Origami

Origami is a Facebook-owned prototyping tool that has been used to create mockups for several apps including Instagram, Messenger, and Paper. While predominantly used for iOS apps due to its integration with Apple's Quartz Composer, Origami also supports the development of interactive Android prototypes. The tool offers designers a suite of gestures and transition animations common to mobile UI patterns — many of which are available via downloadable sample files.

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Origami offers a couple of useful features for interactive prototyping, including plugins for Sketch and Photoshop as well as an extensive documentation library complete with forums, video tutorials, and guides. My personal favourite, however, is its "Export to Code" feature. This option lets you convert your visual design into written code samples for iOS, Android, or web. This allows you to share animation spec snippets with engineers with just a click of a button.

You can quickly test out your interactive mobile prototypes directly within an iOS device with the companion app, Origami Live. All you need to do is download the app from the Apple App Store and plug in your iOS device to your desktop, and voila! — you have an interactive mobile work environment. Unfortunately, this is the extent of Origami's collaborative capabilities as you can't directly share prototypes with individuals operating on devices other than your own.