About Model class

ch05 seems a mountain to climb. For me that not only a newbie in Metal but also a newbie in Swift takes more time to chew.
I notice that the Mesh contain 2 part : mdlMesh and mtkMesh

meshes = zip (mdlMeshes, mtkMeshes).map {
    Mesh(mdlMesh: $0.0, mtkMesh: $0.1)

but In the Render part, only mtkMesh is used

 for submesh in mesh.submeshes {
          let mtkSubmesh = submesh.mtkSubmesh
          renderEncoder.drawIndexedPrimitives(type: .triangle,
                                              indexCount: mtkSubmesh.indexCount,
                                              indexType: mtkSubmesh.indexType,
                                              indexBuffer: mtkSubmesh.indexBuffer.buffer,
                                              indexBufferOffset: mtkSubmesh.indexBuffer.offset)

Why we add those mdlMesh in the class that complicate the code?

Yes, I would agree that Chapter 5 is the hardest chapter in parts 1 and 2. We’ve tried to keep the “Swiftiness” level to a minimum, but sometimes Swift solutions are just too neat to ignore.

In case you’re confused about zip, it will take two arrays and combine them into one array of tuples. So [X,X,X,X] and [Y,Y,Y,Y] becomes [(X,Y),(X,Y),(X,Y),(X,Y)].
When you use .map, the tuple is $0 and each element (X or Y) is $0.0 and $0.1

mdlMesh contains data about materials that you’ll use later.

class Submesh {
  var mtkSubmesh: MTKSubmesh
  init(mdlSubmesh: MDLSubmesh, mtkSubmesh: MTKSubmesh) {
    self.mtkSubmesh = mtkSubmesh

You’ll work on this class in the following two chapters for surface textures and materials.

Model I/O is a convenience API for loading up Metal data, and you won’t use any MDL API in actual rendering. Loading objs etc, used to be a real hassle, and Model I/O abstracts it all away. When initially loading, you need to get all the mdl data into a format that your app will understand. Chapters 6 and 7 are textures and materials, and each submesh (group) could have different materials which you will extract from mdlMesh.

ok, thank you I will keep reading.