定製的 getter 和 setter

可以覆蓋預設屬性 getter 和 setter：

@interface TestClass

@property NSString *someString;

@end

@implementation TestClass

// override the setter to print a message
- (void)setSomeString:(NSString *)newString {
    NSLog(@"Setting someString to %@", newString);
    // Make sure to access the ivar (default is the property name with a _ 
    // at the beginning) because calling self.someString would call the same
    // method again leading to an infinite recursion
    _someString = newString;
}

- (void)doSomething {
    // The next line will call the setSomeString: method
    self.someString = @"Test";
}

@end

這可以用於提供，例如，延遲初始化（通過覆蓋 getter 來設定初始值，如果尚未設定）：

- (NSString *)someString {
    if (_someString == nil) {
        _someString = [self getInitialValueForSomeString];
    }
    return _someString;
}

你還可以建立一個在 getter 中計算其值的屬性：

@interface Circle : NSObject

@property CGPoint origin;
@property CGFloat radius;
@property (readonly) CGFloat area;

@end

@implementation Circle

- (CGFloat)area {
    return M_PI * pow(self.radius, 2);
}

@end