CoreGraphics - Draw stroke of arc - ios

Question

I have a UIBezierPath which is an arc. I only want to draw the stroke to the graphics context (that is, the stroke of the arc, NOT including the line connecting the end and start points of the arc)
Once I’ve done that, I can then add a linear gradient to the context, effectively drawing a gradient on the stroke of an arc.
Here is my drawRect:
let context = UIGraphicsGetCurrentContext()
CGContextSaveGState(context)
CGContextAddPath(context, _progressPathForProgress(_progressToDrawForProgress(progress)).CGPath)
CGContextStrokePath(context)
CGContextClip(context)
let colours = [self.startColour.CGColor, self.endColour.CGColor]
let colourSpace = CGColorSpaceCreateDeviceRGB()
let colourLocations: [CGFloat] = [0.0, 1.0]
let gradient = CGGradientCreateWithColors(colourSpace, colours, colourLocations)
var startPoint = CGPoint.zeroPoint
var endPoint = CGPoint(x: 0, y: bounds.height)
CGContextDrawLinearGradient(context, gradient, startPoint, endPoint, CGGradientDrawingOptions.allZeros)
CGContextRestoreGState(context)
But all this does is add a gradient to the entirety of my UIView. Where have I gone wrong?

Answer 1

The path needs to be closed in order to get the clipping to work.
If you don't want to see the closing line you can set the stroke color to UIColor.clearColor() and then close the path.
CGContextSaveGState(context)
CGContextAddPath(context, _progressPathForProgress(_progressToDrawForProgress(progress)).CGPath)
CGContextStrokePath(context)
CGContextSetStrokeColorWithColor(context, UIColor.clearColor().CGColor)
// close the path, you will need to add some more points here, otherwise
// the end point is simply connected to the start point
CGPathCloseSubpath(...)
CGContextClip(context)
// draw the gradient
I'll explain what I mean by adding more points to close the path. The following images are a screenshot from one of my apps, it is an altitude profile with a gradient below the graph.
Closing the path directly would result in this, meaning the gradient would not be drawn down to the x-axis:
To close the path correctly I add points from the last coordinate (x,y) down to the x axis (x,0), then to (0,0) and finally close the path with the first point, like so:
I don't want to see the closing lines, so I use UIColor.clearColor() here.
Hope you get the idea.