Ios9 - This Application Is Modifying the Autolayout Engine from a Background Thread -- Where

iOS9 - This application is modifying the autolayout engine from a background thread -- where?

This code PSPDFUIKitMainThreadGuard causes assertions on UIKit access outside the main thread

Steps to use:

1. Add to project and compile this file without ARC
2. Move PSPDFAssert definition to the first of the file
3. Comment calling of PSPDFLogError as it is not defined
4. import <UIKit/UIKit.h>

Your app will crash and stop with any attemption to modify any UI element from background thread

For swift use the following code: NBUIKitMainThreadGuard

Xcode7: This application is modifying the autolayout engine from a background thread, which can lead to engine corruption and weird crashes

I don't know what your code is, but I have an idea as to why this is happening. You must be having some UI changes done in a background thread.

Do not change UI from anything but the main thread, it is bound to
make your application unstable, and crash unpredictably.

Use GCD (Grand Central Dispatch).

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
});

Getting the following error: This application is modifying the autolayout engine from a background thread

In iOS all UI code must run on the main thread. You are dispatching to a background thread in order to perform your update check, which is fine, but then trying to update the UI from that same background thread, which is causing the error you are seeing. The solution is to add another dispatch_async inside the first one, wrapping the call to self.showAlertForUpdate(), but dispatching to the main queue (dispatch_get_main_queue(...) instead.

This application is modifying the autolayout engine from a background thread - ios9

How else can I reload the table view without getting this error?

You step out to the main thread in order to call reloadData, like this:

let task = NSURLSession.sharedSession().dataTaskWithURL(url!) {
    (data, response, error) in
    self.jsonResult = try! NSJSONSerialization.JSONObjectWithData(data!, options: NSJSONReadingOptions.MutableContainers) as! NSMutableArray
    dispatch_async(dispatch_get_main_queue()) {
        self.tableView.reloadData()
    }
}

Always do that any time you touch the interface from code that was called on a background thread. You must never never never never touch the interface except on the main thread.

Getting a This application is modifying the autolayout engine from a background thread error?

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.

dispatch_async(dispatch_get_main_queue(){
    // code here
})

Objective-C:

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

Downloading from server: This application is modifying the autolayout engine ...

to update UI you should call Main thread

put this code where you updating ui

swift 3 way

    DispatchQueue.main.async {
        //here you can update UI
    }

swift 2 way

    dispatch_async(dispatch_get_main_queue()) {
       //here you can update UI
    }

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

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.

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