Android 如何检测Firebase侦听器是否完成了数据提取?
我在Android应用程序中使用Firebase ValueEventlistener从数据库中检索一些数据以初始化UI,但我的问题是,我永远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,我将延迟5秒,让侦听器完成数据获取,但有时这还不够,数据仍然为null,并给出null指针异常。我想知道Firebase侦听器何时完成获取数据以更新我的UI 这是一种方法,我可以从中获取数据:Android 如何检测Firebase侦听器是否完成了数据提取?,android,firebase,firebase-realtime-database,Android,Firebase,Firebase Realtime Database,我在Android应用程序中使用Firebase ValueEventlistener从数据库中检索一些数据以初始化UI,但我的问题是,我永远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,我将延迟5秒,让侦听器完成数据获取,但有时这还不够,数据仍然为null,并给出null指针异常。我想知道Firebase侦听器何时完成获取数据以更新我的UI 这是一种方法,我可以从中获取数据: public void GetRequests(){ Retrieve.add
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) {
}
});
}
public void GetRequests(){
Retrieve.addValueEventListener(新的ValueEventListener(){
@凌驾
公共void onDataChange(DataSnapshot DataSnapshot){
Iterable Games=dataSnapshot.getChildren();
用于(com.google.firebase.database.DataSnapshot游戏:游戏){
String Game_Key=Game.child(“GameKey”).getValue(String.class);
String Game_Location=Game.child(“Location”).getValue(String.class);
String UserID\u Sender=game.child(“UserID”).getValue(String.class);
适配器\游戏\键。添加(游戏\键);
Adapter\u UserID\u Sender.add(UserID\u Sender);
}
}
@凌驾
已取消的公共void(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ArrayList传递给ListView的适配器来初始化UI
有人对此有解决方案吗?请按如下方式在
onDataChanged()
方法中执行所有UI更新:
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) {
}
});
ArrayList adArrayList_1=new ArrayList();
DatabaseReference ref=FirebaseDatabase.getInstance().getReference().child(“ads”);
ListView ListView\u 1=(ListView)findViewById(R.id.market\u list\u view\u 1);
MarketArrayAdapter MarketArrayAdapter=新市场ArrayAdapter(
这
adArrayList_1
);
listView_1.setAdapter(marketArrayAdapter);
ref.addValueEventListener(新的ValueEventListener(){
@凌驾
公共void onDataChange(DataSnapshot DataSnapshot){
adArrayList_1.clear();
对于(DataSnapshot快照:DataSnapshot.getChildren()){
Ad=snapshot.getValue(Ad.class);
adArrayList_1.添加(ad);
}
marketArrayAdapter.notifyDataSetChanged();
}
@凌驾
已取消的公共void(DatabaseError DatabaseError){
}
});
下面是我的适配器类
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;
}
公共市场ArrayAdapter(上下文,ArrayList列表){
超级(上下文,0,列表);
}
@非空
@凌驾
公共视图getView(int位置、视图转换视图、视图组父视图){
最终Ad=获取项目(位置);
if(convertView==null){
convertView=LayoutInflater.from(getContext()).flate(R.layout.ad_项,父项,false);
}
ImageView ImageView=(ImageView)convertView.findViewById(R.id.ads\u profile\u pic);
TextView title=(TextView)convertView.findViewById(R.id.ads_title);
TextView价格=(TextView)convertView.findViewById(R.id.ads_价格);
TextView description=(TextView)convertView.findViewById(R.id.ads\u short\u description);
byte[]imageAsBytes=Base64.decode(ad.getaPictureUri().getBytes(),Base64.DEFAULT);
setImageBitmap(BitmapFactory.decodeByteArray(imageAsBytes,0,imageAsBytes.length));
title.setText(ad.getaTitle());
price.setText(String.valueOf(ad.getaPrice());
description.setText(ad.getaDescription());
parent.setOnClickListener(新视图.OnClickListener(){
@凌驾
公共void onClick(视图v){
Intent in=新的Intent(MarketActivity.this、ViewAdActivity.class);
in.putExtra(“key”,ad.getaId());
星触觉(in);
}
});
返回视图;
}
}
请不要忘记将这一行
marketArrayAdapter.notifyDataSetChanged()代码>按照如下方法执行onDataChanged()
方法中的所有UI更新:
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) {
}
});
ArrayList adArrayList_1=new ArrayList();
DatabaseReference ref=FirebaseDatabase.getInstance().getReference().child(“ads”);
ListView ListView\u 1=(ListView)findViewById(R.id.market\u list\u view\u 1);
MarketArrayAdapter MarketArrayAdapter=新市场ArrayAdapter(
这
adArrayList_1
);
listView_1.setAdapter(marketArrayAdapter);
ref.addValueEventListener(新的ValueEventListener(){
@凌驾
公共void onDataChange(DataSnapshot DataSnapshot){
adArrayList_1.clear();
对于(DataSnapshot快照:DataSnapshot.getChildren()){
Ad=snapshot.getValue(Ad.class);
adArrayList_1.添加(ad);
}
marketArrayAdapter.notifyDataSetChanged();
}
@凌驾
已取消的公共void(DatabaseError DatabaseError){
}
});
下面是我的适配器类
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;
}
公共市场ArrayAdapter(上下文,ArrayList列表){
超级(上下文,0,列表);
}
@非空
@凌驾
公共视图getView(int位置、视图转换视图、视图组父视图){
最终Ad=获取项目(位置);
if(convertView==null){
convertView=LayoutInflater.from(getContext()).flate(R.layout.ad_项,父项,false);
}
ImageView ImageView=(ImageView)convertView.findViewById(R.id.ads\u profile\u pic);
TextView title=(TextView)convertView.findViewById(R.id.ads_title);
TextView价格=(TextView)convertView.findViewById(R.id.ads_价格);
TextView description=(TextView)convertView.findViewById(R.id.ads\u short\u d