Android - 当我在打开 GPS 的情况下启动应用程序时出现 NullpointerException

Question

这是我的代码。在打开 GPS 的情况下启动应用程序时发生错误。这也意味着在关闭 GPS 的情况下，该应用程序可以完美运行。

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
        super.onCreateView(inflater, container, savedInstanceState);
        View v = inflater.inflate(R.layout.fragment_map, null, false);        

        map = ((SupportMapFragment) getFragmentManager().findFragmentById(R.id.map))
                .getMap();
        map.setMyLocationEnabled(true);

        LocationManager lm=(LocationManager)getActivity().getSystemService(Context.LOCATION_SERVICE);
        Criteria c = new Criteria();
        String provider = lm.getBestProvider(c, false);

        final TextView switchStatus = (TextView) v.findViewById(R.id.switchStatus);
        Switch mySwitch = (Switch) v.findViewById(R.id.gpsSwitch);

        boolean statusOfGPS = lm.isProviderEnabled(LocationManager.GPS_PROVIDER);
        if (statusOfGPS){
            mySwitch.setChecked(true);
            switchStatus.setText("GPS Switch is currently ON");
        }
        else{               
            mySwitch.setChecked(false);
            switchStatus.setText("GPS Switch is currently OFF");
        }

        mySwitch.setOnCheckedChangeListener(new OnCheckedChangeListener() {

         @Override
         public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {           
          if(isChecked){
           switchStatus.setText("GPS Switch is currently ON");
           Intent viewIntent = new Intent(android.provider.Settings.ACTION_LOCATION_SOURCE_SETTINGS);
           startActivity(viewIntent);         
          }else{
           switchStatus.setText("GPS Switch is currently OFF");
          }           
         }
        });

        //check the current state before we display the screen
        if(mySwitch.isChecked()){
         switchStatus.setText("GPS Switch is currently ON");
        }
        else {
         switchStatus.setText("GPS Switch is currently OFF");
        }

        Location l = lm.getLastKnownLocation(provider);

        if(l!=null)
        {
            onLocationChanged(l);   
        }

        lm.requestLocationUpdates(provider, 20000, 0, this);

        return v;
}

    @Override
    public void onLocationChanged(Location location) {
        TextView ln=(TextView)getView().findViewById(R.id.lng);
        TextView lt=(TextView)getView().findViewById(R.id.lat);

        // Getting latitude of the current location
        double latitude = location.getLatitude();

        // Getting longitude of the current location
        double longitude = location.getLongitude();     

        // Creating a LatLng object for the current location
        LatLng latLng = new LatLng(latitude, longitude);

        Marker myLoc = map.addMarker(new MarkerOptions()
        .position(latLng)
        .title("MyLocation")
        .snippet("I am at ")
        .icon(BitmapDescriptorFactory
            .fromResource(R.drawable.ic_launcher)));

        // Showing the current location in Google Map
        map.moveCamera(CameraUpdateFactory.newLatLng(latLng));

        // Zoom in the Google Map
        map.animateCamera(CameraUpdateFactory.zoomTo(15));  

        ln.setText("long: "+longitude);
        lt.setText("lat: "+latitude);
    }

    @Override
    public void onProviderDisabled(String provider) {
        if (provider.equals("gps")){
            TextView ln=(TextView) getView().findViewById(R.id.lng);
            ln.setText("GPS is OFF");
        }

    }

    @Override
    public void onProviderEnabled(String provider) {
        if (provider.equals("gps")){
            TextView ln=(TextView) getView().findViewById(R.id.lng);
            ln.setText("GPS is ON... wait for updates");
        }

    }

    @Override
    public void onStatusChanged(String provider, int status, Bundle extras) {
        // TODO Auto-generated method stub

    }

这是一个错误日志，但我不知道我的代码的哪一部分给了我一个错误。

    06-22 13:18:21.639: E/AndroidRuntime(26702): FATAL EXCEPTION: main
    06-22 13:18:21.639: E/AndroidRuntime(26702): java.lang.NullPointerException
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at com.gpsocial.fragments.MapFragment.onLocationChanged(MapFragment.java:103)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at com.gpsocial.fragments.MapFragment.onCreateView(MapFragment.java:93)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.support.v4.app.Fragment.performCreateView(Fragment.java:1460)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:911)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1088)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:682)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1444)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:461)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:141)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.support.v4.view.ViewPager.populate(ViewPager.java:1064)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.support.v4.view.ViewPager.populate(ViewPager.java:911)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1432)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.view.View.measure(View.java:16848)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5245)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.view.View.measure(View.java:16848)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5245)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:302)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.view.View.measure(View.java:16848)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5245)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2586)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.view.View.measure(View.java:16848)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2191)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1354)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1537)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1251)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6379)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.view.Choreographer$CallbackRecord.run(Choreographer.java:791)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.view.Choreographer.doCallbacks(Choreographer.java:591)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.view.Choreographer.doFrame(Choreographer.java:561)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:777)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.os.Handler.handleCallback(Handler.java:730)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.os.Handler.dispatchMessage(Handler.java:92)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.os.Looper.loop(Looper.java:137)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at android.app.ActivityThread.main(ActivityThread.java:5493)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at java.lang.reflect.Method.invokeNative(Native Method)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at java.lang.reflect.Method.invoke(Method.java:525)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1209)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1025)
    06-22 13:18:21.639: E/AndroidRuntime(26702):    at dalvik.system.NativeStart.main(Native Method)

有人可以帮忙吗？那将不胜感激。

Answer 1

您不能使用来自onCreateView() 的getView()。它返回 null，正是因为您还没有真正执行完 onCreateView()。

最好的选择可能是将调用延迟到生命周期的后期。例如，使用onStart() 会是一个不错的选择。

另一种可能的解决方法是将要返回的视图分配给成员变量，例如：

private View mMainView;

public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    super.onCreateView(inflater, container, savedInstanceState);
    View v = inflater.inflate(R.layout.fragment_map, null, false);
    mMainView = v;
    ...
}

然后在onLocationChanged() 中使用mMainView。