dynamically change lineWidth of mkpolyline

Question

I have Map in my app in which is moving as per user location.I have successfully drawn a polyline for source and destination. Using following code

- (MKOverlayView *)mapView:(MKMapView *)mapView viewForOverlay:(id < MKOverlay >)overlay{
    
    MKPolylineView *view1 = [[MKPolylineView alloc] initWithOverlay:overlay];
    
        view1.lineWidth = 27.0;
        view1.strokeColor = [UIColor colorWithRed:55.0/255.0 green:168.0/255.0 blue:219.0/255.0 alpha:1];
   
    return view1;
}

But my problem is when map is moving , mean i am changing region of map as user moves , then some times polyline is not shown good some time its show thicker than actual size as you can see in below image.

i am attaching the image below, please let me know what can i do for smooth polyline when map is moving.

EDIT

As Matt suggested i create a subclass of MKPolylineRenderer and implement drawMapRect method as below:

-(void)drawMapRect:(MKMapRect)mapRect zoomScale:(MKZoomScale)zoomScale inContext:(CGContextRef)context{
    
    
    CGMutablePathRef fullPath = CGPathCreateMutable();
    BOOL pathIsEmpty = YES;
    
    //merging all the points as entire path
    for (int i=0;i< self.polyline.pointCount;i++){
        CGPoint point = [self pointForMapPoint:self.polyline.points[i]];
        if (pathIsEmpty){
            CGPathMoveToPoint(fullPath, nil, point.x, point.y);
            pathIsEmpty = NO;
        } else {
            CGPathAddLineToPoint(fullPath, nil, point.x, point.y);
        }
    }
    
    //get bounding box out of entire path.
    CGRect pointsRect = CGPathGetBoundingBox(fullPath);
    CGRect mapRectCG = [self rectForMapRect:mapRect];
    //stop any drawing logic, cuz there is no path in current rect.
    if (!CGRectIntersectsRect(pointsRect, mapRectCG))return;
    
    UIColor *darker = [UIColor blackColor];
    CGFloat baseWidth = 10 / zoomScale;
    
    // draw the dark colour thicker
    CGContextAddPath(context, self.path);
    CGContextSetStrokeColorWithColor(context, darker.CGColor);
    CGContextSetLineWidth(context, baseWidth * 1.5);
    CGContextSetLineCap(context, self.lineCap);
    CGContextStrokePath(context);
    
    // now draw the stroke color with the regular width
    CGContextAddPath(context, self.path);
    CGContextSetStrokeColorWithColor(context, [UIColor redColor].CGColor);
    CGContextSetLineWidth(context, baseWidth);
    CGContextSetLineCap(context, self.lineCap);
    CGContextStrokePath(context);
    
    [super drawMapRect:mapRect zoomScale:zoomScale inContext:context];
}

But Same problem See below image:

ok, i think i got problem , my polyline is added once, but as user's speed i changing zoom level of MKMapView, zoom level is changed , but polyline width is not refresh,

SO how can i dynamically change lineWidth of mkpolyline ??

Answer 1

The problem is that you are setting your lineWidth at a fixed width (a really big fixed width). This gets larger or smaller as the map is zoomed larger or smaller.

Instead, implement your own MKOverlayRenderer subclass, and override drawMapRect:zoomScale:inContext:. It receives a zoomScale parameter, and so you can adjust the width of your line to look good at what the current scale may be.

Answer 2

Subclass MKPolylineRenderer and override applyStrokePropertiesToContext:atZoomScale: so that it ignores the scale, and draws lines at constant width:

@interface ConstantWidthPolylineRenderer : MKPolylineRenderer
@end

@implementation ConstantWidthPolylineRenderer

- (void)applyStrokePropertiesToContext:(CGContextRef)context
                           atZoomScale:(MKZoomScale)zoomScale
{
    [super applyStrokePropertiesToContext:context atZoomScale:zoomScale];
    CGContextSetLineWidth(context, self.lineWidth);
}

@end

Now use it and admire its smooth rendering:

- (MKOverlayRenderer *)mapView:(MKMapView *)mapView rendererForOverlay:(id<MKOverlay>)overlay
{
    MKPolyline *polyline = (MKPolyline *)overlay;
    ConstantWidthPolylineRenderer *renderer = [[ConstantWidthPolylineRenderer alloc] initWithPolyline:polyline];
    renderer.strokeColor = [UIColor redColor];
    renderer.lineWidth = 40;
    return renderer;
}