As someone that's new to Objective-C can someone give me an overview of the retain, assign, copy and any others I'm missing, that follow the @property directive? What are they doing and why would I want to use one over another?
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
.m
@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`
For this reason, it’s faster to access a nonatomic property than an atomic one.
@property (nonatomic)NSString *name;
The setter method will increase retain count of the object, so that it will occupy memory in autorelease pool.
@property (retain)NSString *name;
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.
Compiler will generate a getter, but not a setter.
@property (readonly) NSString *name;
It is opposite of readonly.
@property (readwrite) NSString *name;
Keep in mind retain and assign are basically interchangeable when garbage collection is enabled.
@property (assign) NSInteger year;
It comes with ARC.
@property (nonatomic, strong) AVPlayer *player;
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;
The method should end with a colon.
@property(setter = boolBool:) BOOL finished;
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;