Tags then posts with dispatch groups

dispatchgroups.swift

func posts(forTag tagKey: String, completionBlock: (([Post]) -> ()) {
  let userId = FIRAuth.auth().currentUser?.uid
  let ref = FIRDatabase.database().reference()
  
  let dispatchGroup = DispatchGroup()
  
  var posts = [Post]() //I can put all the posts here as I retrieve them
  
  ref.child("users").child(userId!).child("tags").child(tagKey).observeSingleEvent(of: .value, with: {(tagSnapshot) in
      let tag = Tag(fromSnapshot: tagSnapshot)
                                                                                                      
      dispatchGroup.enter() //starting an async op
      
      for postKey in tags.postKeys {
        ref.child("users").child(userId!).child("posts").child(postKey).observeSingleEvent(of: .value, with: {(postSnapshot)
           
             //Seems that we need to get back to a single thread, since our array will not be thread-safe                                                                                                 
             DispatchQueue.main.async {
                posts.append(Post(fromSnapshot: postSnapshot))
                dispatchGroup.leave() //ending an async op
             }                                                                                             
        })                                                                                           
      }
      
      //wait for all the "entered" aysnc ops to "leave", then run the completion blocks                                                                                                
      dispatchGroup.notify(queue: DispatchQueue.main, execute: {
         completionBlock(posts)
      })                                                         
  })
}