why

LayerTest.playground

//: A Cocoa based Playground to present user interface

import AppKit
import PlaygroundSupport

let nibFile = NSNib.Name("MyView")
var topLevelObjects : NSArray?

Bundle.main.loadNibNamed(nibFile, owner:nil, topLevelObjects: &topLevelObjects)
let views = (topLevelObjects as! Array<Any>).filter { $0 is NSView }

// Present the view in Playground
PlaygroundPage.current.liveView = views[0] as! NSView

class CoolView: NSView {

  override init(frame frameRect: NSRect) {
    super.init(frame: frameRect)

    self.wantsLayer = true

    self.layerContentsRedrawPolicy = .onSetNeedsDisplay
    self.layer?.backgroundColor = .white
  }

  required init?(coder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
  }

}

let v = CoolView(frame: .zero)

let bounds = NSRect(origin: .zero, size: CGSize(width: 150.0, height: 100.0))
v.bounds = bounds
v.setNeedsDisplay(bounds)

print(v.bounds)

v.frame = bounds

print(v.bounds)

let root = views[0] as! NSView

root.addSubview(v)