Annotations with custom callouts

Based on code by Asynchrony Solutions and several authors.
See http://stackoverflow.com/questions/8018841/customize-the-mkannotationview-callout/8019308#8019308

Strategy

We can't create a XIB backed callout, but we can customize the annotation view. So the trick is to add a second annotation when the first is selected, and make the 2nd annotation view look like a callout bubble. This results in a completely customizable virtual callout.

What we see in this image is:

• AnnotationProtocol: protocol to implement by annotations who wish to handle view creation themselves.
• AnnotationViewProtocol: protocol to implement by annotations who wish to handle view selection themselves.
• Annotation & AnnotationView: the real annotation.
• CalloutAnnotation & CalloutAnnotationView: the fake callout bubble.
• BaseCalloutView: MKAnnotationView.
• Content: This class is passed along with the final data to create the callout.

To add an annotation to the map:

CLLocationCoordinate2D coord = CLLocationCoordinate2DMake(0.,0.);
Annotation *ann = [[Annotation alloc]initWithCoordinate:coord]
[self.mapView addAnnotation:ann];

After adding an annotation, the MKMapViewDelegate is invoked, which in turn delegates the view creation to the annotation itself.

-[MKMapViewDelegate mapView:viewForAnnotation:]
  -[Annotation annotationViewInMap] // view creation is delegated to the annotation
    -[AnnotationView initWithAnnotation:reuseIdentifier:] // the view is initialized with the annotation

Now we have a map with a Annotation and AnnotationView on it. Touching the annotation view calls MKMapViewDelegate, which in turn delegates the selection to the view. The view creates a CalloutAnnotation and adds it to the map.

-[MKMapViewDelegate mapView:didSelectAnnotationView:]
  -[AnnotationView didSelectAnnotationViewInMap:]
    -[CalloutAnnotation initWithLat:lon:]

Now we have the same thing, the view creation is delegated to CalloutAnnotation, which creates a CalloutAnnotationView, which uses Core Graphics to create a callout bubble.

CalloutView has a CalloutView.XIB. The code to paint the bubble around the XIB is in the superclass BaseCalloutView.

Classes

AnnotationProtocol
  annotationViewInMap:

AnnotationViewProtocol
  didSelectAnnotationViewInMap:
  didDeselectAnnotationViewInMap:

Annotation        : NSObject         <MKAnnotation, AnnotationProtocol>
AnnotationView    : MKAnnotationView <AnnotationViewProtocol>
BaseCalloutView   : MKAnnotationView <AnnotationViewProtocol>
CalloutAnnotation : NSObject         <MKAnnotation, AnnotationProtocol>
CalloutView       : BaseCalloutView 

Behavior

Annotation        --(added)creates-->    AnnotationView      (the pin)
AnnotationView    --(selected)creates--> CalloutAnnotation   (the annotation acting as delegate to create the bubble)
CalloutAnnotation --(added)creates-->    CalloutView         (the bubble)
CalloutView       --paints-->            <fake callout>      (the bubble painting code)

Usage

To create an annotation:

Content *content = [Content new];
content.iconURL = [[NSBundle mainBundle] URLForResource:@"emoji-ghost" withExtension:@"png"];
content.calloutView = [MyCalloutView class];
content.coordinate = coord;
content.values = [NSDictionary dictionaryWithObjectsAndKeys:@"Booo!",@"title", nil];
Annotation *anno = [[Annotation alloc] initWithContent:content];

Then on MyCalloutView:

#import "CalloutView.h"
#import "CalloutAnnotation.h"
@interface MyCalloutView : CalloutView
@property (nonatomic, retain) IBOutlet UILabel* title;
-(IBAction) handleTouch:(id)sender;
- (id) initWithAnnotation:(CalloutAnnotation*)annotation;
@end

#import "MyCalloutView.h"
@implementation MyCalloutView
@synthesize title = _title;
-(IBAction) handleTouch:(id)sender {
    debug(@"touch %@", sender);
}
- (id)initWithAnnotation:(CalloutAnnotation*)annotation {
    self = [super initWithAnnotation:annotation];
    self.title.text = [annotation.content.values objectForKey:@"title"];
    return self;
}
- (void)dealloc {
    self.title = nil;
    [super dealloc];
}
@end