Draw dotted (not dashed!) line, with IBDesignable in 2017

Set the line cap style to round and set the “on” length to a tiny number.

Swift playground example:

import UIKit
import PlaygroundSupport

let path = UIBezierPath()
path.move(to: CGPoint(x:10,y:10))
path.addLine(to: CGPoint(x:290,y:10))
path.lineWidth = 8

let dashes: [CGFloat] = [0.001, path.lineWidth * 2]
path.setLineDash(dashes, count: dashes.count, phase: 0)
path.lineCapStyle = CGLineCap.round

UIGraphicsBeginImageContextWithOptions(CGSize(width:300, height:20), false, 2)

UIColor.white.setFill()
UIGraphicsGetCurrentContext()!.fill(.infinite)

UIColor.black.setStroke()
path.stroke()

let image = UIGraphicsGetImageFromCurrentImageContext()
let view = UIImageView(image: image)
PlaygroundPage.current.liveView = view

UIGraphicsEndImageContext()

Result:

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For objective-C, using the same example class as in the question, simply add

CGContextSetLineCap(cx, kCGLineCapRound);

before the call to CGContextStrokePath, and change the ra array values to match my Swift code.

Objective-C version of the Swift example above:

UIBezierPath * path = [[UIBezierPath alloc] init];
[path moveToPoint:CGPointMake(10.0, 10.0)];
[path addLineToPoint:CGPointMake(290.0, 10.0)];
[path setLineWidth:8.0];
CGFloat dashes[] = { path.lineWidth, path.lineWidth * 2 };
[path setLineDash:dashes count:2 phase:0];
[path setLineCapStyle:kCGLineCapRound];
UIGraphicsBeginImageContextWithOptions(CGSizeMake(300, 20), false, 2);
[path stroke];
UIImage * image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();