drawing heading angle view with quartz

I just wanted to draw heading angle view on MKMapView like Maps app, Of course I can do that with photoshop or any necessary program. But I don't need to consider retina display or screen differences. Is it possible to draw this sha开发者_如何学JAVApe with quartz or any related built in library and how ?

thanks in advance.

You can ;) To do so, you need to implement this method :

- (MKAnnotationView *)mapView:(MKMapView *)mapView viewForAnnotation:(id <MKAnnotation>)annotation

In this one you will have to create a instance of a subclass of MKAnnotationView only for the user location case of annotation like:

- (MKAnnotationView *)mapView:(MKMapView *)mapView viewForAnnotation:(id <MKAnnotation>)annotation
{
    if ([annotation isKindOfClass:[MKUserLocation class]])
    {
        if (userView == nil) {
            userView=[[[UserLocationView alloc] 
                       initWithAnnotation:annotation reuseIdentifier:@"user"]
                      autorelease];
        }

        [self addObserver:userView forKeyPath:@"userHeading" options:NSKeyValueObservingOptionNew context:NULL] ; 

        return userView ; 
    }

    return nil;
}

Note the KVO statement to let your custom MKAnnotationView that the value has changed in your MKAnnotationView subclass you need to override drawRect; like that for example

- (void)drawRect:(CGRect)rect {

    CGContextRef context = UIGraphicsGetCurrentContext();
    NSAssert(context !=nil, @"Context was nil") ;

    CGContextSetRGBStrokeColor (context,1.0f,0.0f,0.0f,1.0f) ;

    CGContextSetLineWidth(context, 2.0f);

    CGContextMoveToPoint(context, 0, 15);
    CGContextAddLineToPoint(context, 30, 15);

    CGContextMoveToPoint(context, 15,0);
    CGContextAddLineToPoint(context, 15, 30);

    CGContextStrokePath(context);

    CGContextSetLineWidth(context, 4.0f);

    CGContextAddArc (context,15,15,13,0,6.28,0);

    CGContextStrokePath(context);

    hdg = radians([self heading]);

    dx = self.frame.size.width / 2 ;
    dy = self.frame.size.height/ 2 ;

    if (hdg < M_PI) {
        y = (cos(hdg) * dy ) - dy ;
        x = (sin(hdg) * dx ) + dx ;
    } else {
        y = (cos(hdg) * dy ) + dy ;
        x = (sin(hdg) * dx ) + dx ;
    }


    // NSLog(@"Heading:%3f Hdg:%3f X:%3f Y:%3f", heading, hdg, x, y) ;

    CGContextSetRGBStrokeColor (context,1.0f,0.0f,1.0f,1.0f) ;

    CGContextSetLineWidth(context, 2.0f);
    CGContextMoveToPoint(context, dx,dy);
    CGContextAddLineToPoint(context, x, y);


    CGContextStrokePath(context);
}

This draws a red reticle with a purple line showing the heading (in my sample the course is bugged because i am just weak in trigonometry ;) but it draws :)

You of course need to handle the KVO notification like:

-(void) observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    [self setHeading: [[change objectForKey:NSKeyValueChangeNewKey]doubleValue]] ;
    [self setNeedsDisplay] ; 
    NSLog(@"Heading is %f", [self heading]);
}

the -setNeedsDisplay will trigger the redraw.

Make sure that you custom view has a non-zero rect frame, and do NOT set an image to it.

Having not used MKMapView before, but you might be able to subclass it and in your drawRect: method first draw the superclass and then use standard CG drawing methods to draw your heading angle on top of it.

- (void)drawRect:(NSRect)dirtyRect
{
    [super drawRect:dirtyRect];

    // Your drawing goes here
}