The last two are identical; "atomic" is the default behavior (note that it is not actually a keyword; it is specified only by the absence of nonatomic
-- atomic
was added as a keyword in recent versions of llvm/clang).
Assuming that you are @synthesizing the method implementations, atomic vs. non-atomic changes the generated code. If you are writing your own setter/getters, atomic/nonatomic/retain/assign/copy are merely advisory. (Note: @synthesize is now the default behavior in recent versions of LLVM. There is also no need to declare instance variables; they will be synthesized automatically, too, and will have an _
prepended to their name to prevent accidental direct access).
With "atomic", the synthesized setter/getter will ensure that a whole value is always returned from the getter or set by the setter, regardless of setter activity on any other thread. That is, if thread A is in the middle of the getter while thread B calls the setter, an actual viable value -- an autoreleased object, most likely -- will be returned to the caller in A.
In nonatomic
, no such guarantees are made. Thus, nonatomic
is considerably faster than "atomic".
What "atomic" does not do is make any guarantees about thread safety. If thread A is calling the getter simultaneously with thread B and C calling the setter with different values, thread A may get any one of the three values returned -- the one prior to any setters being called or either of the values passed into the setters in B and C. Likewise, the object may end up with the value from B or C, no way to tell.
Ensuring data integrity -- one of the primary challenges of multi-threaded programming -- is achieved by other means.
Adding to this:
atomicity
of a single property also cannot guarantee thread safety when multiple dependent properties are in play.
Consider:
@property(atomic, copy) NSString *firstName;
@property(atomic, copy) NSString *lastName;
@property(readonly, atomic, copy) NSString *fullName;
In this case, thread A could be renaming the object by calling setFirstName:
and then calling setLastName:
. In the meantime, thread B may call fullName
in between thread A's two calls and will receive the new first name coupled with the old last name.
To address this, you need a transactional model. I.e. some other kind of synchronization and/or exclusion that allows one to exclude access to fullName
while the dependent properties are being updated.
Best Answer
The biggest advantage a framework has over static libraries is that they act as a neat way of packaging up the compiled library binary and any related headers. They can be dropped into your project (just like the SDK's built-in frameworks like Foundation and UIKit) and they should just work (most of the time).
Most frameworks contain dynamic libraries; frameworks created in Xcode using the Mac Framework template will create a dynamic library. The iPhone does not support dynamic frameworks which is why it has become common for reusable libraries of iOS code to be distributed as static libraries instead.
Static libraries are fine, but they require a bit of extra work on the part of the user. You need to link your project to the library and you need to copy the header files into your project or reference them somewhere by setting the appropriate header search paths in your build settings.
So: in summary, my opinion is that the best way of distributing your library is as a framework. To create a "static" framework for iOS, you can essentially take a normal framework and replace the binary with your compiled static library. This is how I distribute one of my libraries, Resty and is how I intend to distribute my libraries in the future.
You may want to look at the supplied Rakefile in that project (in case you aren't aware, Rake is Ruby's equivalent of Make). I have a handful of tasks for compiling my project (using
xcodebuild
) and packaging them as a static framework for iOS. You should find this useful.Alternatively, you may wish to use these Xcode 4 templates for creating an iOS framework.
Update 9 Dec 2013: this is a popular answer so I thought I'd edit to say that my first choice for library distribution has changed. My first choice for any third party library as either a consumer or producer is CocoaPods. I distribute my libraries using CocoaPods and offer a precompiled static library with headers as a fallback option.