WeakSingleton.m

+ (id)sharedInstance
{   
    static __weak ASingletonClass *instance;
    ASingletonClass *strongInstance = instance;    
    @synchronized(self) {        
    if (strongInstance == nil) {
            strongInstance = [[[self class] alloc] init];
            instance = strongInstance;
        }
    }   
    return strongInstance;
}