What Cocoa control(s) are sidebar controls as per the human interface guidelines?

Question

The Mac OS X Human Interface Guidelines (HIG) outline certain things you should adhere to for sidebar icons and the reason given is:

OS X applies various effects to sidebar icons

First Question I don't see a standard out-the-box sidebar control that OS X would be applying these effects to. Which controls would these be?

Second Question

HIG also recommend:

Create your icons in three sizes: 16 x 16, 18 x 18, and 32 x 32 pixels (if using PDF).

Why would you need to replicate these three sizes if you are providing vector artwork (pdf) to the application?

[I could perhaps understand if the sizes were not all just pure scalings of the original square 16x16 but given that they are all symmetric to one another...]

Answer 1

The reference to sidebar controls is a reference to the earlier chapter Windows: Source Lists (Sidebars):

A source list (also called a sidebar) is an area of a window, usually set off by a movable splitter, that lets users navigate or select objects in an app. … Typically, users select an object in the source list and then act upon that object in the main part of the window.

API Note

By default, a source list is translucent when you use an NSOutlineView or NSTableView object and set the highlight style to NSTableViewSelectionHighlightStyleSourceList.

Regarding the icon sizes, I'm pretty sure that's just a typo. They dropped the word "not", as in "16 x 16, 18 x 18, and 32 x 32 pixels (if not using PDF)". The subsequent paragraph makes that clear:

If you create your sidebar icons in PDF format, OS X automatically scales your icon for high-resolution displays, so you don't need to provide high-resolution versions. However, if you use PNG format for your icons, you need to supply the following resources: 16x16, 16x16@2x, 18x18, 18x18@2x, 32x32, and 32x32@2x.

Answer 2

What they mean is not clearly stated but it means when you create a sidebar in traditional cocoa fashion. That is, a source list (particularly configured NSOutlineView). In Yosemite that view uses a visual effect view and certain relative styling will be applied to AppKit objects that are NSControls inside of that view. The best example is Finder's sidebar.

As for the second question, not all images used must be vector art and vector art. I'm not even sure those sizes are still meaningful these days if they stil list pixels, unless they're poorly hinting at the smallest size of images that include @2x and @3x versions.