Objective-C Language What are properties?


Example

Here is an example class which has a couple of instance variables, without using properties:

@interface TestClass : NSObject {
    NSString *_someString;
    int _someInt;
}

-(NSString *)someString;
-(void)setSomeString:(NSString *)newString;

-(int)someInt;
-(void)setSomeInt:(NSString *)newInt;

@end


@implementation TestClass

-(NSString *)someString {
    return _someString;
}

-(void)setSomeString:(NSString *)newString {
    _someString = newString;
}

-(int)someInt {
    return _someInt;
}

-(void)setSomeInt:(int)newInt {
    _someInt = newInt;
}

@end

This is quite a lot of boilerplate code to create a simple instance variable. You have to create the instance variable & create accessor methods which do nothing except set or return the instance variable. So with Objective-C 2.0, Apple introduced properties, which auto-generate some or all of the boilerplate code.

Here is the above class rewritten with properties:

@interface TestClass

@property NSString *someString;
@property int someInt;

@end


@implementation testClass

@end

A property is an instance variable paired with auto-generated getters and setters. For a property called someString, the getter and setter are called someString and setSomeString: respectively. The name of the instance variable is, by default, the name of the property prefixed with an underscore (so the instance variable for someString is called _someString, but this can be overridden with an @synthesize directive in the @implementation section:

@synthesize someString=foo;    //names the instance variable "foo"
@synthesize someString;    //names it "someString"
@synthesize someString=_someString;        //names it "_someString"; the default if 
                                           //there is no @synthesize directive

Properties can be accessed by calling the getters and setters:

[testObject setSomeString:@"Foo"];
NSLog(@"someInt is %d", [testObject someInt]);

They can also be accessed using dot notation:

testObject.someString = @"Foo";
NSLog(@"someInt is %d", testObject.someInt);