android - MainActivity 为空

Question

我的 Android 项目有问题。我正在处理 fragment 。其中一个类是 MainActivity 类，它扩展了 FragmentActivity。还有两个扩展 fragment 的类。我想从 fragment 中调用 MainActivity 方法，但每次它都会给我一个 NullPointerException。我已经尝试了一切，但问题仍然存在。

MainActivity 如下所示:

public class MainActivity extends FragmentActivity implements ActionBar.TabListener {

SectionsPagerAdapter mySectionsPagerAdapter;
ViewPager myViewPager;
ActionBar myactionBar;
Schuldet_mir fr_schuldetMir;
Ich_schulde fr_ichSchulde;
public DataBaseHandler myDBHandler;
public String testString = null;

//  ------------------------------------------------------------------------

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

    myactionBar = getActionBar();
    myactionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

    setSectionsPagerAdapter();
    initActionbar();

    initFragments();
}

//  ------------------------------------------------------------------------

@Override
protected void onResume() {
    super.onResume();

    openDB();
    populateListViewFromDB();       
}

//  ------------------------------------------------------------------------

private void setSectionsPagerAdapter() {
    mySectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

    myViewPager = (ViewPager) findViewById(R.id.pager);
    myViewPager.setAdapter(mySectionsPagerAdapter);

    myViewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
                @Override
                public void onPageSelected(int position) {
                    myactionBar.setSelectedNavigationItem(position);
                }
    });     
}

//  ------------------------------------------------------------------------

private void initActionbar() {
    for (int i = 0; i < mySectionsPagerAdapter.getCount(); i++) {
        myactionBar.addTab(myactionBar.newTab()
                   .setText(mySectionsPagerAdapter.getPageTitle(i))
                   .setTabListener(this));
    }
}

//  ------------------------------------------------------------------------

private void initFragments() {

    fr_schuldetMir = (Schuldet_mir)mySectionsPagerAdapter.getItem(0);
    fr_ichSchulde  = (Ich_schulde)mySectionsPagerAdapter.getItem(1);
}

//  ------------------------------------------------------------------------

private void openDB() {
    myDBHandler = new DataBaseHandler(this);
    myDBHandler.open();
}

private void closeDB() {
    myDBHandler.close();
}

public String test() {
    testString = "Test Test TEst!";
    return testString;
}
//  ------------------------------------------------------------------------

private void populateListViewFromDB() {

    fr_schuldetMir.populateListViewFromDB();        
}

//  ------------------------------------------------------------------------

@Override
protected void onDestroy() {
    super.onDestroy();

    closeDB();
}

调用MainActivity方法的 fragment :

public class Schuldet_mir extends Fragment {

public static String ARG_SECTION_NUMBER = "section_number";
ListView listView;
TextView verliehenesGeld;
float _betrag = 0;
float gesamt = 0;   
int iconID;
EditText eintrag_name, eintrag_betrag, eintrag_grund;
String name = "";
String grund = "";
String datum = "";
MainActivity mainActivity = (MainActivity)getActivity();
GetDate date;
public Cursor cursor;

public Schuldet_mir(int position) {
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

    View v = inflater.inflate(R.layout.fragment_schuldet_mir, container, false);

    listView        = (ListView)v.findViewById(R.id.listView1);
    verliehenesGeld = (TextView)v.findViewById(R.id.textView1);
    verliehenesGeld.setText(null);

    return v;
}

//  --------------------------------------------------------------------
public void btnClick_SchulderEintragen(View v) {                        //Button Klick to Add Schuldner 

    openDialog();
}

//  --------------------------------------------------------------------
private void openDialog() {                                                 //Dialog erstellen

    final Dialog dialog = new Dialog(getActivity());                    
    dialog.setContentView(R.layout.dialog1);
    dialog.setTitle("Schulden Hinzufügen");
    dialog.setCancelable(false);

    eintrag_name   = (EditText)dialog.findViewById(R.id.editText_name);
    eintrag_betrag = (EditText)dialog.findViewById(R.id.editText_betrag);
    eintrag_grund  = (EditText)dialog.findViewById(R.id.editText_grund);

    Button hinzufügen = (Button)dialog.findViewById(R.id.button2);
    Button abbrechen = (Button)dialog.findViewById(R.id.button1);

    hinzufügen.setOnClickListener(new OnClickListener() {                   //Buttonklick Hinzufügen 

        @Override
        public void onClick(View v) {

            name = eintrag_name.getText().toString();
            _betrag = Float.valueOf(eintrag_betrag.getText().toString());
            grund = eintrag_grund.getText().toString();
            datum = date.returnDate();
            iconID = R.drawable.user_image3;

            mainActivity.myDBHandler.insertRow(name, _betrag, grund, datum);
            populateListViewFromDB();

            dialog.dismiss();
        }
    });

//      --------------------------------------------------------------------
    abbrechen.setOnClickListener(new OnClickListener() {                  //Buttonklick Abbrechen

        @Override
        public void onClick(View v) {
            dialog.dismiss();                       
        }
    });

    dialog.show();                  
}

//  --------------------------------------------------------------------
@SuppressWarnings("deprecation")
public void populateListViewFromDB() {                                  //ListView mit Werten aus der db befüllen

//          cursor = mainActivity.myDBHandler.getAllRows();

    if(mainActivity == null) {
    Log.e("***TEST***", "MAIN ist NULL");
    }
        mainActivity.startManagingCursor(cursor = mainActivity.myDBHandler.getAllRows());

        String[] dbSpaltenNamen = new String[] {DataBaseHandler.KEY_NAME,       //die spalteneinträge wie
                                                DataBaseHandler.KEY_BETRAG, //angeordnet in die jeweiligen
        //                                              DataBaseHandler.KEY_GRUND,  //id's des custom listzeile zuordnen
                                                DataBaseHandler.KEY_DATUM};

        int[] zuListViewIDs = new int[] {R.id.txt_name,                     //hier die id's. name zu name
                                         R.id.txt_betrag,                   //betrag zu betrag...
                                         R.id.datum};

        SimpleCursorAdapter myCursorAdapter 
                = new SimpleCursorAdapter
                            (getActivity(), R.layout.list_zeile, cursor, dbSpaltenNamen, zuListViewIDs);

        listView.setAdapter(myCursorAdapter);
}

private float gesamtSchulden(EditText betrag) {

    gesamt += Float.valueOf(betrag.getText().toString());
    return gesamt;
}

//  --------------------------------------------------------------------
@Override
public void onAttach(Activity activity) {
    super.onAttach(activity);
    try {
        mainActivity = (MainActivity) activity;
    } 
    catch (ClassCastException e) {
        throw new ClassCastException(activity.toString() +  " Fehler: Keine MainActivity-Instanz!");
    }
}

}

SectionsPagerAdapter:

public class SectionsPagerAdapter extends FragmentPagerAdapter {

    private Schuldet_mir f1;
    private Ich_schulde  f2;

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

    @Override
    public Fragment getItem(int position) {
        //damit nicht immer wieder bei einer Anzeige ein neues fragment instanziiert wird, 
        //wurde diese Methode angepasst. D.h. es werden die vorher erzeugten framgente verwendet
        if(position == 0) {
            if (f1 == null) {               
                f1 = new Schuldet_mir(position);            
            }           
            return f1;      
        }       
        else {
            if (f2 == null) {               
                f2 = new Ich_schulde(position);         
            }           
            return f2;  
        }
    }

    @Override
    public int getCount() {
        // Show 2 total pages.
        return 2;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        switch (position) {
        case 0:
            return "... schuldet mir";
        case 1:
            return "Ich schulde ...";
        }
        return null;
    }
}

NullPointerException :

12-29 11:30:30.172: D/ActivityThread(26852): setTargetHeapUtilization:0.25
12-29 11:30:30.172: D/ActivityThread(26852): setTargetHeapIdealFree:8388608
12-29 11:30:30.172: D/ActivityThread(26852): setTargetHeapConcurrentStart:2097152
12-29 11:30:30.192: V/ActivityThread(26852): Class path: /data/app/debtmanager.mainfolder-2.apk, JNI path: /data/data/debtmanager.mainfolder/lib
12-29 11:30:30.262: E/***TEST***(26852): MAIN ist NULL
12-29 11:30:30.272: D/AndroidRuntime(26852): Shutting down VM
12-29 11:30:30.272: W/dalvikvm(26852): threadid=1: thread exiting with uncaught exception (group=0x410fb2a0)
12-29 11:30:30.272: E/AndroidRuntime(26852): FATAL EXCEPTION: main
12-29 11:30:30.272: E/AndroidRuntime(26852): java.lang.RuntimeException: Unable to resume activity {debtmanager.mainfolder/debtmanager.mainfolder.MainActivity}: java.lang.NullPointerException
12-29 11:30:30.272: E/AndroidRuntime(26852):    at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2608)
12-29 11:30:30.272: E/AndroidRuntime(26852):    at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:2636)
12-29 11:30:30.272: E/AndroidRuntime(26852):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2103)
12-29 11:30:30.272: E/AndroidRuntime(26852):    at android.app.ActivityThread.access$600(ActivityThread.java:138)
12-29 11:30:30.272: E/AndroidRuntime(26852):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1204)
12-29 11:30:30.272: E/AndroidRuntime(26852):    at android.os.Handler.dispatchMessage(Handler.java:99)
12-29 11:30:30.272: E/AndroidRuntime(26852):    at android.os.Looper.loop(Looper.java:137)
12-29 11:30:30.272: E/AndroidRuntime(26852):    at android.app.ActivityThread.main(ActivityThread.java:4872)
12-29 11:30:30.272: E/AndroidRuntime(26852):    at java.lang.reflect.Method.invokeNative(Native Method)
12-29 11:30:30.272: E/AndroidRuntime(26852):    at java.lang.reflect.Method.invoke(Method.java:511)
12-29 11:30:30.272: E/AndroidRuntime(26852):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:790)
12-29 11:30:30.272: E/AndroidRuntime(26852):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:557)
12-29 11:30:30.272: E/AndroidRuntime(26852):    at dalvik.system.NativeStart.main(Native Method)
12-29 11:30:30.272: E/AndroidRuntime(26852): Caused by: java.lang.NullPointerException
12-29 11:30:30.272: E/AndroidRuntime(26852):    at fragments.Schuldet_mir.populateListViewFromDB(Schuldet_mir.java:114)
12-29 11:30:30.272: E/AndroidRuntime(26852):    at debtmanager.mainfolder.MainActivity.populateListViewFromDB(MainActivity.java:105)
12-29 11:30:30.272: E/AndroidRuntime(26852):    at debtmanager.mainfolder.MainActivity.onResume(MainActivity.java:49)
12-29 11:30:30.272: E/AndroidRuntime(26852):    at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1187)
12-29 11:30:30.272: E/AndroidRuntime(26852):    at android.app.Activity.performResume(Activity.java:5265)
12-29 11:30:30.272: E/AndroidRuntime(26852):    at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2598)
12-29 11:30:30.272: E/AndroidRuntime(26852):    ... 12 more

第 114 行是以下代码。

mainActivity.startManagingCursor(cursor = mainActivity.myDBHandler.getAllRows());

我不知道为什么 MainActivity 是 null。希望有人能给我一个解决这个问题的建议。

Answer 1

正如其他人所指出的，您试图在将 Fragment 附加到 Activity 之前引用 Activity。

然而，也就是说，您不应该尝试以您尝试的方式直接从 Fragment 调用 Activity 自己的方法。这是可以做到的，但对于使用 Fragments 进行设计来说，这是一种不好的做法。

Fragment 应该是模块化和可重用的 - 因此 Fragment 不需要知道 Activity 是什么类型托管它或可以调用哪些方法。

相反，您应该在 Fragment 本身中定义一个回调接口(interface)，并让 Activity 实现该接口(interface)和相关方法。

参见 Event Callbacks Fragments 指南。

这样做意味着 Fragment 总是“知道”接口(interface)定义了哪些方法以及如何使用它们。只要使用 Fragment 的所有 Activities 实现了接口(interface)，一切就应该可以正常工作。