如何检测Firebase侦听器是否完成了数据提取?

问题描述

我在Android应用中使用Firebase ValueEventlistener从数据库中检索一些数据以初始化UI，但是我的问题是我不知道何时完成数据提取，这取决于用户的互联网连接速度，我为侦听器完成了5秒钟的延迟以完成数据提取，但是有时这还不够，并且数据仍然为null，从而给出了null指针异常.我想知道Firebase侦听器何时完成数据提取以能够更新我的UI.

I'm using Firebase ValueEventlistener in an Android app to retreive some data from the database to initialize my UI, but my Problem is that I never know when data fetching is done and it depends on the speed of the user's internet connection, I'm making a delay of 5 seconds for the Listeners to finish fetching data but sometimes it's not enough and data is still null giving a null pointer exception. I want to know when the Firebase Listeners finish fetching data to be able to update my UI .

这是我获取数据的一种方法:

This is one Method where I fetch Data :

      public void GetRequests(){

         Retrieve.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
     Iterable<DataSnapshot> Games = dataSnapshot.getChildren();


            for(com.google.firebase.database.DataSnapshot game : Games ){

                String Game_Key = game.child("GameKey").getValue(String.class);
                String Game_Location = game.child("Location").getValue(String.class);
                String UserID_Sender = game.child("UserID").getValue(String.class);
                Adapter_Game_Key.add(Game_Key);
                Adapter_UserID_Sender.add(UserID_Sender);
           }
        }
        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
}

在这里，我尝试从ArrayList中的Firebase侦听器获取数据，以便能够通过将ArrayLists传递到ListView的适配器来初始化UI.

Here I try to get the data from the Firebase Listeners in an ArrayList to be able to initialize the UI by passing the ArrayLists to an Adapter of a ListView.

有人对此有解决方案吗?

Anyone has a solution for this please ?

推荐答案

在onDataChanged()方法内执行所有UI更新，如下所示:

Do all the UI updates inside the onDataChanged() method as follows:

    ArrayList<Ad> adArrayList_1 = new ArrayList<>();
    DatabaseReference ref = FirebaseDatabase.getInstance().getReference().child("ads");

    ListView listView_1 = (ListView) findViewById(R.id.market_list_view_1);

    MarketArrayAdapter marketArrayAdapter = new MarketArrayAdapter(
            this,
            adArrayList_1
    );

    listView_1.setAdapter(marketArrayAdapter);

            ref.addValueEventListener(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                    adArrayList_1.clear();
                    for(DataSnapshot snapshot : dataSnapshot.getChildren()){
                        Ad ad = snapshot.getValue(Ad.class);
                        adArrayList_1.add(ad);

                    }
                    marketArrayAdapter.notifyDataSetChanged();
                }

                @Override
                public void onCancelled(DatabaseError databaseError) {

                }
            });

我的适配器类别在下面

public MarketArrayAdapter(Context context, ArrayList<Ad> list) {
    super(context, 0, list);
}

@NonNull
@Override
public View getView(int position, View convertView, ViewGroup parent) {

    final Ad ad = getItem(position);
    if (convertView == null) {
        convertView = LayoutInflater.from(getContext()).inflate(R.layout.ad_item, parent, false);
    }
    ImageView imageView = (ImageView) convertView.findViewById(R.id.ads_profile_pic);
    TextView title = (TextView) convertView.findViewById(R.id.ads_title);
    TextView price = (TextView) convertView.findViewById(R.id.ads_price);
    TextView description = (TextView) convertView.findViewById(R.id.ads_short_description);

    byte[] imageAsBytes = Base64.decode(ad.getaPictureUri().getBytes(), Base64.DEFAULT);
    imageView.setImageBitmap(BitmapFactory.decodeByteArray(imageAsBytes, 0, imageAsBytes.length));
    title.setText(ad.getaTitle());
    price.setText(String.valueOf(ad.getaPrice()));
    description.setText(ad.getaDescription());

    parent.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent in = new Intent(MarketActivity.this, ViewAdActivity.class);
            in.putExtra("key", ad.getaId());
            startActivity(in);
        }
    });
    return convertView;
}

}

请不要忘记将这行放在marketArrayAdapter.notifyDataSetChanged();

Please dont forget to put this line marketArrayAdapter.notifyDataSetChanged();

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