Android Requesting Permissions at Run Time

public class ThumbnailFragment extends Fragment {

    private static final int MY_PERMISSIONS_REQUEST_READ_EXTERNAL_STORAGE = 1000;
    private List<ImageGroup> imageGroupList;
    private RecyclerView recyclerViewImages;
    private ThumbnailPageAdapter adapter;
    private RecyclerView.LayoutManager layoutManager;

    public ThumbnailFragment() {
        // Required empty public constructor
    }


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

        View view = inflater.inflate(R.layout.fragment_thumbnail, container, false);

        layoutManager = new LinearLayoutManager(getContext());

        adapter = new ThumbnailPageAdapter();

        recyclerViewImages = (RecyclerView) view.findViewById(R.id.recyclerViewImages);
        recyclerViewImages.setLayoutManager(layoutManager);
        recyclerViewImages.setAdapter(adapter);

        // Here, thisActivity is the current activity
        if (ContextCompat.checkSelfPermission(getActivity(),
                Manifest.permission.READ_EXTERNAL_STORAGE)
                != PackageManager.PERMISSION_GRANTED) {

            // Should we show an explanation?
            if (ActivityCompat.shouldShowRequestPermissionRationale(getActivity(),
                    Manifest.permission.READ_EXTERNAL_STORAGE)) {

                // Show an explanation to the user *asynchronously* -- don't block
                // this thread waiting for the user's response! After the user
                // sees the explanation, try again to request the permission.

            } else {

                // No explanation needed, we can request the permission.

                // if we are in activity
/*                ActivityCompat.requestPermissions(getActivity(),
                        new String[]{Manifest.permission.READ_EXTERNAL_STORAGE},
                        MY_PERMISSIONS_REQUEST_READ_EXTERNAL_STORAGE);*/

                // if we are in fragment
                requestPermissions(new String[]{Manifest.permission.READ_EXTERNAL_STORAGE},
                        MY_PERMISSIONS_REQUEST_READ_EXTERNAL_STORAGE);

                // MY_PERMISSIONS_REQUEST_READ_CONTACTS is an
                // app-defined int constant. The callback method gets the
                // result of the request.
            }
        }
        else {

            // if we have permission we can run our permission related task
            ListFiles();
        }

        return view;
    }

    @Override
    public void onRequestPermissionsResult(int requestCode,
                                           String permissions[], int[] grantResults) {
        switch (requestCode) {
            case MY_PERMISSIONS_REQUEST_READ_EXTERNAL_STORAGE: {
                // If request is cancelled, the result arrays are empty.
                if (grantResults.length > 0
                        && grantResults[0] == PackageManager.PERMISSION_GRANTED) {

                    // permission was granted, yay! Do the
                    // permission-related task you need to do.

                    // this will run just after granting permission
                    // not always

                    getActivity().runOnUiThread(new Runnable() {
                        @Override
                        public void run() {
                            ListFiles();
                        }
                    });

                } else {

                    // permission denied, boo! Disable the
                    // functionality that depends on this permission.
                }
                return;
            }

            // other 'case' lines to check for other
            // permissions this app might request
        }
    }

    public void ListFiles()
    {
        final Path path = new Path();
        final Search search = new Search();
        final AddFileHelper addFileHelper = new AddFileHelper(adapter);

        search.setOnListFilesListener(new Search.OnListFilesListener() {
            @Override
            public void onNewFileFound(File file) {
                addFileHelper.addNewFile(file);
            }

            @Override
            public void onListCompleted(List<File> result) {
                //addFileHelper.close();
            }
        });


        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                search.listFiles(path.getDCIMFile());
            }
        });

        thread.start();
    }
}

References
https://developer.android.com/training/permissions/requesting.html