Question

I want to class that should be initialized only once and returns some value which was computed the first time. Is below the right approach ?

@property (nonatomic, retain) NSString *userAgent;

@implementation UserAgent
@synthesize userAgent = _userAgent;


+ (NSString *) userAgentString
{
    UserAgent *thisClass;
    if(self == nil)
    {
        thisClass = [[UserAgent alloc] init];
    }

    if (thisClass.userAgent == nil)
    {
        return @"not initialized";
    }

    return thisClass.userAgent;
}

Answer 1

No.

1. thisClass is a local variable. This means, the value will be reset (to garbage) everytime +userAgentString is called. At least make it static.

2. self‘s meaning is not what you expect inside a class method. Do you mean thisClass?

3. Even with the above fixes, the method isn’t thread-safe, which may or may not be okay.

See Create singleton using GCD's dispatch_once in Objective C and Singleton in iOS 5? as examples to properly construct a singleton.