static method objective c - init once

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;
}

Search for information on singleton's for a good example of how to do this. — lnafziger, May 01 '12 at 18:04
add static before UserAgent *thisClass and you should be fine. `static UserAgent *thisClass;` — Jonathan Cichon, May 01 '12 at 18:04
possible duplicate of [Thread safe instantiation of a singleton](http://stackoverflow.com/questions/2199106/thread-safe-instantiation-of-a-singleton) — Chuck, May 01 '12 at 18:06

score 3 · Accepted Answer · edited May 23 '17 at 11:49

3

No.

thisClass is a local variable. This means, the value will be reset (to garbage) everytime +userAgentString is called. At least make it static.
self's meaning is not what you expect inside a class method. Do you mean thisClass?
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.

edited May 23 '17 at 11:49

Community

1
1

answered May 01 '12 at 18:08

kennytm

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@i_raqz: which is the same as the first answer of http://stackoverflow.com/questions/8796529/singleton-in-ios-5. – kennytm May 01 '12 at 18:19

score 0 · Answer 2 · answered May 01 '12 at 18:07

A couple changes. First, thisClass should be static. Second, you don't have a self pointer in a static method, so you should be using thisClass there. Like so:

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

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

    return thisClass.userAgent;
}

Also, a better approach might be to use a Singleton, as mentioned by a commenter.

score 0 · Answer 3 · edited Jul 24 '12 at 23:15

0

http://www.galloway.me.uk/tutorials/singleton-classes/

This worked..Thanks @Inafziger

edited Jul 24 '12 at 23:15

mattjgalloway

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answered May 01 '12 at 18:16

i_raqz

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static method objective c - init once

3 Answers3