onChildAdded()仅在添加了新的子项或已有子项时被调用.

onChildAdded() will only be called when a new child has been added or a child already exist.

在您的情况下，您没有添加任何子代，因此永远不会调用onChildAdded().这就是 progressDialog不断无限加载

In your case, you don't have any child added, So onChildAdded() is never been called. that's why progressDialog keeps loading infinitely

要检测是否存在子代，请使用ValueEventListener而不是ChildEventListener.这是链接如何使用ValueEventListener

To detect if a child exists, use ValueEventListener instead of ChildEventListener. Here is link how to use ValueEventListener