October 2nd, 2017
Xcode 9 adds Main Thread Checker, a tool that detects when you make AppKit, UIKit, and WebKit calls on a background thread. Main Thread Checker should be enabled automatically when debugging, but you can check your scheme by selecting the Run step and clicking the Diagnostics button at the top of the scheme editor. Make sure the Main Thread Checker checkbox is selected.
You can learn more about Main Thread Checker in the following WWDC video:
Tags: xcode 9