How to create singleton class in objective C

Solution 1:

Okay appDev, you will probably find quite a few different techniques to do this on the web. However, for iOS app development, I think the most convenient way is to do the following:

• Write your method(s) for getting the singleton object. (Recommendation: use dispatch_once thread and GCD for this).

• Wrap your method(s) in a macro and add it to your $Project$-Prefix.pch file.

• Call the one line macro whenever you need singleton object for a class.

Example:

CommonMacros.h:

#define SINGLETON_FOR_CLASS(classname)
+ (id) shared##classname {
    static dispatch_once_t pred = 0;
    static id _sharedObject = nil;
    dispatch_once(&pred, ^{
        _sharedObject = [[self alloc] init];
    });
    return _sharedObject;
}

YourProject-Prefix.pch:

...
#import "CommonMacros.h"
...

YourSingletonClass.m:

...
SINGLETON_FOR_CLASS(YourSingletonClass)
...

Solution 2:

Check out this link for the original source - http://getsetgames.com/2009/08/30/the-objective-c-singleton/

@implementation MySingleton
static MySingleton *_sharedMySingleton = nil;

+(MySingleton *)sharedMySingleton {
    @synchronized([MySingleton class]) {
        if (!_sharedMySingleton)
          _sharedMySingleton = [[self alloc] init];
        return _sharedMySingleton;
    }
    return nil;
}

Solution 3:

I do think this is how we can truly achieve singleton behavior :

@interface SampleSingletonClass : NSObject

+ sharedSampleSingletonClass;

@end


@implementation SampleSingletonClass
static SampleSingletonClass *singletonObject = nil;

+ (id) sharedSampleSingletonClass
{
    if (! singletonObject) {

        singletonObject = [[SampleSingletonClass alloc] init];
    }
    return singletonObject;
}

- (id)init
{
    if (! singletonObject) {

        singletonObject = [super init];
    // Uncomment the following line to see how many times is the init method of the class is called
    // NSLog(@"%s", __PRETTY_FUNCTION__);
    }
    return singletonObject;
}

@end

Here even if one calls init method instead of the intended + (id) SampleSingletonClass; method the actual object is formed just once throughout the app's lifecycle.