What's the difference between the atomic and n

Question

What do atomic and nonatomic mean in property declarations?

@property(nonatomic, retain) UITextField *userName;
@property(atomic, retain) UITextField *userName;
@property(retain) UITextField *userName;

What is the operational difference between these three?

Answer 1

Easiest answer first: There's no difference between your second two examples. By default, property accessors are atomic.

Atomic accessors in a non garbage collected environment (i.e. when using retain/release/autorelease) will use a lock to ensure that another thread doesn't interfere with the correct setting/getting of the value.

See the "Performance and Threading" section of Apple's Objective-C 2.0 documentation for some more information and for other considerations when creating multi-threaded apps.

Answer 2

Atomic means only one thread can access the variable at a time (static type). Atomic is thread-safe, but it is slow.

Nonatomic means multiple threads can access the variable at same time (dynamic type). Nonatomic is thread-unsafe, but it is fast.