Ios what does nonatomic mean

What does the property "Nonatomic" mean?

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 @synthesize d 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.