getActivity() 返回 null - 我第二次打开片段

Question

当我按下按钮时，ShowActivities 片段正在被实例化。我第一次按下它时一切正常。这就是我在 logCat 中得到的：

11-16 23:41:12.560 25874-25874/name.company.newapp E/onCreate: getActivity() == null: false
11-16 23:41:12.560 25874-25874/name.company.newapp E/onCreate: Hash: 1036405628
11-16 23:41:12.570 25874-25874/name.company.newapp E/onCreateView: getActivity() == null: false
11-16 23:41:12.570 25874-25874/name.company.newapp E/onCreateView: Hash: 1036405628
11-16 23:41:12.630 25874-25874/name.company.newapp E/onConnected: getActivity() == null: false
11-16 23:41:12.630 25874-25874/name.company.newapp E/onConnected: Hash: 1036405628

一切正常。我按后退键并返回到上一个片段。现在我再次点击按钮打开ShowActivities片段，现在logCat返回的是：

11-16 23:41:13.890 25874-25874/name.company.newapp E/onCreate: getActivity() == null: false
11-16 23:41:13.890 25874-25874/name.company.newapp E/onCreate: Hash: 252302789
11-16 23:41:13.890 25874-25874/name.company.newapp E/onCreateView: getActivity() == null: false
11-16 23:41:13.890 25874-25874/name.company.newapp E/onCreateView: Hash: 252302789
11-16 23:41:13.900 25874-25874/name.company.newapp E/onConnected: getActivity() == null: true
11-16 23:41:13.900 25874-25874/name.company.newapp E/onConnected: Hash: 1036405628

onConnected 在同一个片段上被调用，尽管 onCreate 正在创建另一个片段。

ShowActivity 片段

    public class ShowActivities extends Fragment implements
            GoogleApiClient.ConnectionCallbacks, GoogleApiClient.OnConnectionFailedListener, LocationListener {


        public ShowActivities() {
        }

        private static GoogleApiClient googleClient;
        private Location mLastLocation;
        private PendingResult<LocationSettingsResult> result;
        private LocationRequest locationRequest;



        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
         Log.e("onCreate"," "+(getActivity() == null));
            if (googleClient==null) {
                googleClient = new GoogleApiClient.Builder(getActivity())
                        .addApi(LocationServices.API)
                        .addConnectionCallbacks(this)
                        .addOnConnectionFailedListener(this)
                        .build();
            }
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                 Bundle savedInstanceState) {
 super.onCreateView(inflater, container, savedInstanceState);
Log.e("onCreateView"," "+(getActivity() == null));
            // Defines the xml file for the fragment
            View view = inflater.inflate(R.layout.upload_activity_fragment, container, false);
            return view;
        }


        @Override
        public void onViewCreated(View view, Bundle savedInstanceState) {

        }


        @Override
        public void onConnected(Bundle bundle) {
            Log.e("onConnected"," "+(getActivity() == null));
            locationRequest = LocationRequest.create();
            locationRequest.setPriority(LocationRequest.PRIORITY_HIGH_ACCURACY);

            LocationSettingsRequest.Builder builder = new LocationSettingsRequest.Builder()
                    .addLocationRequest(locationRequest);

            builder.setAlwaysShow(true);
            result = LocationServices.SettingsApi.checkLocationSettings(googleClient, builder.build());

            if (result != null) {
                result.setResultCallback(new ResultCallback<LocationSettingsResult>() {
                    @Override
                    public void onResult(LocationSettingsResult locationSettingsResult) {
                        final Status status = locationSettingsResult.getStatus();
                        switch (status.getStatusCode()) {
                            case LocationSettingsStatusCodes.SUCCESS:
                                // All location settings are satisfied. The client can initialize location
                                // requests here.

                                break;
                            case LocationSettingsStatusCodes.RESOLUTION_REQUIRED:
                                // Location settings are not satisfied. But could be fixed by showing the user
                                // a optionsDialog.
                                try {
                                    // Show the optionsDialog by calling startResolutionForResult(),
                                    // and check the result in onActivityResult().
                                    if (status.hasResolution()) {
                                        status.startResolutionForResult(getActivity(), 1000);
                                        Log.e("location", "Resolution");
                                    }
                                } catch (IntentSender.SendIntentException e) {
                                    // Ignore the error.
                                    Log.e("location", "Resolution2");
                                }
                                break;
                            case LocationSettingsStatusCodes.SETTINGS_CHANGE_UNAVAILABLE:
                                // Location settings are not satisfied. However, we have no way to fix the
                                // settings so we won't show the optionsDialog.
                                break;
                        }
                    }
                });
            }
        }

        @Override
        public void onActivityResult(int requestCode, int resultCode, Intent data) {
            super.onActivityResult(requestCode, resultCode, data);
            if ((resultCode == Activity.RESULT_OK) && (requestCode == 1000)) {
                mLastLocation = LocationServices.FusedLocationApi.getLastLocation(googleClient);
            }
        }

        @Override
        public void onConnectionSuspended(int i) {

        }

        @Override
        public void onConnectionFailed(ConnectionResult connectionResult) {

        }

        @Override
        public void onLocationChanged(Location location) {
            Log.e("location", "finally");
        }

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

        }

        @Override
        public void onPause() {
            super.onPause();
            if (googleClient!=null) {
                googleClient = null;
            }
        }

    }

这已经解决了。退出片段后，我修改了 onPause 以使 googleClient 为空。

Answer 1

首先，请把onCreateView中的super方法调用改为调用super.onCreateView(...)。

 @Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    // Defines the xml file for the fragment
    View view = inflater.inflate(R.layout.upload_activity_fragment, container, false);
    return view;
}

然后把这块移到onActivityCreated(...)

client = new NightAssetRestClient(getActivity());
    locationQueries = new LocationQueries(getActivity());
    if (googleClient==null) {
        googleClient = new GoogleApiClient.Builder(getActivity())
                .addApi(LocationServices.API)
                .addConnectionCallbacks(this)
                .addOnConnectionFailedListener(this)
                .build();
    }

尽管如此，您仍然需要在 onConnect(...) 中检查 null，因为即使底层 Activity 被销毁，异步进程也可以持有此 Fragment 的引用。

在 onCreate 方法中添加行 setRetainInstance(boolean) 可能会有所帮助。

Answer 2

尝试先通过TAG获取占位符中的Fragment实例，如果没有之前的实例，则创建实例。

    Fragment ShowActivities = getFragmentManager().getFragmentByTag(YOUR_TAG);
    if (showActivities == null) {
        showActivities = new ShowActivities();
        getFragmentManager().beginTransaction()
                .replace(R.id.your_placeholder, showActivities, YOUR_TAG)
                .addToBackStack(null)
                .commit();
    }