iOS: “Main Thread Checker” warning


This guide applies to all frameworks that create or generate iOS applications.

When debugging the iOS version of the app with Smartlook, a warning is encountered which title reads:

Main Thread Checker: UI API called on a background thread: -[UIView drawViewHierarchyInRect:afterScreenUpdates:]

The warning is harmless, and is caused by Smartlook’s capturing screen on the background. The alternative (capturing the screenshot on the main thread) may cause glitches of the app UI – so implementing it this way and having this warning is a necessary trade-off to enable seamless recording functionality.

This warning also might be accompanied by a short freeze of the app during debugging. This freeze does not happen in production builds, and is caused by the Main Thread Checker Xcode diagnostic option.

This option can be switched off in the respective run scheme diagnostic options, see "Detect Improper UI Updates on Background Threads" section of the "Diagnosing Memory, Thread, and Crash Issues Early."