Every view has a border, which by default is invisible. To reveal and style the border you can set the following properties.
width
color
radius
The border itself is actually a property, so you set the above like this:
view.border.width = 10.0
You can even copy an entire border between views:
shape.border = view.border
let img = Image("chop")!
img.center = canvas.center
img.border.width = 10.0
img.border.color = C4Pink
canvas.add(img)