Robin Good's insight:
Briefs is a Mac / iOS app which allows any designer to easily draft and prototype the user interface of any mobile app without needing to write a line of code.
Briefs provides all of the tools needed to animate menus and visual elements so that they react and behave exactly as they would on an iPhone or iPad.
You can prototype on your desktop computer and test an interactive mockup of your design simultaneously.
Designed for designers.
More info: http://giveabrief.com/
Mac App Store: https://purchase.giveabrief.com/ ($199)
Trial version available - limit of 10 scenes and 1 timeline
App Store: https://purchase.giveabrief.com/ (free)
for testing your prototype app