iOS dynamically override subtitle method in id<MKAnnotation> subclass - iphone

Question

I'm in a situation where in the
- (MKAnnotationView *)mapView:(MKMapView *)mapView viewForAnnotation:(id <MKAnnotation>)annotation {
method, I need to dynamically change the implementation of the subtitle method for the annotation object. The reason I need to do this is because I'm doing some computations based on latitudes and longitudes that are changing frequently (which I wish to display as the subtitle) ...so when I first create the id object, it doesn't make sense to do that computation at that time.
How would I dynamically override the subtitle method for my custom id object? Can someone point me in the direction of doing that? Or are there any other approaches I could take?
EDIT:
To be a bit more clear... I want to add the annotation custom object to the map BEFORE figuring out what the title and subtitle should be for that annotation object. I want to wait until the user touches on the annotation on the map..and when it shows the popup, that's where I want to calculate what to show as the subtitle. That's why I thought of dynamically overriding the subtitle method of the custom id object.
Thanks!

Answer 1

If you need to dynamically change the implementation of a method at run time, that might call for an application of strategy pattern.
With C blocks, we can do it in a flexible and quick way. Have your custom annotation delegate its implementation of subtitle to the return value of a block property. Then, in your map view's delegate, define blocks that calculate the subtitle based on your requirements, and assign them to the annotation's property.
Sketch of a custom annotation implementation that delegates its subtitle implementation:
typedef NSString* (^AnnotationImplementationSubtitleBlock)();
#interface AnnotationImplementation : NSObject <MKAnnotation>
#property (nonatomic, copy) AnnotationImplementationSubtitleBlock *subtitleBlock;
#end
#implementation AnnotationImplementation
- (NSString *)subtitle
{
return self.subtitleBlock();
}
// Rest of MKAnnotation methods
#end
Also a sketch of the implementation of the map view delegate method where the blocks are created and assigned:
- (MKAnnotationView *)mapView:(MKMapView *)mapView viewForAnnotation:(id <MKAnnotation>)annotation
{
AnnotationImplementation *customAnnotation = (AnnotationImplementation *)annotation;
if (/* some condition causing you to do it one way */) {
customAnnotation.subtitleBlock = ^{
//calculate the subtitle some way
return calculatedSubtitle;
}
}
else if (/* some condition causing you to do it another way */) {
customAnnotation.subtitleBlock = ^{
//calculate the subtitle the other way
return calculatedSubtitle;
}
}
... rest of method
}