayed by this {@link Fragment}, from the app's 
    * resources. 
    */ 
    @Override 
    public View onCreateView(LayoutInflater inflater, ViewGroup container, 
      Bundle savedInstanceState) { 
     return inflater.inflate(R.layout.fragment_sample, container, false); 
    } 

    // BEGIN_INCLUDE (fragment_onviewcreated) 
    /** 
    * This is called after the {@link #onCreateView(LayoutInflater, ViewGroup, Bundle)} has finished. 
    * Here we can pick out the {@link View}s we need to configure from the content view. 
    * 
    * We set the {@link ViewPager}'s adapter to be an instance of {@link SamplePagerAdapter}. The 
    * {@link SlidingTabLayout} is then given the {@link ViewPager} so that it can populate itself. 
    * 
    * @param view View created in {@link #onCreateView(LayoutInflater, ViewGroup, Bundle)} 
    */ 
    @Override 
    public void onViewCreated(View view, Bundle savedInstanceState) { 
     // BEGIN_INCLUDE (setup_viewpager) 
     // Get the ViewPager and set it's PagerAdapter so that it can display items 
     mViewPager = (ViewPager) view.findViewById(R.id.viewpager); 
     mViewPager.setAdapter(new CustomPagerAdapter(getFragmentManager(),getActivity())); 
     // END_INCLUDE (setup_viewpager) 

     // BEGIN_INCLUDE (setup_slidingtablayout) 
     // Give the SlidingTabLayout the ViewPager, this must be done AFTER the ViewPager has had 
     // it's PagerAdapter set. 
     mSlidingTabLayout = (SlidingTabLayout) view.findViewById(R.id.sliding_tabs); 
     mSlidingTabLayout.setViewPager(mViewPager); 
     // END_INCLUDE (setup_slidingtablayout) 
    } 
    // END_INCLUDE (fragment_onviewcreated) 


    public class CustomPagerAdapter extends FragmentPagerAdapter { 

     protected Context mContext; 

     public CustomPagerAdapter(FragmentManager fm, Context context) { 
      super(fm); 
      mContext = context; 
     } 

     @Override 
     public CharSequence getPageTitle(int position) { 
      return "Item " + (position + 1); 
     } 

     @Override 
     public Fragment getItem(int position) { 

      if(position==0){ 
       // return fragment 
      }else{ 
       ConversationsFragment fragment = new ConversationsFragment(); 
       return fragment; 
      } 
      return null; 
     } 

     @Override 
     public int getCount() { 
      return 3; 
     } 
    } 


} 

這是我ConversationsFragment：

public class ConversationsFragment extends Fragment implements AbsListView.OnItemClickListener { 

// TODO: Rename parameter arguments, choose names that match 
// the fragment initialization parameters, e.g. ARG_ITEM_NUMBER 
private static final String ARG_PARAM1 = "param1"; 
private static final String ARG_PARAM2 = "param2"; 

// TODO: Rename and change types of parameters 
private String mParam1; 
private String mParam2; 

private OnFragmentInteractionListener mListener; 

/** 
* The fragment's ListView/GridView. 
*/ 
private AbsListView mListView; 

/** 
* The Adapter which will be used to populate the ListView/GridView with 
* Views. 
*/ 
private List conversationsListItemList; // at the top of your fragment listW 
private ListAdapter mAdapter; 

// TODO: Rename and change types of parameters 
public static ConversationsFragment newInstance(String param1, String param2) { 
    ConversationsFragment fragment = new ConversationsFragment(); 
    Bundle args = new Bundle(); 
    args.putString(ARG_PARAM1, param1); 
    args.putString(ARG_PARAM2, param2); 
    fragment.setArguments(args); 
    return fragment; 
} 

/** 
* Mandatory empty constructor for the fragment manager to instantiate the 
* fragment (e.g. upon screen orientation changes). 
*/ 
public ConversationsFragment() { 
} 

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

    if (getArguments() != null) { 
     mParam1 = getArguments().getString(ARG_PARAM1); 
     mParam2 = getArguments().getString(ARG_PARAM2); 
    } 

    // TODO: Change Adapter to display your content 
    conversationsListItemList = new ArrayList(); 
    conversationsListItemList.add(new ConversationsListItem("Example 1","conversa")); 
    conversationsListItemList.add(new ConversationsListItem("Example 2","conversa")); 
    conversationsListItemList.add(new ConversationsListItem("Example 3","conversa")); 
    mAdapter = new ConversationsListAdapter(getActivity(), conversationsListItemList); 
    /* mAdapter = new ArrayAdapter<DummyContent.DummyItem>(getActivity(), 
      android.R.layout.simple_list_item_1, android.R.id.text1, DummyContent.ITEMS);*/ 
} 

@Override 
public View onCreateView(LayoutInflater inflater, ViewGroup container, 
         Bundle savedInstanceState) { 
    View view = inflater.inflate(R.layout.fragment_conversations, container, false); 

    // Set the adapter 
    mListView = (AbsListView) view.findViewById(android.R.id.list); 
    ((AdapterView<ListAdapter>) mListView).setAdapter(mAdapter); 

    // Set OnItemClickListener so we can be notified on item clicks 
    mListView.setOnItemClickListener(this); 

    return view; 
} 

@Override 
public void onAttach(Activity activity) { 
    super.onAttach(activity); 
    try { 
     mListener = (OnFragmentInteractionListener) activity; 
    } catch (ClassCastException e) { 
     throw new ClassCastException(activity.toString() 
       + " must implement OnFragmentInteractionListener"); 
    } 
} 

@Override 
public void onDetach() { 
    super.onDetach(); 
    mListener = null; 
} 

@Override 
public void onItemClick(AdapterView<?> parent, View view, int position, long id) { 
    if (null != mListener) { 
     // Notify the active callbacks interface (the activity, if the 
     // fragment is attached to one) that an item has been selected. 
     mListener.onFragmentInteraction(DummyContent.ITEMS.get(position).id); 
    } 
} 

public void setEmptyText(CharSequence emptyText) { 
    View emptyView = mListView.getEmptyView(); 

    if (emptyView instanceof TextView) { 
     ((TextView) emptyView).setText(emptyText); 
    } 
} 

public interface OnFragmentInteractionListener { 
    public void onFragmentInteraction(String id); 
} 

}

截圖：

第一屏：

https://goo.gl/photos/Ze5ThZgevGaALDj47

第二個屏幕：

https://goo.gl/photos/x7JPnESAtZsBT5DC8

而現在這樣的錯誤出現：

06-16 12:32:58.946 12228-12228/com.example.android.slidingtabsbasic E/AndroidRuntime﹕ FATAL EXCEPTION: main 
    Process: com.example.android.slidingtabsbasic, PID: 12228 
    java.lang.NullPointerException 
      at android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:417) 
      at android.support.v4.app.BackStackRecord.add(BackStackRecord.java:412) 
      at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:99) 

Answer 1

我覺得你的代碼的問題是返回View和Fragment

下面的代碼爲我工作

public class CustomPagerAdapter extends FragmentPagerAdapter { 

    protected Context mContext; 

    public CustomPagerAdapter(FragmentManager fm, Context context) { 
     super(fm); 
     mContext = context; 
    } 

    @Override 
    public CharSequence getPageTitle(int position) { 
     return "" + tabsTitle[position]; 
    } 

    @Override 
    public Fragment getItem(int position) { 

     if(position==0){ 
      // return fragment 
     }else{ 
     ConversationsFragment fragment = new ConversationsFragment(); 
       return fragment; 
      } 
     return null; 
    } 

    @Override 
    public int getCount() { 
     return 3; 
    } 
}