In Unity if you click Cmd+B (build and run) while the game is already running in Xcode 7.3 on device then Unity will hang (beach ball) with dialog "Closing open Unity projects in Xcode".

To repro:
1: Create new project or open attached (attached project just has bundle identifier and scene in build settings set)
2: Build for iOS and run on device
3: While running on device go into Unity and press Cmd+B to do a build and run
4: Unity will hang on "Closing open Unity projects in Xcode"