Fragment中调用父Activity中方法

OneFragment:

private MainActivity mainActivity;
 
//onAttach(),当fragment被绑定到activity时被调用(Activity会被传入.).
@Override
public void onAttach(Context context) {
   super.onAttach(context);
   mainActivity = (MainActivity) context;
}

// 切换到TwoFragment
mainActivity.showTwoFragment();

MainActivity:

public void showTwoFragment() {
   if (!(fragmentAll instanceof TwoFragment)) {
      FragmentTransaction fragmentTransaction = getSupportFragmentManager()
            .beginTransaction();
      //如果所有的fragment都不为空的话，把所有的fragment都进行隐藏。最开始进入应用程序，fragment为空时，此方法不执行
      hideFragment(fragmentTransaction);
      //如果这个fragment为空的话，就创建一个fragment，并且把它加到ft中去.如果不为空，就把它直接给显示出来
      if(twoFragment == null){
         twoFragment = new TwoFragment();
         fragmentTransaction.add(R.id.emptyFragment, twoFragment);
      }else {
         fragmentTransaction.show(twoFragment);
      }
      //一定要记得提交
      fragmentTransaction.commit();
   }