binding attribute of a variable to another variable

Question

I have the following class:

@interface Location : RKObject <NSCoding>{
    NSNumber*  _latitude;
    NSNumber*  _longitude;
    NSNumber*  _route_order;
    CLLocationCoordinate2D coordinate;
}

@property (nonatomic, retain) NSNumber* latitude;
@property (nonatomic, retain) NSNumber* longitude;
@property (nonatomic, retain) NSNumber* route_order;

- (id) initWithLatitude:(double) lat  andLongitude:(double) longt;

@end

I would like to bind the coordinate.latitude with _latitude doubleValue and the same thing with longitude. So the value of coordinate.latitude is always set with whatever value is currently at _latitude. Is this possible in objective-C

Vincent Guerci · Accepted Answer · 2011-04-06 09:23:33Z

1

There is no "magical" bind in Objective-C what you could do instead of synthesizing properties is writing accessors (which is what @synthesize really do at compilation in fact), something like that :

@interface Location : RKObject <NSCoding> {
    NSNumber*  _route_order;
    CLLocationCoordinate coordinate;
}

@property (nonatomic, assign) NSNumber* latitude;
@property (nonatomic, assign) NSNumber* longitude;
@property (nonatomic, retain) NSNumber* route_order;

- (id) initWithLatitude:(double) lat  andLongitude:(double) longt;

@end

When implementing accessors instead of synthesizing them, the retain / assign / readonly keyword is somewhat informative. In this example I've used assign because your properties won't actually retain references passed.

In Implementation:

@implementation Location

// note the absence of @synthesize there

- (NSNumber *)latitude  { return [NSNumber numberWithDouble:latitude];  }
- (NSNumber *)longitude { return [NSNumber numberWithDouble:longitude]; }
- (void)setLatitude:(NSNumber *)latitude   { coordinate.latitude  = [latitude doubleValue];  }
- (void)setLongitude:(NSNumber *)longitude { coordinate.longitude = [longitude doubleValue]; }

@end

Another alternative, related to "binding" techniques is KVO, but a bit off-topic imho since this a bit too complex for such a simple case.

edited Apr 6, 2011 at 9:23

answered Apr 6, 2011 at 1:26

Vincent Guerci

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2 Comments

aherlambang Over a year ago

but there's no way to set that coordinate.latitude to latitude?

Vincent Guerci Over a year ago

You can by writing the setter and changing readonly by assign.

Anomie · Accepted Answer · 2011-04-06 01:25:43Z

1

I don't believe there is any way to do what you are asking. But if you misstated your request, and want the latitude property to access the coordinate's latitude (rather than the _latitude ivar), it's a simple matter of writing custom getter and setter methods:

- (NSNumber *)latitude {
    return [NSNumber numberWithDouble:coordinate.latitude];
}

- (void)setLatitude:(NSNumber *)value {
    coordinate.latitude = [value doubleValue];
}

You could then get rid of the _latitude ivar completely.

answered Apr 6, 2011 at 1:25

Anomie

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Comments

Simon Goldeen · Accepted Answer · 2011-04-06 01:13:56Z

0

What it sounds like you want to do is this:

@synthesize latitude = _latitude;

That will make the synthesized setter and getter methods for your latitude property change your _latitude ivar.

answered Apr 6, 2011 at 1:13

Simon Goldeen

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2 Comments

aherlambang Over a year ago

I don't think you're getting my question.. I want coordinate.latitude to bind with NSNumber latitude and same goes for longitude

aherlambang Over a year ago

coordinate is a CLCoordinateLocation2D here, FYI

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