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When declaring the properties in Objective-C, what are the custom options available to configure, which define how the accessor methods would behave?
For example, you can have weak, strong, readonly.
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The goal of the @property directive is to configure how an object can be exposed. If you intend to use a variable inside the class and do not need to expose it to outside classes, then you do not need to define a property for it. Properties are basically the accessor methods.
Categories provide the ability to add functionality to an object without subclassing or changing the actual object. A handy tool, they are often used to add methods to existing classes, such as NSString or your own custom objects.
You can have properties in a protocol, provided every class that conforms to your protocol have a corresponding @synthesize for that property, or provide a getter and setter. But with a class category you generally can't add instance variables to a class.
Basically in Objective-C, we call the function as method. The Objective-C foundation framework provides numerous built-in methods that your program can call. For example, method appendString() to append string to another string.
Here's the short answer:
atomic vs nonatomic primarily ensures that complete values are returned from synthesized getters and that complete values are written by synthesized setters.
readwrite vs readonly determines whether a synthesized property has a synthesized accessor or not (readwrite has a setter and is the default, readonly does not).
assign vs weak vs retain vs copy determines how the synthesized accessors interact with the Objective-C memory management scheme.
And now for the long answer:
Atomic 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.
Assign, weak, retain, copy
In a nutshell, assign vs weak vs retain vs copy determines how the synthesized accessors interact with the Objective-C memory management scheme:
Remember that retain/strong 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.
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