SOLVED: Day 39: Course code throws warning "-[UINavigationController tabBarItem] must be used from main thread only" – 100 Days of Swift – Hacking with Swift forums
![runtime: UI API called from background thread: -[UIApplication applicationState] must be used from main thread only - Stack Overflow runtime: UI API called from background thread: -[UIApplication applicationState] must be used from main thread only - Stack Overflow](https://i.stack.imgur.com/fB8FR.png)
runtime: UI API called from background thread: -[UIApplication applicationState] must be used from main thread only - Stack Overflow
![已解决】Xcode9提示:runtime UI API called from background thread UIApplication currentUserNotificationSettings must be used from main thread only – 在路上 已解决】Xcode9提示:runtime UI API called from background thread UIApplication currentUserNotificationSettings must be used from main thread only – 在路上](https://www.crifan.com/files/pic/uploads/2018/07/153c8a40fe20feb69475e65823ce8679.png)