In iOS, variable property attributes indicate data accessibility and storage considerations,

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- atomic              //default
- nonatomic
- strong = retain        //default
- weak = unsafe_unretained
- retain
- assign                //default
- unsafe_unretained
- copy
- readonly
- readwrite             //default

• atomic (default)
  • Only one thread access the variable (static type)
  • Thread safe, but slow

Example :

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@property (atomic, retain) NSString *name;
@synthesize name;

• nonatomic
  • Multiple threads access the variable (dynamic type)
  • Thread unsafe, but fast
  • Not a default behavior, need to add nonatomic keyword

Example:

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@property (nonatomic, retain) NSString *name;
@synthesize name;

• strong (iOS4 = retain, default)
  • Own the object strongly, keep it in the heap until don’t point to it anymore
  • Can’t dealloc this before aim fine with that same as “retain”
  • Generally, using for UIViewControllers (UI item’s parents)
  • Used with ARC and ARC automatically releases it when beyond its area or the strong reference is invalid

Example:

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@property (nonatomic, strong) ViewController *viewController;
@synthesize viewController;

• weak (In iOS 4 & OS X Snow Leopand = unsafe_unretained )
  • Keep it as long as someone else points to it strongly
  • A “weak” reference is a reference that you don’t retain
  • Can’t own the instance of object
  • When the object is “deallocated”, the weak pointer is automatically set to nil
  • Generally using for IBOutlets (UIViewController’s Childs) because the child object only needs to exist as long as the parent object does

Example :

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@property (nonatomic, weak) IBOutlet UIButton *myButton;
@synthesize myButton;

• retain = strong
  • Old value is released and it is assigned
  • Specifies the new value should be sent “-retain” on assignment and the old value sent “-release”
  • If you write retain it will auto work like strong
  • Methods like “alloc” include an implicit “retain”

Example:

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@property (nonatomic, retain) NSString *name;
@synthesize name;

• assign (default)
  • A property attribute tells the compiler how to synthesize the property’s setter implementation

Example:

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@property (nonatomic, assign) NSString *address;
@synthesize address;

• unsafe_unretained (In iOS 4 & OS X Snow Leopand)
  • An ownership qualifier that tells ARC how to insert retain/release calls
  • The ARC version of assign
  • The old version of weak but not safe

Example:

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@property (nonatomic, unsafe_unretained) NSString *nickName;
@synthesize nickName;

• copy
  • Required when the object is mutable
  • Specifies the new value should be sent “-copy” on assignment and the old value should be sent “-release”
  • Like retain, returns an object which you must explicitly release (e.g., in dealloc) in non-garbage collected environments
  • Need to release the object when finished with it because you are retaining the copy

Example:

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@property (nonatomic, copy) NSArray *myArray;
@synthesize myArray;

• readonly
  • Tell compiler not to generate “setter” method automatically
  • If you specify readonly, only a “getter” method is required in the @implementation block
  • If you use the @synthesize directive in the @implementation block, only the “getter” method is synthesized

Example:

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@property (nonatomic, readonly) NSString *name;
@synthesize name;

• readwrite (default)
  • “setter” and “getter” are both generated
  • Both “setter” and “getter” method are required in the @implementation block
  • If you use the @synthesize directive in the implementation block, both the “setter” and “getter” methods are synthesized

Example:

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@property (nonatomic, readwrite) NSString *name;
@synthesize name;