如何从Android Firebase实时数据库中的onDataChange返回arrayList

Question

我正在尝试从Firebase ArrayList构建并返回DataSnapshot。

我的数据库结构

我有这个ListItem类。

public class ListItem {
    private String title;
    private String message;
    private String image;
    private String type;
    private String userid;

    public String getTitle() {return title;}
    public String getMessage() {return message;}
    public String getImage() {return image;}
    public String getType() {return type;}
    public String getUserid() {return userid;}
    public void setTitle(String title) {this.title = title;}
    public void setMessage(String message) {this.message = message;}
    public void setImage(String image) {this.image = image; }
    public void setType(String type) {this.type = type; }
    public void setUserid(String userid) {this.userid = userid;}
}

我有这种loadNotificationDatabase测试方法，效果很好。

private ArrayList loadNotificationDatabase() {
    ArrayList<ListItem> results = new ArrayList<>();
    ListItem user1 = new ListItem();
    user1.setTitle("Suresh Dasari");
    user1.setMessage("Team Leader");
    user1.setUserid("Hyderabad");
    results.add(user1);
    ListItem user2 = new ListItem();
    user2.setTitle("Rohini Alavala");
    user2.setMessage("Agricultural Officer");
    user2.setUserid("Guntur");
    results.add(user2);
    ListItem user3 = new ListItem();
    user3.setTitle("Trishika Dasari");
    user3.setMessage("Charted Accountant");
    user3.setUserid("Guntur");
    results.add(user3);
    return results;
}

现在，我尝试从Firebase数据库加载数据并返回数组列表，但是它不返回任何内容，因为它始终为空，并且没有错误。 我知道代码编写不正确，任何人都可以帮助我吗？

   public ArrayList loadNotificationDatabase() {
        String userSessionId = "QvdfKO9ek4qlPZJA";
        ArrayList<ListItem> list = new ArrayList<>();
        ValueEventListener valueEventListener = new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                for(DataSnapshot ds : dataSnapshot.getChildren()) {
                    ListItem users = new ListItem();
                    users.setTitle(ds.child("title").getValue(String.class));
                    users.setMessage(ds.child("message").getValue(String.class));
                    users.setUserid(ds.child("user_id").getValue(String.class));
                    list.add(users);

                    String message = ds.child("message").getValue(String.class);
                    String title = ds.child("title").getValue(String.class);
                    String type = ds.child("type").getValue(String.class);
                    String image = ds.child("image").getValue(String.class);
                    String user_id = ds.child("user_id").getValue(String.class);
                    Log.d("loadNotification", title + " / " + message  + " / " + type + " / " + image + " / " + user_id);
                }
                Log.e("loadNotification", list.toString());
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
                Log.e("DatabaseError", databaseError.getMessage());
            }
        };
        notificationDatabase.child(userSessionId).addListenerForSingleValueEvent(valueEventListener);
        return list;
    }

 ArrayList userList = loadNotificationDatabase();
 Log.e("userLists", userList.toString());