如何使用supportfragmentmanager與findfragmentbytag獲得第一個片段

Question

我正在使用viewpager和tabspageradapter。在我的主要活動中，我有一個屬性來安全選擇選項卡中的當前片段：current_tab。它會在選擇新選項卡時更改其值，但是我不知道如何在開始時設置它的值，因爲我需要attribut更新我的片段中的listview。我嘗試了很多東西，但是我在大多數時候都得到了nullpointerexception，誰能告訴我它是如何工作的？

代碼我TabsPagerAdapter的：

public class TabsPagerAdapter extends FragmentPagerAdapter { 

public TabsPagerAdapter(FragmentManager fm){ 
    super(fm); 
} 

@Override 
public Fragment getItem(int index){ 
    switch(index) { 
     case 0: 
      return new AllShowsFragment(); 
     case 1: 
      return new UnwatchedShowsFragment(); 
     case 2: 
      return new WatchedShowsFragment(); 
    } 
    return null; 
} 

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

主要活動的最重要的部分代碼：

public class MainActivity extends FragmentActivity implements 
    ActionBar.TabListener { 

public static FragmentCommunicater fragmentCommunicater; 
public static boolean simple_progress; 
public static int editmode; 
private ViewPager viewPager; 
private TabsPagerAdapter mAdapter; 
private ActionBar actionBar; 
private DatabaseHandler db; 
private Show current_show; 
private static MyFragment current_tab; 
private boolean first_launch=true; 

private String[] tabs = { "All Shows", "Unwatched", "Watched" }; 

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

    db = new DatabaseHandler(this); 
    viewPager = (ViewPager) findViewById(R.id.pager); 
    actionBar = getActionBar(); 
    mAdapter = new TabsPagerAdapter(getSupportFragmentManager()); 

    viewPager.setAdapter(mAdapter); 
    actionBar.setHomeButtonEnabled(false); 
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS); 

    for (String tab_name : tabs){ 
     actionBar.addTab(actionBar.newTab().setText(tab_name) 
       .setTabListener(this)); 
    } 

    viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() { 
     @Override 
     public void onPageSelected(int position) { 
      actionBar.setSelectedNavigationItem(position); 
     } 

     @Override 
     public void onPageScrolled(int arg0, float arg1, int arg2) { 
     } 

     @Override 
     public void onPageScrollStateChanged(int arg0) { 
     } 
    }); 
} 

public void onResume(){ 
    super.onResume(); 
    getSettings(); 
} 

public void getSettings() { 
    SharedPreferences sharedPrefs = PreferenceManager.getDefaultSharedPreferences(this); 
    simple_progress = sharedPrefs.getBoolean("simple_progress_key", false); 
} 

@Override 
public void onTabReselected(Tab tab, FragmentTransaction ft) { 
} 

@Override 
public void onTabSelected(Tab tab, FragmentTransaction ft) { 
    viewPager.setCurrentItem(tab.getPosition()); 
    Fragment page = getSupportFragmentManager().findFragmentByTag("android:switcher:" + R.id.pager + ":" + viewPager.getCurrentItem()); 
    if ((viewPager.getCurrentItem() == 0 || viewPager.getCurrentItem() == 1 || viewPager.getCurrentItem() == 2) && (page != null)) { 
     current_tab = ((MyFragment) page); 
     current_tab.update(); 
    } 
} 

@Override 
public void onTabUnselected(Tab tab, FragmentTransaction ft) { 
} 

public static void update(){ 
    current_tab.update(); 
} 
} 

我的片段類在那裏我得到了新的空指針異常ONY：

public class AllShowsFragment extends MyFragment { 
private SQLiteDatabase newDB; 
private ListView lv; 
private List<Show> showList = new ArrayList<Show>(); 
private int position; 
private DatabaseHandler db; 

@Override 
public View onCreateView(LayoutInflater inflater, ViewGroup container, 
         Bundle savedInstanceState) { 
    View rootView = inflater.inflate(R.layout.fragment_shows, container, false); 
    lv = (ListView) rootView.findViewById(R.id.mainlist); 
    lv.setOnItemLongClickListener(new OnItemLongClickListener() { 
     public boolean onItemLongClick(AdapterView<?> arg0, View arg1, 
             int pos, long id) { 
      position = pos; 
      createClickMenu(); 
      return true; 
     } 
    }); 
    return rootView; 
} 

public void update(){ 
    populateListfromDB(); 
} 

public void populateListfromDB() { 
    db = new DatabaseHandler(this.getActivity()); 
    newDB = db.getWritableDatabase(); //Exception at this line 
    showList.clear(); 

    Cursor cursor = newDB.rawQuery("SELECT id as _id, title, cseason, cepisode, watched FROM " 
      + db.TABLE_SHOWS, null); 

    cursor.moveToFirst(); 

    if (cursor != null & cursor.moveToFirst()) { 
     cursor.moveToFirst(); 
     do { 
      Show show = new Show(Integer.parseInt(cursor.getString(0)), cursor.getString(1), 
        cursor.getString(2), cursor.getString(3), 
        Integer.parseInt(cursor.getString(4))); 
      showList.add(show); 
     } while (cursor.moveToNext()); 
    } 

    myCursorAdapter = new CustomAdapter(this, showList,((MainActivity)this.getActivity()).editmode, 
      ((MainActivity)this.getActivity()).simple_progress); 
    lv.setAdapter(myCursorAdapter); 
} 
} 

例外，前幾行：

 Caused by: java.lang.NullPointerException 
     at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224) 
     at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:164) 
     at wrodarczyk.com.showtracker.AllShowsFragment.populateListfromDB(AllShowsFragment.java:55) 
     at wrodarczyk.com.showtracker.AllShowsFragment.update(AllShowsFragment.java:50) 
     at wrodarczyk.com.showtracker.MainActivity.update(MainActivity.java:183) 
     at wrodarczyk.com.showtracker.MainActivity.onTabSelected(MainActivity.java:134) 
     at com.android.internal.app.ActionBarImpl.selectTab(ActionBarImpl.java:577) 
     at com.android.internal.app.ActionBarImpl.addTab(ActionBarImpl.java:506) 
     at com.android.internal.app.ActionBarImpl.addTab(ActionBarImpl.java:492) 
     at wrodarczyk.com.showtracker.MainActivity.onCreate(MainActivity.java:53) 

Answer 1

當你有一個ViewPager頁面小，固定的號碼，你可以簡單地保持引用自己的預先填充列表：

public TabsPagerAdapter(FragmentManager fm){ 
    super(fm); 
    myTabs.add(new AllShowsFragment()); 
    myTabs.add(new UnWatchedShowsFragment()); 
    // ... 
} 

鑑於此，您getItem()方法只會返回一個來自此數組列表的索引元素。

一旦你有你自己的標籤片段的指數，你可以存儲current_tab在int，你的更新調用是一樣的東西

mAdapter.myTabs.get(current_tab).update();