Why after upgrading to Xcode 4.2 does MKAnnotation display a warning

问题

In Xcode 4.1 there was no problem, but upgrading to Xcode 4.2 I get the following warning:

Property 'title' 'copy' attribute does not match the property inherited from 'MKAnnotation'
Property 'subtitle' 'copy' attribute does not match the property inherited from 'MKAnnotation'

My code:

@interface MyAnnotation : NSObject <MKAnnotation> {
    CLLocationCoordinate2D coordinate;
    NSString *subtitle;  
    NSString *title; 
}
@property (nonatomic, readonly) CLLocationCoordinate2D coordinate;
@property (nonatomic, retain) NSString *subtitle;  
@property (nonatomic, retain) NSString *title; 

-(id)initWithCoordinate:(CLLocationCoordinate2D) coordinate;

@end

回答1:

Change it to:

@property (nonatomic, copy) NSString *subtitle;
@property (nonatomic, copy) NSString *title;

The MKAnnotation protocol declares

@property (nonatomic, readonly, copy) NSString *title;
@property (nonatomic, readonly, copy) NSString *subtitle;

You shouldn't change the storage type of a property, the only change you can / should make is from readonly to readwrite if needed;

回答2:

Try converting you application to ARC using Edit -> Refactor -> Convert to Objective-C ARC

来源：https://stackoverflow.com/questions/7760081/why-after-upgrading-to-xcode-4-2-does-mkannotation-display-a-warning