How to debug custom geometry in SceneKit with Swift

Question

I'm trying to learn how to create custom geometry in SceneKit. However, I've tried to make a triangle and it's not showing anything. I'm at a loss as to how to debug this. Is there a way to figure out if the triangle is valid? I just don't know where to start.

For reference, the playground code in question is below. Note that it is written against Swift 4, but the changes between Swift 3 and Swift 4 are so minor that getting it to compile in Swift 3 is trivial.

import UIKit
import SceneKit

let points = [
    SCNVector3Make(0, 0, 0),
    SCNVector3Make(0, 10, 0),
    SCNVector3Make(10, 0, 0),
]
let indices = [
    0,2,1,
]

let vertexSource = SCNGeometrySource(vertices: points)
let element = SCNGeometryElement(indices: indices, primitiveType: .triangles)
let geo = SCNGeometry(sources: [vertexSource], elements: [element])

Answer 1

When creating custom SCNGeometryElements the type of the indices needs to be Int16¹. I don't think this documented anywhere. But when you change the declaration of the indices too

let indices: [Int16] = [
    0, 2, 1
]

the triangle should appear.

---

Edit

1: As @mnuages has pointed out, SceneKit supports only 32bit Integers as indices. So you can use Int8, Int16 and Int32.