This application is modifying the autolayout engine from a background thread, which can lead to engine corruption and weird crashes

Question

I get this log in the console when I am running my application in is simulator. Haven't seen this in iOS 8. I am not quite sure whats causing this. Has anyone else come across the same issue and if so how was it fixed? or is there any help anyone can provide in regards to this?

Answer 1

Do not change UI from anything but the main thread. While it may appear to work on some OS or devices and not others, it is bound to make your application unstable, and crash unpredictably.

If you must respond to a notification, which can happen in the background, then ensure UIKit invocation takes place on the main thread.

You at least have these 2 options:

Asynchronous Dispatch

Use GCD (Grand Central Dispatch) if your observer can be notified on any thread. You can listen and do work from any thread, and encapsulate UI changes in a dispatch_async:

dispatch_async(dispatch_get_main_queue()) {
    // Do UI stuff here
}

When to use GCD? When you do not control who sends the notification. It can be the OS, a Cocoapod, embedded libraries, etc. Using GCD will woke anytime, every time. Downside: You find yourself re-scheduling the work.

---

Listen on Main Thread

Conveniently, you can specify on which thread you want the observer to be notified, at the time you are registering for notifications, using the queue parameter:

addObserverForName:@"notification"
    object:nil
    queue:[NSOperationQueue mainQueue]
    usingBlock:^(NSNotification *note){
        // Do UI stuff here
    }

When to observe on main thread? When you are both registering and registered. Bu the time you respond to the notification, you are already where you need to be.

---

Post Notification On Main Thread

[self performSelectorOnMainThread:@selector(postNotification:) withObject:notification waitUntilDone:NO];

Hybrid solution which does not guarantee that the observer is only invoked from said method. It allows for lighter observer, at the cost less robust design. Only mentioned here as a solution you should probably avoid.

Answer 2

Swift 3.0

DispatchQueue.main.async {
}

Answer 3

All UI part updation you need to move into MAIN thread of App.

I was calling a createMenuView() into background and i got below error

"This application is modifying the autolayout engine from a background thread, which can lead to engine corruption and weird crashes"

So I called above method into Main thread Using

    DispatchQueue.main.async {
    }

in SWIFT 3.0 and Xcode 8.0

Correct code written below :

RequestAPI.post(postString: postString, url: "https://www.someurl.com") { (succeeded: Bool, msg: String, responceData:AnyObject) -> () in

        if(succeeded) {
            print(items: "User logged in. Registration is done.")

            // Move to the UI thread
            DispatchQueue.main.async (execute: { () -> Void in

                //Set User's logged in
                Util.set_IsUserLoggedIn(state: true)
                Util.set_UserData(userData: responceData)
                self.appDelegate.createMenuView()

            })

        }
        else {
            // Move to the UI thread
            DispatchQueue.main.async (execute: { () -> Void in
                let alertcontroller = UIAlertController(title: JJS_MESSAGE, message: msg, preferredStyle: UIAlertControllerStyle.alert)
                alertcontroller.title = "No Internet"
                alertcontroller.message = FAILURE_MESSAGE
                self.present(alertcontroller, animated: true, completion: nil)
            })
        }
    }

Answer 4

Should try Symbolic Breakpoint to detect the issue:-

Symbol:

[UIView layoutIfNeeded]

Condition:

!(BOOL)[NSThread isMainThread]

Then put your UI Update code in main thread

DispatchQueue.main.async {}

Answer 5

You have code that updates the UI layout from a background thread. Changing the operation queue that you run your code on does not need to be explicit. For example NSURLSession.shared() does not use the main queue when making new requests. To ensure that your code runs on the main thread, I'm using NSOperationQueue's static method mainQueue().

Swift:

NSOperationQueue.mainQueue().addOperationWithBlock(){
    //Do UI stuff here
}

Obj-C:

[NSOperationQueue mainQueue] addOperationWithBlock:^{
    //Do UI stuff here
}];

Answer 6

Same issue is happened in my case,i have to change the code in following way then it works fine. In ViewDidLoad,call this method by using main thread,

[self performSelectorOnMainThread:@selector(setUpTableRows) withObject:nil waitUntilDone:YES];