问题
I want to get data from my Firebase Firestore database. I have a collection called user and every user has collection of some objects of the same type (My Java custom object). I want to fill my ArrayList with these objects when my Activity is created.
private static ArrayList<Type> mArrayList = new ArrayList<>();;
In onCreate():
getListItems();
Log.d(TAG, "onCreate: LIST IN ONCREATE = " + mArrayList);
*// it logs empty list here
Method called to get items to list:
private void getListItems() {
mFirebaseFirestore.collection("some collection").get()
.addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
@Override
public void onSuccess(QuerySnapshot documentSnapshots) {
if (documentSnapshots.isEmpty()) {
Log.d(TAG, "onSuccess: LIST EMPTY");
return;
} else {
for (DocumentSnapshot documentSnapshot : documentSnapshots) {
if (documentSnapshot.exists()) {
Log.d(TAG, "onSuccess: DOCUMENT" + documentSnapshot.getId() + " ; " + documentSnapshot.getData());
DocumentReference documentReference1 = FirebaseFirestore.getInstance().document("some path");
documentReference1.get().addOnSuccessListener(new OnSuccessListener<DocumentSnapshot>() {
@Override
public void onSuccess(DocumentSnapshot documentSnapshot) {
Type type= documentSnapshot.toObject(Type.class);
Log.d(TAG, "onSuccess: " + type.toString());
mArrayList.add(type);
Log.d(TAG, "onSuccess: " + mArrayList);
/* these logs here display correct data but when
I log it in onCreate() method it's empty*/
}
});
}
}
}
}
}).addOnFailureListener(new OnFailureListener() {
@Override
public void onFailure(@NonNull Exception e) {
Toast.makeText(getApplicationContext(), "Error getting data!!!", Toast.LENGTH_LONG).show();
}
});
}
回答1:
The get() operation returns a Task<> which means it is an asynchronous operation. Calling getListItems() only starts the operation, it does not wait for it to complete, that's why you have to add success and failure listeners.
Although there's not much you can do about the async nature of the operation, you can simplify your code as follows:
private void getListItems() {
mFirebaseFirestore.collection("some collection").get()
.addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
@Override
public void onSuccess(QuerySnapshot documentSnapshots) {
if (documentSnapshots.isEmpty()) {
Log.d(TAG, "onSuccess: LIST EMPTY");
return;
} else {
// Convert the whole Query Snapshot to a list
// of objects directly! No need to fetch each
// document.
List<Type> types = documentSnapshots.toObjects(Type.class);
// Add all to your list
mArrayList.addAll(types);
Log.d(TAG, "onSuccess: " + mArrayList);
}
})
.addOnFailureListener(new OnFailureListener() {
@Override
public void onFailure(@NonNull Exception e) {
Toast.makeText(getApplicationContext(), "Error getting data!!!", Toast.LENGTH_LONG).show();
}
});
}
回答2:
Try this..Working fine.Below function will get Realtime Updates from firebse as well..
db = FirebaseFirestore.getInstance();
db.collection("dynamic_menu").addSnapshotListener(new EventListener<QuerySnapshot>() {
@Override
public void onEvent(QuerySnapshot documentSnapshots, FirebaseFirestoreException e) {
if (e !=null)
{
}
for (DocumentChange documentChange : documentSnapshots.getDocumentChanges())
{
String isAttendance = documentChange.getDocument().getData().get("Attendance").toString();
String isCalender = documentChange.getDocument().getData().get("Calender").toString();
String isEnablelocation = documentChange.getDocument().getData().get("Enable Location").toString();
}
}
});
More reference :https://firebase.google.com/docs/firestore/query-data/listen
If You do not want realtime updates refer Below Document
https://firebase.google.com/docs/firestore/query-data/get-data
回答3:
Here is a simplified example:
Create a collection "DownloadInfo" in Firebase.
And add a few documents with these fields inside it:
file_name (string), id (string), size (number)
Create your class:
public class DownloadInfo {
public String file_name;
public String id;
public Integer size;
}
Code to get list of objects:
FirebaseFirestore db = FirebaseFirestore.getInstance();
db.collection("DownloadInfo")
.get()
.addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
@Override
public void onComplete(@NonNull Task<QuerySnapshot> task) {
if (task.isSuccessful()) {
if (task.getResult() != null) {
List<DownloadInfo> downloadInfoList = task.getResult().toObjects(DownloadInfo.class);
for (DownloadInfo downloadInfo : downloadInfoList) {
doSomething(downloadInfo.file_name, downloadInfo.id, downloadInfo.size);
}
}
}
} else {
Log.w(TAG, "Error getting documents.", task.getException());
}
}
});
回答4:
This is the code to get the list. Since this is an async task, it takes time that's why the list size shows empty at first. But including the source for the cache data will enable the previous list(and also its size) to be in memory until next task is performed.
Source source = Source.CACHE;
firebaseFirestore
.collection("collectionname")
.get(source)
.addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
@Override
public void onSuccess(QuerySnapshot documentSnapshots) {
if (documentSnapshots.isEmpty()) {
return;
} else {
// Convert the whole Query Snapshot to a list
// of objects directly! No need to fetch each
// document.
List<ModelClass> types = documentSnapshots.toObjects(ModelClass.class);
// Add all to your list
mArrayList.addAll(types);
}
}
})
.addOnFailureListener(new OnFailureListener() {
@Override
public void onFailure(@NonNull Exception e) {
}
});
来源：https://stackoverflow.com/questions/46706433/firebase-firestore-get-data-from-collection