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Fragment does not work
public class MainActivity extends FragmentActivity {
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .replace(R.id.fragment_placeholder, new MyFragment())
                .commit();
    }

public class MyFragment extends ListFragment implements
    LoaderCallbacks<Void> {

public static final String TAG = "Test_Tuo";

private MyAdapter mAdapter;
private ArrayList<String> mItems;
private LayoutInflater mInflater;
private Button mBtnReload;

private boolean mFirstRun = true;
private final Handler mHandler = new Handler();

private static final String[] WORDS = { "Lorem", "ipsum", "dolor", "sit",
        "amet", "consectetur", "adipiscing", "elit", "Fusce", "pharetra",
        "luctus", "sodales" };

private static final int SIZE = 12;
private static final int SLEEP = 2000;

private final int wordBarColor = R.color.word_bar;
private Resources mRes;

@Override
public void onActivityCreated(Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
    // this is really important in order to save the state across screen
    // configuration changes for example
    setRetainInstance(true);

    // LoaderManager.enableDebugLogging(true);

    mRes = getResources();
    mInflater = LayoutInflater.from(getActivity());

    mBtnReload = (Button) getActivity().findViewById(R.id.btn);
    mBtnReload.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            // first time
            if (mFirstRun) {
                Log.d(TAG, "Parte mFirstRun");
                mFirstRun = false;
                mBtnReload.setText(mRes.getString(R.string.restart));

                startLoading();
            }
            // already started once
            else {
                Log.d(TAG, "Parte restart");
                restartLoading();
            }
        }
    });

    // you only need to instantiate these the first time your fragment is
    // created; then, the method above will do the rest
    if (mAdapter == null) {
        mItems = new ArrayList<String>();
        mAdapter = new MyAdapter(getActivity(), mItems);
    }
    getListView().setAdapter(mAdapter);

    // ---- magic lines starting here -----
    // call this to re-connect with an existing
    // loader (after screen configuration changes for e.g!)
    LoaderManager lm = getLoaderManager();
    if (lm.getLoader(0) != null) {
        lm.initLoader(0, null, this);
    }
    // ----- end magic lines -----

    if (!mFirstRun) {
        mBtnReload.setText(mRes.getString(R.string.restart));
    }
}

protected void startLoading() {
    showDialog();

    // first time we call this loader, so we need to create a new one
    getLoaderManager().initLoader(0, null, this);
}

protected void restartLoading() {
    showDialog();

    mItems.clear();
    mAdapter.notifyDataSetChanged();
    getListView().invalidateViews();

    // --------- the other magic lines ----------
    // call restart because we want the background work to be executed
    // again
    Log.d(TAG, "restartLoading(): re-starting loader");
    getLoaderManager().restartLoader(0, null, this);
    // --------- end the other magic lines --------
}

@Override
public Loader<Void> onCreateLoader(int id, Bundle args) {
    AsyncTaskLoader<Void> loader = new AsyncTaskLoader<Void>(getActivity()) {

        @Override
        public Void loadInBackground() {
            try {
                // simulate some time consuming operation going on in the
                // background
                Log.d(TAG, "Parte il caricamento dei dati");

                for (int i = 0; i < SIZE; ++i) {
                    mItems.add(WORDS[i]);
                }

                Thread.sleep(SLEEP);
            } catch (InterruptedException e) {
                Log.d(TAG, "eccezione"+e.toString());
            }
            return null;
        }
    };
    // somehow the AsyncTaskLoader doesn't want to start its job without
    // calling this method
    loader.forceLoad();
    return loader;
}

@Override
public void onLoadFinished(Loader<Void> loader, Void result) {

    mAdapter.notifyDataSetChanged();
    hideDialog();
    Log.d(TAG, "onLoadFinished(): done loading!");

}

@Override
public void onLoaderReset(Loader<Void> loader) {
}

private class MyAdapter extends ArrayAdapter<String> {

    public MyAdapter(Context context, List<String> objects) {
        super(context, R.layout.item_list, R.id.text, objects);
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        View view = convertView;
        Wrapper wrapper;

        if (view == null) {
            view = mInflater.inflate(R.layout.item_list, null);
            wrapper = new Wrapper(view);
            view.setTag(wrapper);
        } else {
            wrapper = (Wrapper) view.getTag();
        }

        wrapper.getTextView().setText(getItem(position));
        wrapper.getBar().setBackgroundColor(
                getResources().getColor(wordBarColor));
        return view;
    }

}

// use an wrapper (or view holder) object to limit calling the
// findViewById() method, which parses the entire structure of your
// XML in search for the ID of your view
private static class Wrapper {
    private final View mRoot;
    private TextView mText;
    private View mBar;

    public Wrapper(View root) {
        mRoot = root;
    }

    public TextView getTextView() {
        if (mText == null) {
            mText = (TextView) mRoot.findViewById(R.id.text);
        }
        return mText;
    }

    public View getBar() {
        if (mBar == null) {
            mBar = mRoot.findViewById(R.id.bar);
        }
        return mBar;
    }
}

public static class MyAlertDialog extends DialogFragment {
    /*
     * All subclasses of Fragment must include a public empty constructor.
     * The framework will often re-instantiate a fragment class when needed,
     * in particular during state restore, and needs to be able to find this
     * constructor to instantiate it. If the empty constructor is not
     * available, a runtime exception will occur in some cases during state
     * restore.
     */
    public MyAlertDialog() {
    }

    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {
        ProgressDialog progress = new ProgressDialog(getActivity());
        progress.setMessage(getString(R.string.loading));
        return progress;
    }
}

private void showDialog() {
    FragmentTransaction ft = getFragmentManager().beginTransaction();
    Fragment prev = getFragmentManager().findFragmentByTag("dialog");
    if (prev != null) {
        ft.remove(prev);
    }

    // Create and show the dialog.
    DialogFragment newFragment = new MyAlertDialog();
    newFragment.show(ft, "dialog");
}

private void hideDialog() {
    mHandler.post(new Runnable() {

        @Override
        public void run() {
            FragmentTransaction ft = getFragmentManager()
                    .beginTransaction();
            Fragment prev = getFragmentManager()
                    .findFragmentByTag("dialog");
            if (prev != null) {
                ft.remove(prev).commit();
            }
        }
    });
}

<?xml version="1.0" encoding="utf-8"?>
 <LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent">
<FrameLayout
    android:id="@+id/fragment_placeholder"
    android:layout_width="fill_parent"
    android:layout_height="0dp"
    android:layout_weight="1" />
<Button
    android:id="@+id/btn"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/start"/>