Android getContactsFromFirebase()方法返回空列表 public List getContactsFromFirebase(){ FirebaseDatabase.getInstance().getReference().child(“用户”) .addListenerForSingleValueEvent(新的ValueEventListener(){ @凌驾 公共void onDataChange(DataSnapshot DataSnapshot){ 对于(DataSnapshot快照:DataSnapshot.getChildren()){ Users user=snapshot.getValue(Users.class); 断言用户!=null; 字符串contact_found=user.getPhone_number(); mContactsFromFirebase.add(找到联系人); Log.i(“Test”,mContactsFromFirebase.toString()); } } @凌驾 已取消的公共void(DatabaseError DatabaseError){ } }); 从Firebase返回mcontacts; }
我似乎找不到错误。在上面的代码中,当我调用日志时,我从Android getContactsFromFirebase()方法返回空列表 public List getContactsFromFirebase(){ FirebaseDatabase.getInstance().getReference().child(“用户”) .addListenerForSingleValueEvent(新的ValueEventListener(){ @凌驾 公共void onDataChange(DataSnapshot DataSnapshot){ 对于(DataSnapshot快照:DataSnapshot.getChildren()){ Users user=snapshot.getValue(Users.class); 断言用户!=null; 字符串contact_found=user.getPhone_number(); mContactsFromFirebase.add(找到联系人); Log.i(“Test”,mContactsFromFirebase.toString()); } } @凌驾 已取消的公共void(DatabaseError DatabaseError){ } }); 从Firebase返回mcontacts; },android,firebase,firebase-realtime-database,Android,Firebase,Firebase Realtime Database,我似乎找不到错误。在上面的代码中,当我调用日志时,我从mContactsFromFirebase获取值,但是getContactsFromFirebase()方法返回一个空列表。您能帮我一下吗?数据是从Firebase异步加载的。由于从服务器获取数据可能需要一些时间,因此主Android代码将继续,Firebase将在数据可用时调用您的onDataChange 这意味着当您返回mContactsFromFirebase时,它仍然是空的。查看这一点的最简单方法是放置一些日志语句: public L
mContactsFromFirebase
获取值,但是getContactsFromFirebase()
方法返回一个空列表。您能帮我一下吗?数据是从Firebase异步加载的。由于从服务器获取数据可能需要一些时间,因此主Android代码将继续,Firebase将在数据可用时调用您的onDataChange
这意味着当您返回mContactsFromFirebase
时,它仍然是空的。查看这一点的最简单方法是放置一些日志语句:
public List<String> getContactsFromFirebase(){
FirebaseDatabase.getInstance().getReference().child("Users")
.addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
Users user = snapshot.getValue(Users.class);
assert user != null;
String contact_found = user.getPhone_number();
mContactsFromFirebase.add(contact_found);
Log.i("Test", mContactsFromFirebase.toString());
}
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
});
return mContactsFromFirebase;
}
运行此代码时,它将打印:
在附加侦听器之前
附加侦听器后
在Ondata变化中
这可能不是您期望的输出顺序。正如您可以看到的那样,在调用回调后,在onDataChange
之前调用该行。这就解释了为什么您返回的列表是空的,或者(更准确地说)当您返回它时它是空的,并且只在以后才被填充
有几种方法可以处理这种异步加载
最简单的解释是将所有返回列表的代码放入
onDataChange
方法。这意味着此代码仅在加载数据后执行。以最简单的形式:
System.out.println("Before attaching listener");
FirebaseDatabase.getInstance().getReference().child("Users")
.addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
System.out.println("In onDataChange");
}
@Override
public void onCancelled(DatabaseError databaseError) {
throw databaseError.toException(); // don't ignore errors
}
});
System.out.println("After attaching listener");
但还有更多的方法,包括使用自定义回调(类似于Firebase自己的
ValueEventListener
):
Java:
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
Users user = snapshot.getValue(Users.class);
assert user != null;
String contact_found = user.getPhone_number();
mContactsFromFirebase.add(contact_found);
System.out.println("Loaded "+mContactsFromFirebase.size()+" contacts");
}
}
public interface UserListCallback {
void onCallback(List<Users> value);
}
interface UserListCallback {
fun onCallback(value:List<Users>)
}
public void getContactsFromFirebase(final UserListCallback myCallback) {
databaseReference.child(String.format("users/%s/name", uid)).addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
Users user = snapshot.getValue(Users.class);
assert user != null;
String contact_found = user.getPhone_number();
mContactsFromFirebase.add(contact_found);
System.out.println("Loaded "+mContactsFromFirebase.size()+" contacts");
}
myCallback.onCallback(mContactsFromFirebase);
}
@Override
public void onCancelled(DatabaseError databaseError) {
throw databaseError.toException();
}
});
}
fun getContactsFromFirebase(myCallback:UserListCallback) {
databaseReference.child(String.format("users/%s/name", uid)).addListenerForSingleValueEvent(object:ValueEventListener() {
fun onDataChange(dataSnapshot:DataSnapshot) {
for (snapshot in dataSnapshot.getChildren())
{
val user = snapshot.getValue(Users::class.java)
assert(user != null)
val contact_found = user.getPhone_number()
mContactsFromFirebase.add(contact_found)
System.out.println("Loaded " + mContactsFromFirebase.size() + " contacts")
}
myCallback.onCallback(mContactsFromFirebase)
}
fun onCancelled(databaseError:DatabaseError) {
throw databaseError.toException()
}
})
getContactsFromFirebase(new UserListCallback() {
@Override
public void onCallback(List<Users> users) {
System.out.println("Loaded "+users.size()+" contacts")
}
});
Kotlin:
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
Users user = snapshot.getValue(Users.class);
assert user != null;
String contact_found = user.getPhone_number();
mContactsFromFirebase.add(contact_found);
System.out.println("Loaded "+mContactsFromFirebase.size()+" contacts");
}
}
public interface UserListCallback {
void onCallback(List<Users> value);
}
interface UserListCallback {
fun onCallback(value:List<Users>)
}
public void getContactsFromFirebase(final UserListCallback myCallback) {
databaseReference.child(String.format("users/%s/name", uid)).addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
Users user = snapshot.getValue(Users.class);
assert user != null;
String contact_found = user.getPhone_number();
mContactsFromFirebase.add(contact_found);
System.out.println("Loaded "+mContactsFromFirebase.size()+" contacts");
}
myCallback.onCallback(mContactsFromFirebase);
}
@Override
public void onCancelled(DatabaseError databaseError) {
throw databaseError.toException();
}
});
}
fun getContactsFromFirebase(myCallback:UserListCallback) {
databaseReference.child(String.format("users/%s/name", uid)).addListenerForSingleValueEvent(object:ValueEventListener() {
fun onDataChange(dataSnapshot:DataSnapshot) {
for (snapshot in dataSnapshot.getChildren())
{
val user = snapshot.getValue(Users::class.java)
assert(user != null)
val contact_found = user.getPhone_number()
mContactsFromFirebase.add(contact_found)
System.out.println("Loaded " + mContactsFromFirebase.size() + " contacts")
}
myCallback.onCallback(mContactsFromFirebase)
}
fun onCancelled(databaseError:DatabaseError) {
throw databaseError.toException()
}
})
getContactsFromFirebase(new UserListCallback() {
@Override
public void onCallback(List<Users> users) {
System.out.println("Loaded "+users.size()+" contacts")
}
});
然后这样称呼它:
Java:
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
Users user = snapshot.getValue(Users.class);
assert user != null;
String contact_found = user.getPhone_number();
mContactsFromFirebase.add(contact_found);
System.out.println("Loaded "+mContactsFromFirebase.size()+" contacts");
}
}
public interface UserListCallback {
void onCallback(List<Users> value);
}
interface UserListCallback {
fun onCallback(value:List<Users>)
}
public void getContactsFromFirebase(final UserListCallback myCallback) {
databaseReference.child(String.format("users/%s/name", uid)).addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
Users user = snapshot.getValue(Users.class);
assert user != null;
String contact_found = user.getPhone_number();
mContactsFromFirebase.add(contact_found);
System.out.println("Loaded "+mContactsFromFirebase.size()+" contacts");
}
myCallback.onCallback(mContactsFromFirebase);
}
@Override
public void onCancelled(DatabaseError databaseError) {
throw databaseError.toException();
}
});
}
fun getContactsFromFirebase(myCallback:UserListCallback) {
databaseReference.child(String.format("users/%s/name", uid)).addListenerForSingleValueEvent(object:ValueEventListener() {
fun onDataChange(dataSnapshot:DataSnapshot) {
for (snapshot in dataSnapshot.getChildren())
{
val user = snapshot.getValue(Users::class.java)
assert(user != null)
val contact_found = user.getPhone_number()
mContactsFromFirebase.add(contact_found)
System.out.println("Loaded " + mContactsFromFirebase.size() + " contacts")
}
myCallback.onCallback(mContactsFromFirebase)
}
fun onCancelled(databaseError:DatabaseError) {
throw databaseError.toException()
}
})
getContactsFromFirebase(new UserListCallback() {
@Override
public void onCallback(List<Users> users) {
System.out.println("Loaded "+users.size()+" contacts")
}
});
getContactsFromFirebase(新的UserListCallback(){
@凌驾
公用void onCallback(列出用户){
System.out.println(“已加载”+用户.size()+联系人”)
}
});
Kotlin:
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
Users user = snapshot.getValue(Users.class);
assert user != null;
String contact_found = user.getPhone_number();
mContactsFromFirebase.add(contact_found);
System.out.println("Loaded "+mContactsFromFirebase.size()+" contacts");
}
}
public interface UserListCallback {
void onCallback(List<Users> value);
}
interface UserListCallback {
fun onCallback(value:List<Users>)
}
public void getContactsFromFirebase(final UserListCallback myCallback) {
databaseReference.child(String.format("users/%s/name", uid)).addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
Users user = snapshot.getValue(Users.class);
assert user != null;
String contact_found = user.getPhone_number();
mContactsFromFirebase.add(contact_found);
System.out.println("Loaded "+mContactsFromFirebase.size()+" contacts");
}
myCallback.onCallback(mContactsFromFirebase);
}
@Override
public void onCancelled(DatabaseError databaseError) {
throw databaseError.toException();
}
});
}
fun getContactsFromFirebase(myCallback:UserListCallback) {
databaseReference.child(String.format("users/%s/name", uid)).addListenerForSingleValueEvent(object:ValueEventListener() {
fun onDataChange(dataSnapshot:DataSnapshot) {
for (snapshot in dataSnapshot.getChildren())
{
val user = snapshot.getValue(Users::class.java)
assert(user != null)
val contact_found = user.getPhone_number()
mContactsFromFirebase.add(contact_found)
System.out.println("Loaded " + mContactsFromFirebase.size() + " contacts")
}
myCallback.onCallback(mContactsFromFirebase)
}
fun onCancelled(databaseError:DatabaseError) {
throw databaseError.toException()
}
})
getContactsFromFirebase(new UserListCallback() {
@Override
public void onCallback(List<Users> users) {
System.out.println("Loaded "+users.size()+" contacts")
}
});
getContactsFromFirebase(对象:UserListCallback(){
趣味onCallback(用户:列表){
System.out.println(“已加载”+用户.size()+联系人”)
}
})
它不像同步加载数据那样简单,但它的优点是运行时不会阻塞主线程
这个话题以前已经讨论过很多次了,所以我建议你也看看下面的一些问题:
- 这个
- (在这里,我解释了在某些情况下如何获得同步数据加载,但通常不能)
- (我第一次使用日志语句解释发生了什么)
- (其中Doug展示了一种将
任务
API与Firebase数据库结合使用的酷而复杂的方法)
- (我从中借用了一些回调语法)
onDataChange
这意味着当您返回mContactsFromFirebase
时,它仍然是空的。查看这一点的最简单方法是放置一些日志语句:
public List<String> getContactsFromFirebase(){
FirebaseDatabase.getInstance().getReference().child("Users")
.addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
Users user = snapshot.getValue(Users.class);
assert user != null;
String contact_found = user.getPhone_number();
mContactsFromFirebase.add(contact_found);
Log.i("Test", mContactsFromFirebase.toString());
}
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
});
return mContactsFromFirebase;
}
运行此代码时,它将打印:
在附加侦听器之前
附加侦听器后
在Ondata变化中
这可能不是您期望的输出顺序。正如您可以看到的那样,在调用回调后,在onDataChange
之前调用该行。这就解释了为什么您返回的列表是空的,或者(更准确地说)当您返回它时它是空的,并且只在以后才被填充
有几种方法可以处理这种异步加载
最简单的解释是将所有返回列表的代码放入
onDataChange
方法。这意味着此代码仅在加载数据后执行。以最简单的形式:
System.out.println("Before attaching listener");
FirebaseDatabase.getInstance().getReference().child("Users")
.addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
System.out.println("In onDataChange");
}
@Override
public void onCancelled(DatabaseError databaseError) {
throw databaseError.toException(); // don't ignore errors
}
});
System.out.println("After attaching listener");
但还有更多的方法,包括使用自定义回调(类似于Firebase自己的
ValueEventListener
):
Java:
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
Users user = snapshot.getValue(Users.class);
assert user != null;
String contact_found = user.getPhone_number();
mContactsFromFirebase.add(contact_found);
System.out.println("Loaded "+mContactsFromFirebase.size()+" contacts");
}
}
public interface UserListCallback {
void onCallback(List<Users> value);
}
interface UserListCallback {
fun onCallback(value:List<Users>)
}
public void getContactsFromFirebase(final UserListCallback myCallback) {
databaseReference.child(String.format("users/%s/name", uid)).addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
Users user = snapshot.getValue(Users.class);
assert user != null;
String contact_found = user.getPhone_number();
mContactsFromFirebase.add(contact_found);
System.out.println("Loaded "+mContactsFromFirebase.size()+" contacts");
}
myCallback.onCallback(mContactsFromFirebase);
}
@Override
public void onCancelled(DatabaseError databaseError) {
throw databaseError.toException();
}
});
}
fun getContactsFromFirebase(myCallback:UserListCallback) {
databaseReference.child(String.format("users/%s/name", uid)).addListenerForSingleValueEvent(object:ValueEventListener() {
fun onDataChange(dataSnapshot:DataSnapshot) {
for (snapshot in dataSnapshot.getChildren())
{
val user = snapshot.getValue(Users::class.java)
assert(user != null)
val contact_found = user.getPhone_number()
mContactsFromFirebase.add(contact_found)
System.out.println("Loaded " + mContactsFromFirebase.size() + " contacts")
}
myCallback.onCallback(mContactsFromFirebase)
}
fun onCancelled(databaseError:DatabaseError) {
throw databaseError.toException()
}
})
getContactsFromFirebase(new UserListCallback() {
@Override
public void onCallback(List<Users> users) {
System.out.println("Loaded "+users.size()+" contacts")
}
});
Kotlin:
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
Users user = snapshot.getValue(Users.class);
assert user != null;
String contact_found = user.getPhone_number();
mContactsFromFirebase.add(contact_found);
System.out.println("Loaded "+mContactsFromFirebase.size()+" contacts");
}
}
public interface UserListCallback {
void onCallback(List<Users> value);
}
interface UserListCallback {
fun onCallback(value:List<Users>)
}
public void getContactsFromFirebase(final UserListCallback myCallback) {
databaseReference.child(String.format("users/%s/name", uid)).addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
Users user = snapshot.getValue(Users.class);
assert user != null;
String contact_found = user.getPhone_number();
mContactsFromFirebase.add(contact_found);
System.out.println("Loaded "+mContactsFromFirebase.size()+" contacts");
}
myCallback.onCallback(mContactsFromFirebase);
}
@Override
public void onCancelled(DatabaseError databaseError) {
throw databaseError.toException();
}
});
}
fun getContactsFromFirebase(myCallback:UserListCallback) {
databaseReference.child(String.format("users/%s/name", uid)).addListenerForSingleValueEvent(object:ValueEventListener() {
fun onDataChange(dataSnapshot:DataSnapshot) {
for (snapshot in dataSnapshot.getChildren())
{
val user = snapshot.getValue(Users::class.java)
assert(user != null)
val contact_found = user.getPhone_number()
mContactsFromFirebase.add(contact_found)
System.out.println("Loaded " + mContactsFromFirebase.size() + " contacts")
}
myCallback.onCallback(mContactsFromFirebase)
}
fun onCancelled(databaseError:DatabaseError) {
throw databaseError.toException()
}
})
getContactsFromFirebase(new UserListCallback() {
@Override
public void onCallback(List<Users> users) {
System.out.println("Loaded "+users.size()+" contacts")
}
});
然后这样称呼它:
Java:
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
Users user = snapshot.getValue(Users.class);
assert user != null;
String contact_found = user.getPhone_number();
mContactsFromFirebase.add(contact_found);
System.out.println("Loaded "+mContactsFromFirebase.size()+" contacts");
}
}
public interface UserListCallback {
void onCallback(List<Users> value);
}
interface UserListCallback {
fun onCallback(value:List<Users>)
}
public void getContactsFromFirebase(final UserListCallback myCallback) {
databaseReference.child(String.format("users/%s/name", uid)).addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
Users user = snapshot.getValue(Users.class);
assert user != null;
String contact_found = user.getPhone_number();
mContactsFromFirebase.add(contact_found);
System.out.println("Loaded "+mContactsFromFirebase.size()+" contacts");
}
myCallback.onCallback(mContactsFromFirebase);
}
@Override
public void onCancelled(DatabaseError databaseError) {
throw databaseError.toException();
}
});
}
fun getContactsFromFirebase(myCallback:UserListCallback) {
databaseReference.child(String.format("users/%s/name", uid)).addListenerForSingleValueEvent(object:ValueEventListener() {
fun onDataChange(dataSnapshot:DataSnapshot) {
for (snapshot in dataSnapshot.getChildren())
{
val user = snapshot.getValue(Users::class.java)
assert(user != null)
val contact_found = user.getPhone_number()
mContactsFromFirebase.add(contact_found)
System.out.println("Loaded " + mContactsFromFirebase.size() + " contacts")
}
myCallback.onCallback(mContactsFromFirebase)
}
fun onCancelled(databaseError:DatabaseError) {
throw databaseError.toException()
}
})
getContactsFromFirebase(new UserListCallback() {
@Override
public void onCallback(List<Users> users) {
System.out.println("Loaded "+users.size()+" contacts")
}
});
getContactsFromFirebase(新的UserListCallback(){
@凌驾
公用void onCallback(列出用户){
System.out.println(“已加载”+用户.size()+联系人”)
}
});
Kotlin:
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
Users user = snapshot.getValue(Users.class);
assert user != null;
String contact_found = user.getPhone_number();
mContactsFromFirebase.add(contact_found);
System.out.println("Loaded "+mContactsFromFirebase.size()+" contacts");
}
}
public interface UserListCallback {
void onCallback(List<Users> value);
}
interface UserListCallback {
fun onCallback(value:List<Users>)
}
public void getContactsFromFirebase(final UserListCallback myCallback) {
databaseReference.child(String.format("users/%s/name", uid)).addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
Users user = snapshot.getValue(Users.class);
assert user != null;
String contact_found = user.getPhone_number();
mContactsFromFirebase.add(contact_found);
System.out.println("Loaded "+mContactsFromFirebase.size()+" contacts");
}
myCallback.onCallback(mContactsFromFirebase);
}
@Override
public void onCancelled(DatabaseError databaseError) {
throw databaseError.toException();
}
});
}
fun getContactsFromFirebase(myCallback:UserListCallback) {
databaseReference.child(String.format("users/%s/name", uid)).addListenerForSingleValueEvent(object:ValueEventListener() {
fun onDataChange(dataSnapshot:DataSnapshot) {
for (snapshot in dataSnapshot.getChildren())
{
val user = snapshot.getValue(Users::class.java)
assert(user != null)
val contact_found = user.getPhone_number()
mContactsFromFirebase.add(contact_found)
System.out.println("Loaded " + mContactsFromFirebase.size() + " contacts")
}
myCallback.onCallback(mContactsFromFirebase)
}
fun onCancelled(databaseError:DatabaseError) {
throw databaseError.toException()
}
})
getContactsFromFirebase(new UserListCallback() {
@Override
public void onCallback(List<Users> users) {
System.out.println("Loaded "+users.size()+" contacts")
}
});
getContactsFromFirebase(对象:UserListCallback(){
趣味onCallback(用户:列表){
System.out.println(“已加载”+用户.size()+联系人”)
}
})
它不像同步加载数据那样简单,但它的优点是运行时不会阻塞主线程
这个话题以前已经讨论过很多次了,所以我建议你也看看下面的一些问题:
- 这个
- (在这里,我解释了在某些情况下如何获得同步数据加载,但通常不能)
- (我第一次使用日志语句解释发生了什么)
- (其中Doug展示了一种将
任务
API与Firebase数据库结合使用的酷而复杂的方法)
- (我从中借用了一些回调语法)
mContactsFromFirebase
是ca