无法从ondatachange方法获取值 [英] can't get values out of ondatachange method
问题描述
我目前正在开发一个android应用程序,我使用firebase作为数据库,但是在获取onDataChange方法中的变量时,我将它们分配给一个全局变量,我得到了空变量,但是当我在onDataChange中调用这些变量时方法它们不是null。
pre $ public class PositionateMarkerTask extends AsyncTask {
public ArrayList< Location> arrayList = new ArrayList<>();
public void connect(){
//设置连接参数
最终Firebase ref = new Firebase(https://test.firebaseio.com/test);
查询查询= ref.orderByChild(longitude);
//从数据库获取数据
query.addListenerForSingleValueEvent(new ValueEventListener(){
$ b $ @Override
public void onDataChange(DataSnapshot dataSnapshot) {
//检查用户是否存在
if(dataSnapshot.exists()){
for(DataSnapshot userSnapshot:dataSnapshot.getChildren()){
//获取每个用户其中包含目标用户名
位置位置= userSnapshot.getValue(Location.class);
arrayList.add(location);
//如果密码为true,数据将被存储在将会启动sharedPreferences文件和Home活动
$ b $ else {
System.out.println(not found);
}
}
@覆盖
public void onCancelled(FirebaseError firebaseError){
System.out.println(problem);
}
});
$ b $ @覆盖
保护对象doInBackground(Object [] params){
connect();
返回null;
$ b $ @覆盖
保护无效onPostExecute(对象o){
super.onPostExecute(o);
System.out.println(首先是+ arrayList.get(0).getLongitude());
$ b $ / code $ / $ p
欢迎使用异步编程,这会混淆你一直认为是真实的一切。 : - )
Firebase会在后台自动检索/同步数据库。工作发生在一个单独的线程上,所以你不需要和 AsyncTask
。但不幸的是,这也意味着你不能等待数据。
我通常建议你将代码从先做A再到B重新编码为每当我们得到A,我们用它做B。
在你的情况下,你想获得数据,然后打印第一个项目的经度。 Reframed是:每当你收到数据,打印第一项的经度。
$ p $ 查询查询= ref.orderByChild(经度);
query.addListenerForSingleValueEvent(new ValueEventListener(){
@Override $ b $ public void onDataChange(DataSnapshot dataSnapshot){$ b $ if(dataSnapshot.exists()){$ b (DataSnapshot userSnapshot:dataSnapshot.getChildren()){
位置位置= userSnapshot.getValue(Location.class);
arrayList.add(location);
}
System.out.println(first long is+ arrayList.get(0).getLongitude());}
else {
System.out.println(not found);
$ b @Override
public void onCancelled(FirebaseError firebaseError){
System.out.println(problem);
}
});
这里有几点需要注意:
-
如果您只对第一个项目感兴趣,则可以将查询限制为一个项目:
query = ref.orderByChild(longitude)。limitToFirst 1)
。这将获取更少的数据。我推荐使用addValueEventListener()
来代替addListenerForSingleValueEvent()
。前者将继续同步数据。这意味着如果您插入/更改列表中项目的经度,您的代码将自动被重新触发并打印(可能)新的第一个项目。
I'm currently developing an android app where I'm using firebase as database but when in got the variable in the onDataChange method and I assign them to a global variables I got null variables but when I call those variables in the onDataChange method they are not null.
public class PositionateMarkerTask extends AsyncTask {
public ArrayList<Location> arrayList= new ArrayList<>();
public void connect() {
//setting connexion parameter
final Firebase ref = new Firebase("https://test.firebaseio.com/test");
Query query = ref.orderByChild("longitude");
//get the data from the DB
query.addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
//checking if the user exist
if(dataSnapshot.exists()){
for (DataSnapshot userSnapshot : dataSnapshot.getChildren()) {
//get each user which has the target username
Location location =userSnapshot.getValue(Location.class);
arrayList.add(location);
//if the password is true , the data will be storaged in the sharedPreferences file and a Home activity will be launched
}
}
else{
System.out.println("not found");
}
}
@Override
public void onCancelled(FirebaseError firebaseError) {
System.out.println("problem ");
}
});
}
@Override
protected Object doInBackground(Object[] params) {
connect();
return null;
}
@Override
protected void onPostExecute(Object o) {
super.onPostExecute(o);
System.out.println("the firs long is"+arrayList.get(0).getLongitude());
}
}
Welcome to asynchronous programming, which messes up everything you always thought was true. :-)
Firebase retrieves/synchronizes with the database automatically in the background. The work happens on a separate thread, so you don't need and AsyncTask
. But unfortunately this also means you can't wait for the data.
I typically recommend that you reframe your code from "first do A, then to B" to "whenever we get A, we do B with it".
In your case, you want to get the data and then print the longitude of the first item. Reframed that is: whenever you receive data, print the longitude of the first item.
Query query = ref.orderByChild("longitude");
query.addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
if(dataSnapshot.exists()){
for (DataSnapshot userSnapshot : dataSnapshot.getChildren()) {
Location location =userSnapshot.getValue(Location.class);
arrayList.add(location);
}
System.out.println("the first long is"+arrayList.get(0).getLongitude()); }
else{
System.out.println("not found");
}
}
@Override
public void onCancelled(FirebaseError firebaseError) {
System.out.println("problem ");
}
});
A few things to note here:
if you're only interested in the first item, you can limit the query to one item:
query = ref.orderByChild("longitude").limitToFirst(1)
. This will retrieve less data.I recommend using
addValueEventListener()
instead ofaddListenerForSingleValueEvent()
. The former will keep synchronizing the data. This means that if you insert/change the longitude of an item in the list, your code will automatically get retriggered and print the (potentially) new first item.
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