如何从Firebase中检索特定的数据列表 [英] How to retrieve specific list of data from firebase
问题描述
我是新来的 firebase 和 nosql 数据库
<我试图从Firebase中检索一些特定的数据。我有一个节点大学，并且该节点有许多唯一的标识符作为具有更多数据的节点。我想从每个节点检索 name 。
请看看这个。
到目前为止，我尝试过使用 addChildEventListener ，但是这只能侦听第一个孩子。我调试过它，它只显示大学的第一个孩子的值节点。
myRef.addChildEventListener（new ChildEventListener（）{
@Override $ b $ public void onChildAdded（DataSnapshot dataSnapshot，String s）{
List< University> university = new ArrayList< >（）;
Map< String，Object> td =（HashMap< String，Object>）dataSnapshot.getValue（）;
Log.d（TAG，dataSnapshot.getValue （）.toString（））;
}
@Override
public void onChildChanged（DataSnapshot dataSnapshot，String s）{
}
@Override
public void onChildRemoved（DataSnapshot dataSnapshot）{
$ b @Override
public void onChildMoved（DataSnapshot dataSnapshot，String s）{
}
@Override
public void onCancelled （DatabaseError databaseError）{
}
}）;
我也试过 addValueEventListener 。这听整个节点，并返回整个数据，但我无法提取名称，因为它包含唯一的标识符。
请引导我进入正确的方向。
myRef.addValueEventListener（new ValueEventListener（）{
@Override
public void onDataChange（DataSnapshot dataSnapshot） {
University university = dataSnapshot.getValue（University.class）;
列表< University>大学=（List< University>）dataSnapshot.getValue（University.class）;
* // * （DataSnapshot dsp：dataSnapshot.getChildren（））{
lst.add（dsp.getKey（））;
}
Log.d（TAG，lst.toString（））; * // *
Map< String，Object> td =（HashMap< String，Object> ）dataSnapshot.getValue（）;
* // // List& LT;对象> values = new ArrayList< Object>（td.values（））;
列表< String> list = new ArrayList< String>（）;
for（int i = 0; i< values.size（）; i ++）{
list.add（（String）values.get（i））;
} * // *
Log.d（TAG，university.toString（））;
}
@Override
public void onCancelled（DatabaseError databaseError）{
$ b}）;
addValueEventListener 从Firebase检索所有大学 列表< University> universityList = new ArrayList<>（）;
myRef.addValueEventListener（new ValueEventListener（）{
@Override
public void onDataChange（DataSnapshot snapshot）{
universityList.clear（）;
for（DataSnapshot postSnapshot： snapshot.getChildren（））{
University university = postSnapshot.getValue（University.class）;
universityList.add（university）;
//在这里你可以访问name属性像university.name
}
}
@Override
public void onCancelled（DatabaseError databaseError）{
System.out.println （读取失败：+ firebaseError.getMessage（））;
}
}）;
或者如果您使用 addChildEventListener
列表< University> universityList = new ArrayList<>（）;
myRef.addChildEventListener（new ChildEventListener（）{
@Override
public void onChildAdded（DataSnapshot dataSnapshot，String s）{
University university = dataSnapshot.getValue（University.class）;
universityList.add（university）;
Log.i（TAG，add university name =+ university.name）;
...
}
。 ..
}
I am new to firebase and nosql database
I am trying to retrieve some specific data from a Firebase. I have a node universities and that node has many unique identifier as node which has so more data into it. I want to retrieve name from each node.
Please have a look at this.
What I have tried so far: I tried using addChildEventListener but this only listens first child. I've debugged it and it was only showing the values of the first child of universities node.
myRef.addChildEventListener(new ChildEventListener() {
@Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
List<University> university = new ArrayList<>();
Map<String, Object> td = (HashMap<String, Object>) dataSnapshot.getValue();
Log.d("TAG",dataSnapshot.getValue().toString());
}
@Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
}
@Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
}
@Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
});
I have also tried addValueEventListener. This listens the whole node and return whole data, but I am unable to extract "name" for it since it contains unique identifiers.
Please guide me into the right directions.
myRef.addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
University university = dataSnapshot.getValue(University.class);
List<University> universities = (List<University>) dataSnapshot.getValue(University.class);
*//*List<String> lst = new ArrayList<String>();
for(DataSnapshot dsp : dataSnapshot.getChildren()){
lst.add(dsp.getKey());
}
Log.d("TAG",lst.toString());*//*
Map<String, Object> td = (HashMap<String, Object>) dataSnapshot.getValue();
*//*List<Object> values = new ArrayList<Object>(td.values());
List<String> list=new ArrayList<String>();
for(int i=0; i<values.size(); i++){
list.add((String) values.get(i));
}*//*
Log.d("TAG", university.toString());
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
});
If you using addValueEventListener for retrieve all university from Firebase
List<University> universityList = new ArrayList<>();
myRef.addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot snapshot) {
universityList.clear();
for (DataSnapshot postSnapshot: snapshot.getChildren()) {
University university = postSnapshot.getValue(University.class);
universityList.add(university);
// here you can access to name property like university.name
}
}
@Override
public void onCancelled(DatabaseError databaseError) {
System.out.println("The read failed: " + firebaseError.getMessage());
}
});
Or if you using addChildEventListener
List<University> universityList = new ArrayList<>();
myRef.addChildEventListener(new ChildEventListener() {
@Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
University university = dataSnapshot.getValue(University.class);
universityList.add(university);
Log.i(TAG,"add university name = " + university.name);
...
}
...
}
这篇关于如何从Firebase中检索特定的数据列表的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
