为什么片段具有默认构造函数? [英] why fragment have default constructor?

问题描述

片段中有默认构造函数，我想知道它的用途以及它提供的功能吗?并且我在没有代码的情况下运行代码，并且在删除代码时找不到任何错误

public class SongListFragment extends Fragment {

   private static final String SONG_IDS = "song_ids";

   // TODO: Rename and change types of parameters
   private int[] songIds;
   private OnFragmentInteractionListener mListener;

   public SongListFragment() {
      // Required empty public constructor
   }

   // TODO: Rename and change types and number of parameters
   public static SongListFragment newInstance(int[] songIds) {
      SongListFragment fragment = new SongListFragment();
      Bundle args = new Bundle();
      args.putIntArray(SONG_IDS, songIds);
      fragment.setArguments(args);
      return fragment;
   }

   @Override
   public void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      if (getArguments() != null) {
         songIds = getArguments().getIntArray(SONG_IDS);
      }
   }

   @Override
   public View onCreateView(
      LayoutInflater inflater,
      ViewGroup      container,
      Bundle         savedInstanceState )
   {
      // Inflate the layout for this fragment
      return inflater.inflate(R.layout.fragment_song_list, container, false);
   }

   // TODO: Rename method, update argument and hook method into UI event
   public void onButtonPressed(Uri uri) {
      if (mListener != null) {
         mListener.onSongSelected(10);
      }
   }

   @Override
   public void onAttach(Context context) {
      super.onAttach(context);
      if (context instanceof OnFragmentInteractionListener) {
         mListener = (OnFragmentInteractionListener) context;
      }
      else {
         throw new RuntimeException( context.toString() +
            " must implement OnFragmentInteractionListener");
      }
   }

   @Override
   public void onDetach() {
      super.onDetach();
      mListener = null;
   }


   public interface OnFragmentInteractionListener {

      public void onSongSelected(int songId);
      }
   }

推荐答案

请参阅此问题和评论/答案.简而言之，Fragments需要为Android系统使用no-args构造函数来实例化它们(我相信活动历史记录管理器会执行此操作，等等).

See this question and comments / answers. In short, Fragments need to have a no-args constructor for the Android system to instantiate them (I believe the activity history manager does this, etc).

如果构造函数是显式的(如未更改的示例中所示)，则确实可以确保在添加其他构造函数的情况下no-args构造函数可以正常工作，并且注释起着提醒作用(即，或原始作者并未真正了解该语言的目的和/或工作方式.

If the constructor is explicit, as in the unaltered example, then it's really there to ensure the no-args constructor works if other constructors are added, and the comment serves as a reminder (that or the original author didn't really understand the purpose and/or how the language works).

如果no-args构造函数可能是隐式的-即在源代码中省略了它，并且没有声明其他构造函数-则将根据

If the no-args constructor may be implicit - ie it is omitted in the source and there are no other constructors declared - then one is created behind the scenes as per the JLS (this is what happened when you deleted the constructor in your example):

如果一个类不包含构造函数声明，则使用默认值没有正式参数且没有throws子句的构造函数是隐式声明.

If a class contains no constructor declarations, then a default constructor with no formal parameters and no throws clause is implicitly declared.

如果要声明的类是原始类Object，则默认构造函数的主体为空.否则，默认构造函数只调用no的超类构造函数争论.

If the class being declared is the primordial class Object, then the default constructor has an empty body. Otherwise, the default constructor simply invokes the superclass constructor with no arguments.

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