android - Android 片段兼容性问题

Question

我有片段的主要活动

public class MainActivity extends Activity implements OnClickListener {

    Fragment fragment;
    Button btn1, btn2,btn3;

    @Override
    public void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        btn1 = (Button)findViewById(R.id.button1);
        btn2 = (Button)findViewById(R.id.button2);

        FragmentManager fm = getFragmentManager();
        FragmentTransaction ft = fm.beginTransaction();

        MyFragment myfragment = new MyFragment();
        ft.add(R.id.myFragment, myfragment);
        ft.commit();
    }

    @Override
    public void onClick(View v){

        Fragment fragment = null;
        if(v == btn1){
            fragment = new Fragment1();
        }else if(v == btn2){
            fragment = new Fragment2();
        }

        FragmentTransaction transcation = getFragmentManager().beginTransaction();
        transcation.replace(R.id.myFragment,fragment);
        transcation.addToBackStack(null);
        transcation.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_CLOSE);
        transcation.commit();
    }
}

和三个类似的简单片段

public class Fragment2 extends Fragment {


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate( R.layout.fragment2, container, false);
    }
}

它适用于 HC 和 ICS，但如果我尝试在旧版本 2.3 上运行。它崩溃了....我也添加了支持的库，但不起作用。还有一个问题......请向我推荐有助于从正常开发转向片段的好教程或视频。