Firebase addValueEventListener未被触发

上周我一直在从Firebase数据库中检索数据时遇到问题。 我搜遍了StackOverflow和google以获得答案，要么我不理解正在呈现的解决方案，要么他们根本不适合我。 一切都被成功调用，直到我到达valueEventListener。 我记录了所有内容，但它从未触发过。 error handling程序不会返回任何错误。

public class DBHandler { private FirebaseAuth mAuth = FirebaseAuth.getInstance(); private String key; public ArrayListtips,tables,times; private DatabaseReference mDatabase; private DatabaseReference mRootRef = FirebaseDatabase.getInstance().getReference(); private DatabaseReference mPostRef = mRootRef.child("user-posts"); private DatabaseReference mIdRef = mPostRef.child(mAuth.getCurrentUser().getUid()); private DatabaseReference mKeyRef;//move down private DatabaseReference mTimeRef; private DatabaseReference mTipRef; private DatabaseReference mTableRef; public DBHandler(){ //Constructor } public DBHandler(String key) { tips(key); Log.d("Test for tips()", "Triggered"); } public void tips(String key){ mKeyRef = mIdRef.child(key); mTipRef = mKeyRef.child("tips"); Log.d("Test for reference", "mTipRef: " + mTipRef.getRef()); mTipRef.addValueEventListener(new ValueEventListener() { @Override public void onDataChange(DataSnapshot dataSnapshot) { Log.d("Test for listener", "Triggered"); ArrayList placeholder= new ArrayList(); placeholder = dataSnapshot.getValue(ArrayList.class); Log.d("Test for placeholder", "placeholder: " + placeholder); tipsHolder(placeholder); } @Override public void onCancelled(DatabaseError databaseError) { } }); } private void tipsHolder(ArrayList placeholder) { this.tips = placeholder; } public void times(String key){ } public void tables(String key){ } public void tablesHolder(ArrayList placeholder){ } public void timesHolder(ArrayList placeholder){ } } 

提前感谢任何试图帮助我解决这一问题的人。