Global Variables in iPhone Objective-C
If you studied any programming in school you were probably told to never use global variables because ultimately it can cause you end up with a messy slob of code that is difficult to maintain, and this holds true in iPhone Objective-C.
While that is correct, the truth of the matter is there are many situations when you want to have a global variable, as often there are variables that don’t really belong to any objects, and you want to change them from many locations within your application. You could of course place variables in your AppDelegate and access them through there, but this is not a good solution and can turn things into a great big mess which is the reason why many college professors forbid the use of global variables in all languages (not simply because they may have never actually coded anything). The solution to this is to create a class using the Singleton design pattern. For example:
@interface VariableStore : NSObject
{
// Place any "global" variables here
}
// message from which our instance is obtained
+ (VariableStore *)sharedInstance;
@end
@implementation VariableStore
+ (VariableStore *)sharedInstance
{
// the instance of this class is stored here
static VariableStore *myInstance = nil;
// check to see if an instance already exists
if (nil == myInstance) {
myInstance = [[[self class] alloc] init];
// initialize variables here
}
// return the instance of this class
return myInstance;
}
@end
Whenever you need to access a variable assuming you have a getter defined as the variable name simply use:
[[VariableStore sharedInstance] variableName]
If the variable is a primitive, and a global constant then there is really no need to use a singleton as above you can use a preprocessor #define statement. For example:
#define VARIABLE value
Now when you need to share a variable between many objects this should make for much simpler, and cleaner code.
If you’re transitioning to Objective-C from C/C++/Java you might want to check out my Objective-C Cheat Sheet.
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thanks for this!
I'm new to iPhone / objective c. I would ask, if these are global, do you need to make accessors and setters thread safe? Or are most apps single threaded unless/until you explicitly start another thread? Is the iPhone even multithreaded?
Also, you could define class methods as getter and setter methods, but then you are doing twice the work. Although I did read that an instance (a class as well,perhaps) can get a message and pass it along to another object to process it. If you could do this for all messages not implemented, just send all unhandled messages sent to the class over to the instance… Something to think about…
Hi Brian,
there are no real “global” variables in Objective-c, the purpose of the singleton in this case is so that we can access it from anywhere, and access the variables in there, the getters and setters are made in the singleton class's property statements. You can set the properties to be atomic and the getters/setters created will be thread safe.
I still get duplicate symbol error during build for the singleton class name, VariableStore, when include the same header file with globals in multiple files. Any way to resolve that? Thanks,
I'm having trouble accessing the variable after doing this. I get a few warnings:
– “VariableStore may not respond to variableName (messages without a matching method signature will be assumed to return id and accept '…' as arguments)
– initialization makes integer from pointer without a cast
Are there some steps missing from the post? I'm trying to get a handle on the singleton concept.
Thanks,
Sean
Nice, thanks for sharing this
Thanks so much for this tip. I've been trying for a while to get the singleton to work.
Thanks so much for this tip. I've been trying for a while to get the singleton to work.
i have searched for a solution like this, but – maybe i interpreted it wrong – i don’t get the initialization at // initialize variables here to work
in the .m i have made this:
@implementation VariableStore
//synthesize [3]
@synthesize globalerstring;
+ (VariableStore *)sharedInstance
{….
in the .h
#import
@interface VariableStore : NSObject
{
// Place any “global” variables here [1]
NSString *globalerstring;
}
// message from which our instance is obtained
+ (VariableStore *)sharedInstance;
//properties [2]
@property (nonatomic,retain) NSString *globalerstring;
@end
maybe i haven’t understood the principle in the right way. could anybody do one example for any variable ?
btw: who is the thief ?
http://www.picksourcecode.com/ps/ct/161183.php
(@mod: you dont have to make this post public)
That post came out several months after mine so clearly they just copied. Thanks for pointing that out.
Hi, I’m having the same problem as Sean:
– “VariableStore may not respond to variableName (messages without a matching method signature will be assumed to return id and accept ‘…’ as arguments)
I’m clearly missing some step, but which one?
Thanks
Daniel
Hi, I’m pretty new with Obj C. Every class that I know, always have dealloc method. Should I put dealloc in this Singleton? Thanks.
Great stuff, concise and simple.
I make use of 4 global variable parameters. Input once and used throughout three view controllers. Never modified. I tried to use your singleton and sought advice from Stack Overflow on how best to get four UITextfields (the parameters) into the singleton variables. I came in for some criticism for a) using a singleton for such a piffling requirement and b) for not complying with the Apple recommended Design Pattern for a singleton. Can’t win. Don’t use Globals and don’t use Singletons for program-wide use.
I ended up using 4 Globals and haven’t looked back. So, when is it correct/incorrect to use a singleton?
Hi
Really thanks it works and better than my F…ing global … Thks a lot