我正在编写一个函数，该函数采用groupchatID（字符串），并为该群聊返回收件人列表（[String]）。我正在努力与功能的异步部分。当我运行该函数时，它将正确地将我要查找的用户名数组打印到控制台。虽然，当我调用该函数并尝试打印返回的值时，它始终是一个空数组，因为该函数会在Firebase调用完成之前返回该数组。我正在尝试使用回调，但是我不太了解所有语法。请看一下，让我知道需要更改的内容。

功能：

func GetRecipientsFor(GroupChatID :String , completion: @escaping ([String]) -> ()) {
var returnArray: [String] = [""]
rootRef.child("chatMembers").child(GroupChatID).observeSingleEvent(of: .value, with: { (snapshot) in
    for child in snapshot.children.allObjects {
        var append = child as! FIRDataSnapshot
        returnArray.append((append.key as String))
        print("Return Array Currently Contains: \(returnArray)")
        //The above printout works properly and when the for loop finishes, the array is exactly as I want it
    }
    completion(returnArray)
    //BUT, this portion returns an empty array
})
}

我如何调用该函数：

GetRecipientsFor(GroupChatID: gchatID) { (result) -> () in
        print(result)
    }

新功能调用

var recipients : [String] = [""]
DispatchQueue.main.async {
    GetRecipientsFor(GroupChatID: gchatID) { result in
    print(result) //PRINTS CORRECTLY!!!
    recipients = result
    }
}
print(recipients) //PRINTS A BLANK ARRAY