Sketch is great for wireframing apps, and is getting a lot more useful with a new plugin named Silver, which lets you build actual working prototypes with navigation.
After you get some app pages populated in Sketch, Silver takes over, using familiar control flow tools for linking pages and navigation. A simple drag-and-drop scheme puts your app to actual use, and has editable text fields and Core OS functionality to let users access photo albums, the camera or hack out text messages.
Of course, this won’t actually build the app for you, but it’s handy for knowing how it will actually perform, and if your app views are disparate or flow nicely. You can even hook an actual phone up to your computer to experience your design.