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Create singleton using GCD's dispatch_once in Objective-C

If you can target iOS 4.0 or above

Using GCD, is it the best way to create singleton in Objective-C (thread safe)?

+ (instancetype)sharedInstance
{
    static dispatch_once_t once;
    static id sharedInstance;
    dispatch_once(&once, ^{
        sharedInstance = [[self alloc] init];
    });
    return sharedInstance;
}
like image 544
Ryan Avatar asked Apr 19 '11 17:04

Ryan


4 Answers

This is a perfectly acceptable and thread-safe way to create an instance of your class. It may not technically be a "singleton" (in that there can only ever be 1 of these objects), but as long as you only use the [Foo sharedFoo] method to access the object, this is good enough.

like image 137
Dave DeLong Avatar answered Oct 19 '22 11:10

Dave DeLong


instancetype

instancetype is just one of the many language extensions to Objective-C, with more being added with each new release.

Know it, love it.

And take it as an example of how paying attention to the low-level details can give you insights into powerful new ways to transform Objective-C.

Refer here: instancetype


+ (instancetype)sharedInstance
{
    static dispatch_once_t once;
    static id sharedInstance;

    dispatch_once(&once, ^
    {
        sharedInstance = [self new];
    });    
    return sharedInstance;
}

+ (Class*)sharedInstance
{
    static dispatch_once_t once;
    static Class *sharedInstance;

    dispatch_once(&once, ^
    {
        sharedInstance = [self new];
    });    
    return sharedInstance;
}
like image 36
Zelko Avatar answered Oct 19 '22 10:10

Zelko


MySingleton.h

@interface MySingleton : NSObject

+(instancetype)sharedInstance;

+(instancetype)alloc __attribute__((unavailable("alloc not available, call sharedInstance instead")));
-(instancetype)init __attribute__((unavailable("init not available, call sharedInstance instead")));
+(instancetype)new __attribute__((unavailable("new not available, call sharedInstance instead")));
-(instancetype)copy __attribute__((unavailable("copy not available, call sharedInstance instead")));

@end

MySingleton.m

@implementation MySingleton

+(instancetype)sharedInstance {
    static dispatch_once_t pred;
    static id shared = nil;
    dispatch_once(&pred, ^{
        shared = [[super alloc] initUniqueInstance];
    });
    return shared;
}

-(instancetype)initUniqueInstance {
    return [super init];
}

@end
like image 34
Sergey Petruk Avatar answered Oct 19 '22 11:10

Sergey Petruk


You can avoid that the class be allocated with overwriting the alloc method.

@implementation MyClass

static BOOL useinside = NO;
static id _sharedObject = nil;


+(id) alloc {
    if (!useinside) {
        @throw [NSException exceptionWithName:@"Singleton Vialotaion" reason:@"You are violating the singleton class usage. Please call +sharedInstance method" userInfo:nil];
    }
    else {
        return [super alloc];
    }
}

+(id)sharedInstance
{
    static dispatch_once_t p = 0;
    dispatch_once(&p, ^{
        useinside = YES;
        _sharedObject = [[MyClass alloc] init];
        useinside = NO;
    });   
    // returns the same object each time
    return _sharedObject;
}
like image 6
i-developer Avatar answered Oct 19 '22 12:10

i-developer