@property retain, assign, copy, nonatomic in Objective-C

Before you know about the attributes of @property, you should know what is the use of @property.

@property offers a way to define the information that a class is intended to encapsulate. If you declare an object/variable using @property, then that object/variable will be accessible to other classes importing its class.
If you declare an object using @property in the header file, then you have to synthesize it using @synthesize in the implementation file. This makes the object KVC compliant. By default, compiler will synthesize accessor methods for this object.
accessor methods are : setter and getter.

Example: .h

@interface XYZClass : NSObject
@property (nonatomic, retain) NSString *name;
@end

@implementation XYZClass
@synthesize name;
@end

Now the compiler will synthesize accessor methods for name.

XYZClass *obj=[[XYZClass alloc]init];
NSString *name1=[obj name]; // get 'name'
[obj setName:@"liza"]; // first letter of 'name' becomes capital in setter method

List of attributes of @property

atomic, nonatomic, retain, copy, readonly, readwrite, assign, strong, getter=method, setter=method, unsafe_unretained
atomic is the default behavior. If an object is declared as atomic then it becomes thread-safe. Thread-safe means, at a time only one thread of a particular instance of that class can have the control over that object.

If the thread is performing getter method then other thread cannot perform setter method on that object. It is slow.

@property NSString *name; //by default atomic`
@property (atomic)NSString *name; // explicitly declared atomic`

nonatomic is not thread-safe. You can use the nonatomic property attribute to specify that synthesized accessors simply set or return a value directly, with no guarantees about what happens if that same value is accessed simultaneously from different threads.

For this reason, it’s faster to access a nonatomic property than an atomic one.

@property (nonatomic)NSString *name;

retain is required when the attribute is a pointer to an object.

The setter method will increase retain count of the object, so that it will occupy memory in autorelease pool.

@property (retain)NSString *name;

copy If you use copy, you can't use retain. Using copy instance of the class will contain its own copy.

Even if a mutable string is set and subsequently changed, the instance captures whatever value it has at the time it is set. No setter and getter methods will be synthesized.

@property (copy) NSString *name;

now,

NSMutableString *nameString = [NSMutableString stringWithString:@"Liza"];    
xyzObj.name = nameString;    
[nameString appendString:@"Pizza"];

name will remain unaffected.

readonly If you don't want to allow the property to be changed via setter method, you can declare the property readonly.

Compiler will generate a getter, but not a setter.

@property (readonly) NSString *name;

readwrite is the default behavior. You don't need to specify readwrite attribute explicitly.

It is opposite of readonly.

@property (readwrite) NSString *name;

assign will generate a setter which assigns the value to the instance variable directly, rather than copying or retaining it. This is best for primitive types like NSInteger and CGFloat, or objects you don't directly own, such as delegates.

Keep in mind retain and assign are basically interchangeable when garbage collection is enabled.

@property (assign) NSInteger year;

strong is a replacement for retain.

It comes with ARC.

@property (nonatomic, strong) AVPlayer *player;

getter=method If you want to use a different name for a getter method, it’s possible to specify a custom name by adding attributes to the property.

In the case of Boolean properties (properties that have a YES or NO value), it’s customary for the getter method to start with the word “is”

@property (getter=isFinished) BOOL finished;

setter=method If you want to use a different name for a setter method, it’s possible to specify a custom name by adding attributes to the property.

The method should end with a colon.

@property(setter = boolBool:) BOOL finished;

unsafe_unretained There are a few classes in Cocoa and Cocoa Touch that don’t yet support weak references, which means you can’t declare a weak property or weak local variable to keep track of them. These classes include NSTextView, NSFont and NSColorSpace,etc. If you need to use a weak reference to one of these classes, you must use an unsafe reference.

An unsafe reference is similar to a weak reference in that it doesn’t keep its related object alive, but it won’t be set to nil if the destination object is deallocated.

@property (unsafe_unretained) NSObject *unsafeProperty;

If you need to specify multiple attributes, simply include them as a comma-separated list, like this:

@property (readonly, getter=isFinished) BOOL finished;

The article linked to by MrMage is no longer working. So, here is what I've learned in my (very) short time coding in Objective-C:

nonatomic vs. atomic - "atomic" is the default. Always use "nonatomic". I don't know why, but the book I read said there is "rarely a reason" to use "atomic". (BTW: The book I read is the BNR "iOS Programming" book.)

readwrite vs. readonly - "readwrite" is the default. When you @synthesize, both a getter and a setter will be created for you. If you use "readonly", no setter will be created. Use it for a value you don't want to ever change after the instantiation of the object.

retain vs. copy vs. assign

"assign" is the default. In the setter that is created by @synthesize, the value will simply be assigned to the attribute. My understanding is that "assign" should be used for non-pointer attributes.
"retain" is needed when the attribute is a pointer to an object. The setter generated by @synthesize will retain (aka add a retain count) the object. You will need to release the object when you are finished with it.
"copy" is needed when the object is mutable. Use this if you need the value of the object as it is at this moment, and you don't want that value to reflect any changes made by other owners of the object. You will need to release the object when you are finished with it because you are retaining the copy.

@property retain, assign, copy, nonatomic in Objective-C

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