java在Firebase侦听器中设置单例属性值

Question

我目前正在测试Firebase以及一个我计划在整个应用程序生命周期中访问的单例模型。我现在被一些看起来很琐碎的事情缠住了，但我一辈子都搞不懂。我有一个我使用的模型示例：firebase中的书签

public class BookSingleton {



private static BookSingleton model;

private ArrayList<BookMark> bookmarks = new ArrayList<BookMark>();


public static BookSingleton getModel()
{
    if (model == null)
    {
        throw new IllegalStateException("The model has not been initialised yet.");
    }

    return model;
}


public ArrayList<Bookmark> theBookmarkList()
{
    return this.bookmarks;
}


public void setBookmarks(ArrayList<Bookmark> bookmarks){
    this.bookmarks = bookmarks;
}


public void loadModelWithDataFromFirebase(){
    Firebase db = new Firebase(//url);
    Firebase bookmarksRef = fb.child(//access correct child);


    final ArrayList<Bookmark> loadedBookmarks = new ArrayList<Bookmark>();
    bookmarksRef.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
                    //getting all properties from firebase...
                    Bookmark bookmark = new Bookmark(//properties here);
                    loadedBookmarks.add(bookmark);



                }
            }
            //bookmarks still exist here at this point
            setBookmarks(loadedBookmarks);

        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {

        }
    });
    //by now loadedBookmarks is empty
    //this is probably the issue?
    //even without this line bookmarks is still not set in mainactivity
    setBookmarks(loadedBookmarks);
}

现在，当我用Singleton集合的实例启动mainActivity时，我得到一个空错误，因为我编写的从firebase加载模型数据的函数显然没有设置任何内容

比如： MainActivity

public class MainActivity extends AppCompatActivity {

private BookSingleton theModel;



@Override
protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);

    // Load the model
    theModel = BookSingleton.getModel(this);
      //manually setting this works 
      //        ArrayList<Book> bookSamples = new ArrayList<Book>;
      //        bookSamples.add(aBookSample);

    theModel.loadModelWithSampleData(bookSamples);
    //should have set the singleton model property Bookmarks to the results from firebase

    theModel.loadModelWithDataFromFirebase();
    //returns 0
    Log.d(TAG, "" + theModel.theBookmarkList().size());


    setContentView(R.layout.activity_main);

    //......rest of code

我怎样才能做到这一点