What does the property "Nonatomic" mean?

ios objective-c iphone cocoa properties
user100051 picture user100051 · May 4, 2009 · Viewed 70.5k times · Source

What does "nonatomic" mean in this code?

@property(nonatomic, retain) UITextField *theUsersName;

What is the difference between atomic and nonatomic?

Thanks

Answer

Jesse Rusak picture Jesse Rusak · May 4, 2009

Take a look at the Apple Docs.

Basically, if you say nonatomic, and you generate the accessors using @synthesize, then if multiple threads try to change/read the property at once, badness can happen. You can get partially-written values or over-released/retained objects, which can easily lead to crashes. (This is potentially a lot faster than an atomic accessor, though.)

If you use the default (which is atomic; there used to be no keyword for this, but there is now), then the @synthesized methods use an object-level lock to ensure that multiple reads/writes to a single property are serialized. As the Apple docs point out, this doesn't mean the whole object is thread-safe, but the individual property reads/writes are.

Of course, if you implement your own accessors rather than using @synthesize, I think these declarations do nothing except express your intent as to whether the property is implemented in a threadsafe manner.

Related questions

What are best practices that you use when writing Objective-C and Cocoa?

I know about the HIG (which is quite handy!), but what programming practices do you use when writing Objective-C, and more specifically when using Cocoa (or CocoaTouch).

Read More
Getting a list of files in a directory with a glob

For some crazy reason I can't find a way to get a list of files with a glob for a given directory. I'm currently stuck with something along the lines of: NSString *bundleRoot = [[NSBundle mainBundle] bundlePath]; NSArray *dirContents = [[NSFileManager defaultManager] …

Read More
NSURLRequest setting the HTTP header

I need to set the HTTP header for a request. In the documentation for the NSURLRequest class I didn't find anything regarding the HTTP header. How can I set the HTTP header to contain custom data?

Read More