Is it possible to inherit from MKPolyline

Question

I'm building a MapKit based app for iPhone.

I have a number of MKPolylines added to the map.

However, instead of having a MKPolyline, I would like to have my own Model class conforming to the MKOverlay protocol added to the map so that I can access the model properties when creating the corresponding view in mapView:viewForOverlay.

The problem is that I can't find the way to inherit from MKPolyline because it doesn't have any init methods that I can call from the subclass' init. You can only create them using the convenience methods.

How can I bring together the model properties and the MKPolyline behaviour?

Answer 1

MANIAK_dobrii's code is the way to go but I found I had to implement some additional MKMultiPoint methods to get it to work, here are my complete header and implementation files for an AnchorLine class I used:-

Header AnchorLine.h

#import <MapKit/MapKit.h>

@interface AnchorLine : NSObject <MKOverlay> {
    MKPolyline* polyline;
}

@property (nonatomic, retain) MKPolyline* polyline;

+ (AnchorLine*)initWithPolyline: (MKPolyline*) line;
@end

Implementation AnchorLine.m

#import "AnchorLine.h"

@implementation AnchorLine

@synthesize polyline;


+ (AnchorLine*)initWithPolyline: (MKPolyline*) line {
    AnchorLine* anchorLine = [[AnchorLine alloc] init];
    anchorLine.polyline = line;
    return [anchorLine autorelease];
}

- (void) dealloc {
    [polyline release];
    polyline = nil;
    [super dealloc];
}

#pragma mark MKOverlay
//@property (nonatomic, readonly) CLLocationCoordinate2D coordinate;
- (CLLocationCoordinate2D) coordinate {
    return [polyline coordinate];
}

//@property (nonatomic, readonly) MKMapRect boundingMapRect;
- (MKMapRect) boundingMapRect {
    return [polyline boundingMapRect];
}

- (BOOL)intersectsMapRect:(MKMapRect)mapRect {
    return [polyline intersectsMapRect:mapRect];
}

- (MKMapPoint *) points {
    return [polyline points];
}


-(NSUInteger) pointCount {
    return [polyline pointCount];
}

- (void)getCoordinates:(CLLocationCoordinate2D *)coords range:(NSRange)range {
    return [polyline getCoordinates:coords range:range];
}

@end

Hope that helps someone.

Answer 2

You can set an associated object attribute of the class. This allows you to bind an instance variable to an existing class. Make sure you properly clean up after yourself.

Answer 3

It is true that MKPolyline doesn't have its own init method. In fact the only class in MKPolyline's inheritance chain that does have an init method is NSObject.

So when I subclassed MKPolyline I just overrode the init method defined by NSObject...

-(id) init {
    self = [super init];
    if(self) {
        //my initialization here
    }
    return self;
}

Then when you want to instantiate your subclass with coordinates you might do something like this...

-MyPolyline* myPolyline = (MyPolyline*)[MyPolyline polylineWithCoordinates:coordinates count:coordinateCount];