Main Thread Checker: UI API called on a background thread: -[UIApplication applicationState]

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It’s hard to find the UI code which is not executed in main thread sometimes. You can use the trick below to locate it and fix it.

  1. Choose Edit Scheme -> Diagnostics, tick Main Thread Checker.

    Xcode 11.4.1

    Click the small arrow next to the Main Thread Checker to create a Main Thread Checker breakpoint.
    enter image description here

    Previous Xcode

    Tick on Pause on issues.
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  2. Run your iOS application to reproduce this issue. (Xcode should pause on the first issue.)
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  3. Wrap the code that modify the UI in DispatchQueue.main.async {}
    enter image description here

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