Fragment在Android应用中的作用有哪些

 ActivityUtils.startLogin(this, new ActivityUtils.ActivityResultListner() {
  @Override
  public void loginsuccess() {
  ((Button)v).setText("您已登陆");
  }

  @Override
  public void logincancel() {
  Toast.makeText(MainActivity.this,"您已取消登陆",0).show();
  }
 });

//用abstract class我们在不需要cancel回掉时可以不重写
 public abstract static class ActivityResultListner {
 public void loginsuccess() {
 }

 public void logincancel() {
 }
 }

 public static class MyFragment extends Fragment {

 public static final int LOGIN = 123;

 ActivityResultListner listener;

 public void setListener(ActivityResultListner listener) {
  this.listener = listener;
 }

 @Override
 public void onActivityResult(int requestCode, int resultCode, Intent data) {
  super.onActivityResult(requestCode, resultCode, data);
  if (requestCode == LOGIN) {
  if (resultCode == RESULT_OK) {
   if (listener != null)
   listener.loginsuccess();
  } else {
   if (listener != null)
   listener.logincancel();
  }
  }
 }

 public static void startLogin(FragmentActivity context, ActivityResultListner listener) {
 //先看activity是否添加过该fragment， 添加根据Tag找出 ，没有就添加
 FragmentManager manager = context.getSupportFragmentManager();
 MyFragment myFragment = null;
 Fragment loginf = manager.findFragmentByTag(MyFragment.LOGIN + "");
 if (loginf == null) {
  myFragment = new MyFragment();
  manager.beginTransaction().add(myFragment, MyFragment.LOGIN + "").commit();
  //这句是让commit立即生效，不然运行会报错，fragment还没有被attach
  manager.executePendingTransactions();
 } else {
  myFragment = (MyFragment) loginf;
 }
 //设置监听
 myFragment.setListener(listener);
 Intent intent = new Intent(context, loginActivity.class);
 myFragment.startActivityForResult(intent, MyFragment.LOGIN);
 }