“This application is modifying the autolayout engine from a background thread”

Question

I am having an error of:

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

whenever I try to load an image view into a table view. The relevant code is as follows:

override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    let cellIdentifier = "ArticleCell"
    let cell = tableView.dequeueReusableCellWithIdentifier(cellIdentifier, forIndexPath: indexPath) as! ArticleCell


    let a: Article = self.articlesArray[indexPath.row] as! Article
    //let a: Article = self.newArticlesArray[indexPath.row] as! Article

    cell.titleLabel.text = a.title
    cell.descLabel.text = a.desc

    let data = NSData(contentsOfURL: a.thumbnail)
    cell.iView.image = UIImage(data: data!)

    return cell
}

func downloadArticles(){
    //JSON parsing methods here
        self.syncCompleted = true
        dispatch_async(dispatch_get_main_queue()) {
            self.tableView.reloadData()
        }
    }
    task.resume()
}

Every time I click the image in my table view the image moves to the left of the cell and if the image I loaded is not the size of the UIImageView then it resizes it all.

How would I fix this error?

I have tried the dispatch_get_main_queue code, but it does not seem to be doing much for me.

Answer 1

Let's identify exact execution path which leads to auto layout engine corruption. For this you need to add symbolic breakpoint to following:

+[NSException raise:format:]

You can do this following way:

Then:

After that once your app get stuck on the breakpoint, please grab and share the stack trace. You can do this using lldb console:

(lldb) bt
* thread #4: tid = 0x2bbc48, 0x000000010ba5ebf0 CoreFoundation`+[NSException raise:format:], queue = 'com.apple.root.default-qos', stop reason = breakpoint 1.1
  * frame #0: 0x000000010ba5ebf0 CoreFoundation`+[NSException raise:format:]
    frame #1: 0x0000000108dd5b6b Foundation`_AssertAutolayoutOnMainThreadOnly + 79
    frame #2: 0x0000000108dd5a62 Foundation`-[NSISEngine _optimizeWithoutRebuilding] + 49
    frame #3: 0x0000000108c34c0f Foundation`-[NSISEngine optimize] + 46
    frame #4: 0x0000000108c3b875 Foundation`-[NSISEngine constraintDidChangeSuchThatMarker:shouldBeReplacedByMarkerPlusDelta:] + 313
    frame #5: 0x0000000108c3b6f2 Foundation`-[NSISEngine tryToChangeConstraintSuchThatMarker:isReplacedByMarkerPlusDelta:undoHandler:] + 440
    frame #6: 0x0000000108c27715 Foundation`-[NSLayoutConstraint _tryToChangeContainerGeometryWithUndoHandler:] + 484
    frame #7: 0x0000000108c27274 Foundation`-[NSLayoutConstraint _setSymbolicConstant:constant:] + 422
    frame #8: 0x0000000108ac35d5 iosapp`ViewController.(self=0x00007fe86a6abc80) -> ()).(closure #1) + 133 at ViewController.swift:26
    frame #9: 0x0000000108ac3627 iosapp`thunk + 39 at ViewController.swift:0
    frame #10: 0x000000010c775d9d libdispatch.dylib`_dispatch_call_block_and_release + 12
    frame #11: 0x000000010c7963eb libdispatch.dylib`_dispatch_client_callout + 8
    frame #12: 0x000000010c77eb2f libdispatch.dylib`_dispatch_root_queue_drain + 1829
    frame #13: 0x000000010c77e405 libdispatch.dylib`_dispatch_worker_thread3 + 111
    frame #14: 0x000000010cad34de libsystem_pthread.dylib`_pthread_wqthread + 1129
    frame #15: 0x000000010cad1341 libsystem_pthread.dylib`start_wqthread + 13
(lldb) 

Once you've done this, we can identify exact place which causes the issue.