[swift] Getting a "This application is modifying the autolayout engine from a background thread" error?

Been encountering this error a lot in my OS X using swift:

"This application is modifying the autolayout engine from a background thread, which can lead to engine corruption and weird crashes. This will cause an exception in a future release."

I have a my NSWindow and I'm swapping in views to the contentView of the window. I get the error when I try and do a NSApp.beginSheet on the window, or when I add a subview to the window. Tried disabling autoresize stuff, and I don't have anything using auto layout. Any thoughts?

Sometimes it's fine and nothing happens, other times it totally breaks my UI and nothing loads

This question is related to swift macos autolayout background-thread

The answer is

It needs to be placed inside a different thread that allows the UI to update as soon as execution of thread function completes:

Modern Swift:

DispatchQueue.main.async {
    // Update UI

Older versions of Swift, pre Swift 3.

    // code here


dispatch_async(dispatch_get_main_queue(), ^{
    // code here

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