Fragment(11)FragmentTransaction的 add 注意事项

getFragmentManager().beginTransaction().add(R.id.frgmt_top, new TopFrgmt()).commit()

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MActivity"
    tools:ignore="MergeRootFrame" >
</FrameLayout>

public class MActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        if (savedInstanceState == null) {
            getFragmentManager().beginTransaction()
                    .add(R.id.container, new MainFrgmt()).commit();
        }
    }
}

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/LinearLayout1"
    ... >
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal" >
       ...
    </LinearLayout>
    <LinearLayout
        android:id="@+id/frgmt_top"
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:layout_marginTop="18dp"
        android:gravity="center_horizontal"
        android:background="#ff09a23d"
        android:orientation="horizontal" >

        <TextView
            android:id="@+id/textView2"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="top" />
    </LinearLayout>

</LinearLayout>

public class MainFrgmt extends Fragment implements OnClickListener {
    private LinearLayout top, center, bottom;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.frgmt_main, container, false);
　　　　 ...return rootView;
    }
    @Override
    public void onClick(View v) {
        FragmentManager fmgr = getFragmentManager();
        FragmentTransaction ft = fmgr.beginTransaction();
        switch (v.getId()) {
        case R.id.tog_top:
            if (tog_top.isChecked()) {
                ft.add(R.id.frgmt_top, new TopFrgmt());
            } else {
                Fragment frmg = fmgr.findFragmentById(R.id.frgmt_top);
                ft.remove(frmg);
            }
            break;
　　//...