用Swift查询朋友的朋友

I'm trying to pull a list of my friends and my friend's friends using Firebase. Right now, I'm pulling my list of friends and then looping through their friends to pull their user ID. I would like to append it to a single array with the output ["friendID1","friendID2", "friendID3", "friendID4","friendID5"]. My code so far:I'm trying to pull a list of my friends and my