Making global mutable variables that are available to all classes in Objective C

Question

I am developing a game which needs variables that need to be accessed by different classes. I wanted to know if there is a way to make global mutable variables with Objective C?

Answer 1

There's no problem creating global mutable variables in Objective-C.

In a header file:

extern NSMutableArray *gMyArray;
extern NSMutableDictionary *gMyDictionary;

In your application delegate source file:

NSMutableArray *gMyArray;
NSMutableDictionary *gMyDictionary;

In applicationDidFinishLaunching:

gMyArray = [[NSMutableArray alloc] init];
gMyDictionary = [[NSMutableDictionary alloc] init];

Then just #import the header file in every source file you want to access the global.

Answer 2

The usual approach is to provide a singleton class which provides access to the required variables. And rather than exposing a raw variable that is mutated directly by the caller, you should really encapsulate the operations. Having true global variables creates too much coupling and is a bad code smell.

For example, a singleton lets you write this:

[[NetworkTrafficStats instance] addNetworkTraffic:bytes_sent];

which is much better to maintain than:

extern unsigned gTotalBytesSent;
//...
gTotalBytesSent += bytes_sent;

Worth reading:

• Singleton Design
• The Objective-C Singleton
• ADC: Singletons