android - 未从 firebase 填充的 fragment 内的 ListView ，但我可以将数据保存到可能的 firebase DB

Question

我有一个 MainActivity.Java 类，它使用 View 寻呼机来呈现我的 fragment 。其中一个 fragment 称为 DashBoardFrag，它应该从我的 firebase 数据库中选取数据并将其填充到 ListView 中。目前我可以发送数据并将其保存到 firebas 但我无法检索。检索应该在 DashBoardFrag 加载或显示给用户时发生。当我运行我的应用程序时，它不会崩溃，但再次没有显示任何内容。这是我在 DashBoardFrag fragment 中的 onCreateView 方法

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the fragment_dash_board for this fragment
    View rootView = inflater.inflate(R.layout.fragment_dash_board, container, false);
    newsListView = (ListView) rootView.findViewById(R.id.newslist_view);

    dbref = FirebaseDatabase.getInstance().getReference();
    dbOperationsHelper = new DBOperationsHelper(dbref);

    /**
     * SET ADAPTER
     */

     //dbAdapter = new DBAdapter(getActivity().getApplicationContext(),dbOperationsHelper.retrieveNews());
    arrayAdapter = new ArrayAdapter<String>(getActivity(), android.R.layout.simple_list_item_1,dbOperationsHelper.retrieveNews());

    //Toast.makeText(getActivity().getApplicationContext(), "MUST NOT BE EMPTY", Toast.LENGTH_SHORT).show();
     newsListView.setAdapter(dbAdapter);


    return rootView;

}

这是我的 DBOperationsHelper 类

public class DBOperationsHelper {
DatabaseReference dbref;
Boolean saved;
ArrayList<String> newsArraylist = new ArrayList<>();

public DBOperationsHelper(DatabaseReference db){
    this.dbref = db;
}

public Boolean save(DBModel news){
    if(news == null){
        saved = false;
    }else{
        try {
            dbref.child("DBModel").push().setValue(news);
            saved = true;
        } catch (DatabaseException e) {

            e.printStackTrace();
            saved = false;
        }
    }
    return saved;
}

private void fetchData(DataSnapshot dataSnapshot){
    newsArraylist.clear();
    for(DataSnapshot ds : dataSnapshot.getChildren()){
        String news = ds.getValue(DBModel.class).getNewsHead();
        newsArraylist.add(news);
    }
}

/**
 * RETRIEVE NEWS
 */
public ArrayList<String> retrieveNews(){
   dbref.addChildEventListener(new ChildEventListener() {
       @Override
       public void onChildAdded(DataSnapshot dataSnapshot, String s) {
         fetchData(dataSnapshot);
       }

       @Override
       public void onChildChanged(DataSnapshot dataSnapshot, String s) {
           fetchData(dataSnapshot);
       }

       @Override
       public void onChildRemoved(DataSnapshot dataSnapshot) {

       }

       @Override
       public void onChildMoved(DataSnapshot dataSnapshot, String s) {

       }

       @Override
       public void onCancelled(DatabaseError databaseError) {

       }
   });
    return newsArraylist;
}
}

这是我的 DBModel 类

public class DBModel {
boolean bool;
String name, email, username, pass, newsHead, newsBody;

public void setIsLogin(boolean bl){
    this.bool = bl;

}
public boolean getIsLogin(){
    return this.bool;
}

public void setEmail(String email){
    this.email = email;

}
public String getEmail(){
    return this.email;
}

public void setUsername(String username){
    this.username = username;

}
public String getUsername(){
    return this.username;
}

public void setPass(String pass){
    this.pass = pass;

}
public String getPass(){
    return this.pass;
}

public void setNewsHead(String nws_name){
    this.newsHead = nws_name;

}

public String getNewsHead() {
    return newsHead;
}

public void setNewsBody(String newsBody) {
    this.newsBody = newsBody;
}

public String getNewsBody() {
    return newsBody;
}
}

Answer 1

最后我设法使用 FirebaseRecyclerAdapter 来填充我的回收器(从使用 ListView 更改为 recyclerview)。

这是我做的。

1.更改我的 DBModel(NewsModel) 对象类，使其具有 getters 和 setters 用于我想要显示的详细信息和如图所示检索

   public class NewsModel {
    String email, username, pass, newsHead, newsBody;


public NewsModel() {
}

public NewsModel(String newsHead, String newsBody) {
    this.newsHead = newsHead;
    this.newsBody = newsBody;

}

public String getNewsHead() {
    return newsHead;
}

public void setNewsHead(String nws_name){
    this.newsHead = nws_name;

}

public String getNewsBody() {
    return newsBody;
}

public void setNewsBody(String newsBody) {
    this.newsBody = newsBody;
}


}

2.通过创建一个扩展 RecyclerViewHolder 的内部静态类来编辑我的 fragment ，我用它来设置文本字段的值，如图所示

DatabaseReference dbref;
FirebaseRecyclerAdapter<NewsModel,NewsModelVH> firebasenewsRecycleAdapter ;
RecyclerView newsrecyclerView;
LinearLayoutManager nwlinearLayoutManager;
ArrayList<NewsModel> newsArraylist = new ArrayList<>();



public static class NewsModelVH extends RecyclerView.ViewHolder{

    public TextView newsHead, newsBody;

    public NewsModelVH(View itemView) {
        super(itemView);
        this.newsHead = (TextView) itemView.findViewById(R.id.listview_item_title);
        this.newsBody = (TextView) itemView.findViewById(R.id.listview_item_short_description);
       }
    }// End NewsModelVH class

public static final String NEWS= "NewsModel";

并编辑我的 onCreateView 方法以使用 FirebaseRecyclerAdapter，它使用 DBModel 类 DatabaseReference 和 RecyccerViewHolder，如图所示

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the fragment_dash_board for this fragment
    View rootView = inflater.inflate(R.layout.fragment_news_rv, container, false);
    newsrecyclerView =(RecyclerView) rootView.findViewById(R.id.rv_noticeboard);
    nwlinearLayoutManager = new LinearLayoutManager(getActivity());
    nwlinearLayoutManager.setStackFromEnd(true);

    dbref = FirebaseDatabase.getInstance().getReference();
    firebasenewsRecycleAdapter = new FirebaseRecyclerAdapter<NewsModel, NewsModelVH>(
            NewsModel.class,
            R.layout.fragment_dashboard_imagetext,
            NewsModelVH.class,
            dbref.child(NEWS)) {
        @Override
        protected void populateViewHolder(NewsModelVH viewHolder, NewsModel model, int position) {
            viewHolder.newsHead.setText(model.getNewsHead());
            viewHolder.newsBody.setText(model.getNewsBody());
        }
    };

    firebasenewsRecycleAdapter.registerAdapterDataObserver(new RecyclerView.AdapterDataObserver(){
        @Override
         public void onItemRangeInserted(int positionStart, int itemCount){
            super.onItemRangeInserted(positionStart, itemCount);
            int newsCount = firebasenewsRecycleAdapter.getItemCount();
            int lastVisiblePosition = nwlinearLayoutManager.findLastVisibleItemPosition();
            if(lastVisiblePosition == -1 || (positionStart>= (newsCount -1) && lastVisiblePosition == (positionStart -1))){
                newsrecyclerView.scrollToPosition(positionStart);
            }
        }
    });
    newsrecyclerView.setLayoutManager(nwlinearLayoutManager);
    newsrecyclerView.setAdapter(firebasenewsRecycleAdapter);

    /**
     * SET ADAPTER
     */

    ProgressBar newsprogressBar = (ProgressBar) rootView.findViewById(R.id.progress_bar);
    newsprogressBar.setVisibility(View.VISIBLE);

    Log.v("RETRIEVE", " dbOperationsHelper.retrieveNews() NEWS=" + dbref);

    return rootView;

}

仍然让我头疼的一件事是为什么要显示数据，而我既没有使用 dbref.addValueEventListener(new ValueEventListener() 也没有使用 ref.addChildEventListener(new ChildEventListener()

好吧，有些事情告诉我 future 可能会限制我实现某些事情......目前还不确定是什么，但我会在那个时候知道。但是如果有人对此有解释也可以分享信息。

否则，希望这对某人有所帮助，这样您就不会在适配器之间切换两天(ArrayAdapter、ArrayList、CustomeAdapter , FirebaseListAdapter 和 FirebaseRecyclerAdapter) 来寻找解决方案。