From Xojo Documentation
| This class is only available on the macOS platform.
For cross-platform compatibility, use ＃If...＃Endif with the Target... specifiers to make sure you will not attempt to use this class on an incompatible platform.
Used to manage the icons for your application and application windows in the macOS Dock.
The DockItem class is only for macOS. It enables you to manipulate the dock item associated with your application or with the application's windows. To manipulate the application's dock icon, use the DockItem property of the Application class. To manipulate a window’s dock icon, use the DockItem property of the Window class.
Use the methods of the Graphics class to modify the appearance of the icon. Since a macOS icon is intended to be scaled automatically, you should design it as at least a 128x128 pixel icon and perhaps as large as 1024x1024 for newer version of macOS.
The DockItem class has two methods, UpdateNow and ResetIcon. ResetIcon resets the icon to its original state. Call the UpdateNow method to redraw the icon. You can also use the ClearRect property of the Graphics class to start over from a blank icon. Anything you can do with a Graphics object you are able to do with the DockItem's Graphics property, such as drawing in a picture or using a Group2D.