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
{
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.
Dave is correct, that is perfectly fine. You may want to check out Apple's docs on creating a singleton for tips on implementing some of the other methods to ensure that only one can ever be created if classes choose NOT to use the sharedFoo method.
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
@interface className : NSObject{
+(className*)SingleTonShare;
}
@implementation className
+(className*)SingleTonShare{
static className* sharedObj = nil;
static dispatch_once_t once = 0;
dispatch_once(&once, ^{
if (sharedObj == nil){
sharedObj = [[className alloc] init];
}
});
return sharedObj;
}
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;
}
To create thread safe singleton you can do like this:
@interface SomeManager : NSObject
+ (id)sharedManager;
@end
/* thread safe */
@implementation SomeManager
static id sharedManager = nil;
+ (void)initialize {
if (self == [SomeManager class]) {
sharedManager = [[self alloc] init];
}
}
+ (id)sharedManager {
return sharedManager;
}
@end
and this blog explain singleton very well singletons in objc/cocoa