This tool wraps imagemagick to provide a convenient utility for generating macOS/iPhone app icons that conform to Apple's standard. You should feed it a large square image (>= 1024x1024), or (preferrably) an svg file, and get all appropriate sizes plus a proper json index in a AppIcon.appiconset folder, which can be directly copied into the Assets.xcassets folder in your Xcode project.

Type genicon or genicon -h in terminal:

  $ genicon

   genicon: A command line tool for generating icons for Cocoa apps.
   © Kaiyin Zhong 2016

   Usage:
   genicon [options] INPUT OUTPUT_FOLDER

   --help, -h Print this message.
   --mac Generate icons for macOS.
   --iphone Generate icons for iPhone. When neither --mac nor --iphone is given, mac icons are generated by default.

You need to have imagemagick installed with librsvg enabled, something like:

brew install imagemagick --with-librsvg

Only tested on macOS Sierra.

Download and unzip the executable from the release page and put it anywhere in your PATH, /usr/local/bin for example.