Why Use Properties?

Question

So I've been programming on Objective-C for over a year now, and I can't seem to understand the use for properties. I have searched the internet a few times but never really found a good explaniation. I understand how to create them:

@property (something, something) something *variableName;
@syntheize variableName;

But should I make all my instance variables properties. To me, from what I know, it seems like a waste of code. But when I look at code online, sometimes I see like 25 properties in one class. Which I think is a waste. The only time I ever use them is when passing info from a UITableView cell selected to a detail viewController. For that, I use:

@property (copy) NSString *myString;

Can you also explain what: nonatomic, copy, retain, assign, etc. mean.

Thanks

Answer 1

These properties are convenience methods for creating getters and setters.

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Atmoic v Nonatomic

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.

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.

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Assign, retain, copy

In a nutshell, assign vs retain vs copy determines how the synthesized accessors interact with the Objective-C memory management scheme:

• assign is the default and simply performs a variable assignment
• retain specifies the new value should be sent -retain on assignment and the old value sent release
• copy specifies the new value should be sent -copy on assignment and the old value sent release.

Remember that retain is done on the created object (it increases the reference count) whereas copy creates a new object. The difference, then, is whether you want to add another retain to the object or create an entirely new object.