Create our commands and show the buffer

buffer.swift

// STEP 5: create a buffer of actual render commands
let encoder = buffer?.makeRenderCommandEncoder(descriptor: rpDesc)
encoder?.setRenderPipelineState(renderPipeline!)
encoder?.setVertexBuffer(vertexArray, offset: 0, index: 0)
encoder?.drawPrimitives(type: .triangle, vertexStart: 0, vertexCount: 3)
encoder?.endEncoding()

// show the buffer.
if let drawable = drawable {
    buffer?.present(drawable)
    buffer?.commit()
}