Icon Set Studio is the spiritual successor of the acclaimed Icon Set Creator.

Unlike its minimalistic predecessor, where you had to choose an input image and output directory each time, Icon Set Studio is a document-based cocoa app: you create a "project file” that persists all input and output configuration between app launches (including multiple "targets" on each supported platform). 

Icons can be exported directly into your Xcode project’s asset catalog directory, so you don’t need to copy files manually.