Question

So my clients iphone app has balloned from 5 or so classes to over 25 in the last few weeks. With such a large (for the iphone anyway) class structure I’ve been utilizing the Singleton class for accessing global variables. The problem is that whenever I need to access a variable outside of the class I’m working on, I have a choice of either modifying the code to pass a reference to the variable or just throw it in the singleton. Of course throwing it in the singleton is a lot less code and effort, however, now the singleton is beginning to look like a mish-mosh of unrelated globals.

My question is how appropriate is it to maintain a large singleton object with unrelated global vars and what is generally the “best practice” way of sharing data across classes in a “non-trivial” iphone app?

Many thanks in advance for your advice!

Answer 1

I have done a detailed analysis of this problem in the context of a decoupled design. I’ll try to summarize my points:

1. If your Singleton carries a significant global state, don’t use Singleton. This includes persistent storage such as Databases, Files etc.
2. In cases, where dependency on a Singleton Object is not obvious by the classes name, the dependency should be injected. The need to inject Singleton Instances into classes proves a wrong usage of the pattern (see point 1).
3. A Singleton’s life-cycle is assumed to be the same as the application’s. Most Singleton implementations are using a lazy-load mechanism to instantiate themselves. This is trivial and their life-cycle is unlikely to change, or else you shouldn’t use Singleton.