在活动中调用片段方法

Question

我想在我的MainActivity中调用片段方法。这个片段连接到ViewPager在我的MainActivity

我已经调用的方法在我的MainActivity

mViewPager = (ViewPager) findViewById(R.id.pager); 
     setupViewPager(mViewPager); 

private void setupViewPager(ViewPager mViewPager) { 
     mSectionsPagerAdapter = new SectionsPagerAdapter(this, getSupportFragmentManager()); 
     mSectionsPagerAdapter.addFragment(new FeedsFragment(), "Around You"); 
     mSectionsPagerAdapter.addFragment(new InboxFragment(), "Shares"); 
     mViewPager.setAdapter(mSectionsPagerAdapter); 
    }  

@Override 
    public void onLocationChanged(Location location) { 
     currentLocation = location; 
     if (lastLocation != null 
       && geoPointFromLocation(location) 
       .distanceInKilometersTo(geoPointFromLocation(lastLocation)) < 0.01) { 
      // If the location hasn't changed by more than 10 meters, ignore it. 
      return; 
     } 
     lastLocation = location; 
     if (!hasSetUpInitialLocation) { 
      // Zoom to the current location. 
      hasSetUpInitialLocation = true; 
     } 
       FeedsFragment fragment = (FeedsFragment) getSupportFragmentManager().findFragmentById(R.id.feeds_fragment); 
     if(fragment != null) { 
      fragment.doFeedsQuery(); 
     } 

    } 

这是我SectionsPagerAdapter

protected Context mContext; 
    private final List<Fragment> mFragmentList = new ArrayList<>(); 
    private final List<String> mFragmentTitleList = new ArrayList<>(); 

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

    @Override 
    public Fragment getItem(int position) { 
     // getItem is called to instantiate the fragment for the given page. 
     // Return a DummySectionFragment (defined as a static inner class 
     // below) with the page number as its lone argument. 

     return mFragmentList.get(position); 
    } 

    @Override 
    public int getCount() { 
     return mFragmentList.size(); 
    } 

    public void addFragment(Fragment fragment, String title) { 
     mFragmentList.add(fragment); 
     mFragmentTitleList.add(title); 
    } 

    @Override 
    public CharSequence getPageTitle(int position) { 

     return mFragmentTitleList.get(position); 
    } 

} 

这是我从我的片段调用方法

public void doFeedsQuery() { 
      Location myLoc = (MainActivity.currentLocation == null) ? MainActivity.lastLocation : MainActivity.currentLocation; 
     // If location info is available, load the data 
     if (myLoc != null) { 
     // Refreshes the list view with new data based 
     // usually on updated location data. 
      feedsQueryAdapter.loadObjects(); 
     } 
    } 

这是我打电话给片段的资源ID

​​

这里是我的logcat

java.lang.NullPointerException: Attempt to invoke virtual method 'void io.wyntr.peepster.Fragments.FeedsFragment.doFeedsQuery()' on a null object reference 
                 at io.wyntr.peepster.Activities.MainActivity.onLocationChanged(MainActivity.java:687) 
                 at com.google.android.gms.location.internal.zzk$zzb.handleMessage(Unknown Source) 
                 at android.os.Handler.dispatchMessage(Handler.java:102) 
                 at android.os.Looper.loop(Looper.java:168) 
                 at android.app.ActivityThread.main(ActivityThread.java:5845) 
                 at java.lang.reflect.Method.invoke(Native Method) 

我不知道我在哪里出了问题。

Answer 1

您只能在初始化后调用片段的方法。

例子：

FragmentManager fragmentManager = getFragmentManager() 
FragmentTransaction fragmentTransaction =fragmentManager.beginTransaction(); 

Fragment yourFragment = new FeedsFragment(); 
fragmentTransaction.add(R.id.fragment_container, yourFragment); 
fragmentTransaction.commit(); 

yourFragment.doFeedsQuery(); 

有大量的文档超过这一点，并在计算器等相关问题。

Answer 2

你的问题似乎是在这里：

FeedsFragment fragment = (FeedsFragment) getSupportFragmentManager().findFragmentById(R.id.feeds_fragment); 

findFragmentById()将试图找到在XML指定id既然您使用的是动态方法，你没有一个。你应该改用为findFragmentByTag这样的：

FeedsFragment fragment = (FeedsFragment) getSupportFragmentManager().findFragmentByTag("Around You"); 

Answer 3

请使用以下FeedsFragment片段的代码中获取片段对象。

int size = mSectionsPagerAdapter.getCount(); 
    for (int i = 0; i < size; i++) { 
     Fragment fragment = mSectionsPagerAdapter.getItem(i); 
     if (fragment != null && fragment instanceof FeedsFragment) { 
      ((FeedsFragment)fragment).doFeedsQuery(); 
     } 
    } 

Answer 4

针对您的问题的两种解决方案。创建您的片段的实例作为班级成员，另一个已经告知Hiren Dabhi。

private FeedsFragment feedFragment;//Use as class instance. 

private void setupViewPager(ViewPager mViewPager) { 
     mSectionsPagerAdapter = new SectionsPagerAdapter(this, getSupportFragmentManager()); 
     mSectionsPagerAdapter.addFragment(feedFragment, "Around You"); 
     mSectionsPagerAdapter.addFragment(new InboxFragment(), "Shares"); 
     mViewPager.setAdapter(mSectionsPagerAdapter); 
    } 

，并简单地检查它不是空，isVisibleToUser然后调用你的方法

if(feedFragment != null && feedFragment.isVisible()) 
    feedFragment.doFeedsQuery();