为什么不是我的UIGraphicsImageRenderer绘图（iOS 10）？

Question

UIGraphicsImageRenderer is the new iOS 10 way to create an image context and draw into it. I'm trying to use UIGraphicsImageRenderer to draw a simple image:

let size = CGSize(width:100,height:100)
let f = UIGraphicsImageRendererFormat()
f.opaque = false
let r = UIGraphicsImageRenderer(size:size, format: f)
let im = r.image { _ in
    let p = UIBezierPath(ovalIn: CGRect(origin: CGPoint.zero, size: size))
    UIColor.blue().setFill()
    p.fill()
}
let iv = UIImageView(image:im)
iv.frame.origin = CGPoint(x: 200, y: 200)
self.view.addSubview(iv)

I expect to see a blue filled circle. But I don't see anything. What am I doing wrong?

PS I'm using Xcode 8 Seed 3, if it matters.

Answer 1

The problem is that that's not how you make a UIGraphicsImageRendererFormat. The documentation is a bit unclear on this point, but in fact you should be saying this:

let size = CGSize(width:100,height:100)
let f = UIGraphicsImageRendererFormat.default() // *
f.opaque = false
let r = UIGraphicsImageRenderer(size:size, format: f)
let im = r.image { _ in
    let p = UIBezierPath(ovalIn: CGRect(origin: CGPoint.zero, size: size))
    UIColor.blue().setFill()
    p.fill()
}
let iv = UIImageView(image:im)
iv.frame.origin = CGPoint(x: 200, y: 200)
self.view.addSubview(iv)

Your code will then work perfectly.

The problem with the previous code is that UIGraphicsImageRendererFormat() creates a completely unconfigured UIGraphicsImageRendererFormat, which is thus pretty much useless (unless you set all of its properties yourself).