iOS9 This application is modifying the autolayout engine from a background thread, which can lead to engine corruption and weird crashes

Question

I have just dowloaded the latest XCode (7.1 beta) and have started playing around with iOS9.

I had an app working perfectly with no errors in iOS8 but now I get the following error from overriding the drawRect method in a UITableViewCell class :

"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."

here is the backtrace :

Stack:(
0   CoreFoundation                      0x000000010a749f65 __exceptionPreprocess + 165
1   libobjc.A.dylib                     0x0000000109dcfdeb objc_exception_throw + 48
2   CoreFoundation                      0x000000010a749e9d +[NSException raise:format:] + 205
3   Foundation                          0x0000000109b442e5 _AssertAutolayoutOnMainThreadOnly + 79
4   Foundation                          0x00000001099a4ece -[NSISEngine withBehaviors:performModifications:] + 31
5   UIKit                               0x000000010b9d425b -[UIView(AdditionalLayoutSupport) _withAutomaticEngineOptimizationDisabledIfEngineExists:] + 58
6   UIKit                               0x000000010b9d4d9e -[UIView(AdditionalLayoutSupport) updateConstraintsIfNeeded] + 254
7   UIKit                               0x000000010b702760 -[UITableViewCellContentView updateConstraintsIfNeeded] + 185
8   UIKit                               0x000000010b9d5ab3 -[UIView(AdditionalLayoutSupport) _updateConstraintsAtEngineLevelIfNeeded] + 272
9   UIKit                               0x000000010b1e6274 -[UIView(Hierarchy) _updateConstraintsAsNecessaryAndApplyLayoutFromEngine] + 159
10  UIKit                               0x000000010b1f5d84 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 710
11  QuartzCore                          0x000000010ae1059a -[CALayer layoutSublayers] + 146
12  QuartzCore                          0x000000010ae04e70 _ZN2CA5Layer16layout_if_neededEPNS_11TransactionE + 366
13  QuartzCore                          0x000000010ae04cee _ZN2CA5Layer28layout_and_display_if_neededEPNS_11TransactionE + 24
14  QuartzCore                          0x000000010adf9475 _ZN2CA7Context18commit_transactionEPNS_11TransactionE + 277
15  QuartzCore                          0x000000010ae26c0a _ZN2CA11Transaction6commitEv + 486
16  QuartzCore                          0x000000010ae2737c _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv + 92
17  CoreFoundation                      0x000000010a675967 __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 23
18  CoreFoundation                      0x000000010a6758d7 __CFRunLoopDoObservers + 391
19  CoreFoundation                      0x000000010a66ae4c CFRunLoopRunSpecific + 524
20  CoreFoundation                      0x000000010a71e011 CFRunLoopRun + 97
21  SDWebImage                          0x000000010971773c -[SDWebImageDownloaderOperation start] + 1868
22  Foundation                          0x0000000109961e47 __NSOQSchedule_f + 194
23  libdispatch.dylib                   0x000000010d93849b _dispatch_client_callout + 8
24  libdispatch.dylib                   0x000000010d91e8ec _dispatch_queue_drain + 2215
25  libdispatch.dylib                   0x000000010d91de0d _dispatch_queue_invoke + 601
26  libdispatch.dylib                   0x000000010d920a56 _dispatch_root_queue_drain + 1420
27  libdispatch.dylib                   0x000000010d9204c5 _dispatch_worker_thread3 + 111
28  libsystem_pthread.dylib             0x000000010dc80a9d _pthread_wqthread + 729
29  libsystem_pthread.dylib             0x000000010dc7e3dd start_wqthread + 13
)

Here is the drawRect Method :

override func drawRect(rect: CGRect) {

    super.drawRect(rect)

    let cellRect = rect

    // Values
    let buttonBoxX = CELL_MARGIN + CELL_MARGIN/2
    let buttonBoxY = cellRect.height - buttonBoxHeight
    let buttonBoxWidth = rect.width - CELL_MARGIN * 3


    // Set Button Box
    let buttonBoxRect = CGRectMake(buttonBoxX, buttonBoxY, buttonBoxWidth, buttonBoxHeight )
    let buttonBox = UIBezierPath(roundedRect: buttonBoxRect, byRoundingCorners: [.BottomRight, .BottomLeft], cornerRadii: CGSize(width: CORNER_RADIUS, height: CORNER_RADIUS)) // Create the path
    UIColor.whiteColor().setFill() // Set the Fill to be white

    buttonBox.fill()


}

My understanding (cf. this question) is that complicated calculations etc should be done on a background thread and the the update of the UI on the main thread because UI is not thread safe.

However, IF i use the following :

override func drawRect(rect: CGRect) {

    super.drawRect(rect)

    let cellRect = rect

    // Values
    let buttonBoxX = CELL_MARGIN + CELL_MARGIN/2
    let buttonBoxY = cellRect.height - buttonBoxHeight
    let buttonBoxWidth = rect.width - CELL_MARGIN * 3


    // Set Button Box
    let buttonBoxRect = CGRectMake(buttonBoxX, buttonBoxY, buttonBoxWidth, buttonBoxHeight )
    let buttonBox = UIBezierPath(roundedRect: buttonBoxRect, byRoundingCorners: [.BottomRight, .BottomLeft], cornerRadii: CGSize(width: CORNER_RADIUS, height: CORNER_RADIUS)) // Create the path
    UIColor.whiteColor().setFill() // Set the Fill to be white

    dispatch_async(dispatch_get_main_queue(), {
        buttonBox.fill()
    })


}

I now get a CGContext error…

<Error>: CGContextSaveGState: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
<Error>: CGContextSetFlatness: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
<Error>: CGContextAddPath: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
<Error>: CGContextDrawPath: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
<Error>: CGContextRestoreGState: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
<Error>: CGContextSaveGState: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
<Error>: CGContextSetFlatness: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
<Error>: CGContextAddPath: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
<Error>: CGContextDrawPath: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
<Error>: CGContextRestoreGState: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.

Any suggestions ?

SOLUTION : OK. here is the deal. I was loading an image into a UIImageView asynchronously, but not "painting" it in the UI on the main thread but on a background thread. Furthermore; I was calling setNeedsDisplay on the UITableViewCell after adding the image to the UI, thus calling the drawRect method again… but this time on the background thread.

Answer 1

The first error you note doesn't look like it's related to your drawRect: code. Your drawRect: looks fine. It isn't doing anything particularly complicated. If your path were more complex, you'd probably want to cache it, but it's probably fine as-is. More likely you're trying to modify the UI somewhere else on a background thread. It may only spring up when you override drawRect: because that changes how the UI update happens (without a custom drawRect:, the system may be able to apply simple transforms). You need to look for where you're making a UIKit call on a background thread.

When in doubt, if it starts with UI, you probably can't use it on a background thread. That's not completely true (you can create a UIBezierPath on a background thread, and you can even draw it into a non-screen context), but as a first approximation, it's a good thing to look for.

This code is simply incorrect:

dispatch_async(dispatch_get_main_queue(), {
    buttonBox.fill()
})

The call to drawRect: had better already be on the main queue (so dispatching to the main queue is not helpful). If it's not, see above. By the time this block executes, the draw cycle is over, so there's no context to draw into any more.

Answer 2

Basically 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 states that you shouldn't update UI, especially autolayout related part in non-UI threads. To fix use dispatch_async(dispatch_get_main_queue()) { // your code }, or

Swift 3:

---

DispatchQueue.main.async {
    buttonBox.fill()
}