What to use instead of singleton server manager?

Question

API

Why is it bad practice to make a server-manager singleton? How to do it right and why?

Answer 1

First time I hear about this. Another thing is how to write it and for what purposes it is, what are the restrictions. It is necessary to answer such questions:
From my experience, I came to the conclusion that it is best to use a queue (if you plan to implement for iOS less than 7, then use NSOperationQueue + NSOperation, if 7+ then NSURLSession + NSURLSessionDownloadTask) And, depending on the answer, set the maximum number of executable downloads.
this question is relevant if you implement NSURLSessionDownloadTask on iOS 7+ - can be designed to be loaded into background.
From my experience, I found a more or less universal solution, after the request has received a response, it is immediately processed inside the manager and used by NSNotificationCenter to notify all the necessary controllers.