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We all know UI calls have to be run in the main thread.

But sometimes mistake happen, and you will see this in debug log:

This application is modifying the autolayout engine from a background thread after the engine was accessed from the main thread. This can lead to engine corruption and weird crashes.

There is a easy way to trace which part of your code is casuing the problem.

1. Add Symbolic Breakpoint
2. Symbol: [UIView layoutIfNeeded]
3. Condition: !(BOOL)[NSThread isMainThread]

There are other “symbols” to catch. Create a symbolic breakpoint for each of these symbols:

• [UIView updateConstraintsIfNeeded]
• [UIView setNeedsUpdateConstraints]
• [UIView setNeedsDisplay]
• [UIView setNeedsLayout]

Run the app, and Xcode will break at the offending code.

Lastly, the fix as we all know:

DispatchQueue.main.async {
    // The UI call
}