ArrayList 不会存储 Firebase 数据库列表

Question

我想将我的数据库子项存储在我的模型的数组列表中。

型号：

public class testInformation {
private String taskName;
private String answer;

public testInformation() {
}

public String getTaskName() {
    return taskName;
}

public void setTaskName(String taskName) {
    this.taskName = taskName;
}

public String getAnswer() {
    return answer;
}

public void setAnswer(String answer) {
    this.answer = answer;
}
}

在片段的onCreate()方法中，我调用了一个GetDataFromDatabase()方法，如下所示：

public void GetDataFromDatabase(){
    mFirebaseDatabase= FirebaseDatabase.getInstance();
    myref=mFirebaseDatabase.getReference().child("solutionKey");
    myref.keepSynced(true);
    myref.addListenerForSingleValueEvent(new ValueEventListener() {

        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for (DataSnapshot ds:dataSnapshot.getChildren()){
                testInformation testInformation=new testInformation();
                testInformation.setTaskName(ds.getKey().toString());
                testInformation.setAnswer(ds.getValue().toString());
                results.add(testInformation);
            }
        }
        @Override
        public void onCancelled(DatabaseError databaseError) {
        }
    });
}

我有一个名为results 的公共ArrayList<testInformation> 变量，用于存储数据库中的数据，但即使在调用GetDataFromDatabase() 之后，results 列表似乎是空的，但是当我在OnDataChange() 方法处设置断点，它似乎被正确的数据填满了。

我的问题是，如何从OnDataChange() 中获取已填满的results 列表以使用它？

编辑：这个列表不会被修改或改变，所以我只需要得到它一次，这就是我使用addListenerForSingleValueEvent()方法的原因。

Answer 1

这就是代码的结构：

public class EndOfTestFragment extends Fragment {


    private FirebaseDatabase mFirebaseDatabase;
    private DatabaseReference myref;
public ArrayList<testInformation> results;

...

@Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
...
GetDataFromDatabase();
...
}
public void GetDataFromDatabase(){
        mFirebaseDatabase= FirebaseDatabase.getInstance();
        myref=mFirebaseDatabase.getReference().child("solutionKey");
        myref.keepSynced(true);
        myref.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                for (DataSnapshot ds:dataSnapshot.getChildren()){
                    testInformation testInformation=new testInformation();
                    testInformation.setTaskName(ds.getKey().toString());
                    testInformation.setAnswer(ds.getValue().toString());
                    results.add(testInformation);
                }
            }
            @Override
            public void onCancelled(DatabaseError databaseError) {
            }
        });
    }

Answer 2

public class EndOfTestFragment extends Fragment {
private FirebaseDatabase mFirebaseDatabase;
private DatabaseReference myref;
public ArrayList<testInformation> results;

//...

@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    //...
    GetDataFromDatabase();// This returns before results.add(...) is called
    // Whatever code is after this line gets run before results list has data
    /* LABEL#1 */
    // The results list is probably NOT populated yet so don't use it here
}
public void GetDataFromDatabase() {
    mFirebaseDatabase= FirebaseDatabase.getInstance();
    myref=mFirebaseDatabase.getReference().child("solutionKey");
    myref.keepSynced(true);

    myref.addListenerForSingleValueEvent(new ValueEventListener() {// This function returns immediately, does not wait for data from Firebase...
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            /* LABEL#2 */
            for (DataSnapshot ds:dataSnapshot.getChildren()){
                testInformation testInformation=new testInformation();
                testInformation.setTaskName(ds.getKey().toString());
                testInformation.setAnswer(ds.getValue().toString());
                results.add(testInformation);
            }
            // The results list is populated now, do whatever you want with it here
        }
        @Override
        public void onCancelled(DatabaseError databaseError) {
        }
    });
}

LABEL#1中的代码在 LABEL#2

之前执行

只要数据可用，就会在后台调用onDataChange(...) 函数。
addListenerForSingleValueEvent() 不等待数据，立即返回。