Greg Combs Greg Combs - 4 months ago 5
iOS Question

AutoLayout: removeFromSuperview / removeConstraints throws exception and crashes hard

We use auto layout constraints selectively, primarily to position labels in relation to editable field elements (UITextView, UITextField, typically). However, since implementing auto layout for these fields, we're seeing a nasty exception and crash whenever we're unloading views, deallocating, etc. The exceptions are happening as it's attempting to remove the constraints from a view before unloading it.

Our view/controller hierarchy is as such:

UITableViewController (plain style, but with cell appearance to mimic grouped style)
--> UITableViewCell
----> UIViewController (container for editable form)
------> UICollectionViewController (editable form)
--------> UICollectionViewCell
-----------> UIViewController (editable field)
--------------> UILabel (field label) **HAS CONSTRAINTS**
--------------> UITextView / UITextField (field value) **HAS CONSTRAINTS**


Many times when the upper level table cells are being deallocated/replaced/reloaded, we see a huge exception and then crash as it's trying to deallocate/unload the view hierarchy within.

I've attempted to mitigate the crash by catching the exception (no help) and also by forcefully removing all of the constraints on the affected view and all of the subviews prior to deallocation/unload (in
viewWillDisappear:
) and it doesn't seem to help. I've even tried to remove these constraints one by one to see if there's one in particular that's causing the trouble but all of them are blowing up when we call
removeConstraint:
or
removeConstraints:
on a container in preparation for disappearing.

I'm baffled! Here's a snippet of our exception -- roughly about 3000 lines have been chopped out of it, so if you need more, just ask.

Exception while deallocating view: { Rows:
0x18911270.posErrorMarker == 4 + 1*0x18911270.negError + 1*0x189112f0.marker + -1*0x189113f0.negError + 1*0x189113f0.posErrorMarker + 1*0x18911a60.marker + -0.5*0x1892dae0.negError + 0.5*0x1892dae0.posErrorMarker + 1*0x18951520.negError + -1*0x18951520.posErrorMarker + -0.5*0x18958090.negError + 0.5*0x18958090.posErrorMarker
0x189112b0.negError == 12 + 1*0x189112b0.posErrorMarker + -1*0x189112f0.marker + 1*0x189113f0.negError + -1*0x189113f0.posErrorMarker + -1*0x18911a60.marker + 1*0x18925530.marker + 0.5*0x1892dae0.negError + -0.5*0x1892dae0.posErrorMarker + 1*0x1893e080.marker + 0.5*0x18958090.negError + -0.5*0x18958090.posErrorMarker + 1*0x18963640.marker
0x18911370.negError == 9 + -1*0x189112f0.marker + 1*0x18911370.posErrorMarker + 1*0x18925530.marker + 1*0x1892dae0.negError + -1*0x1892dae0.posErrorMarker + 1*0x1893e080.marker + 1*0x18963640.marker
0x189113b0.slackMarker == 2 + -1*0x189107d0.marker + 1*0x18910b90.negError + -1*0x18910b90.posErrorMarker +

........ EXPLETIVES DELETED .........

UITableView:0xca2b000.contentHeight == 36 + 1*0xc221c00.marker
UITableView:0xca2b000.contentWidth == 704 + 1*0xc239470.marker
UITableView:0xca2b000.minX == 0 + 1*0xc2a23f0.marker + -0.5*0xc2a2590.marker
UITableView:0xca2b000.minY == 0 + 1*0xc2a25d0.marker + -0.5*0xc2a2630.marker
UITableViewCellContentView:0x18ab13d0.Height == 174 + 1*0x18abd4f0.marker
UITableViewCellContentView:0x18ab13d0.Width == 704 + 1*0x18abd470.marker

........ EXPLETIVES DELETED .........

<NSAutoresizingMaskLayoutConstraint:0x18988bc0 h=-&- v=-&- UIView:0x18911e50.midY == UIView:0x1892d0c0.midY> Marker:0x18988bc0.marker
<NSAutoresizingMaskLayoutConstraint:0x18994b40 h=-&- v=-&- UIView:0xc4a6fb0.midX == UIView:0xc4b4990.midX> Marker:0x18994b40.marker
<NSAutoresizingMaskLayoutConstraint:0x18998480 h=-&- v=-&- UIView:0x18915180.width == UIView:0xc4c5970.width> Marker:0x18998480.marker
<NSAutoresizingMaskLayoutConstraint:0x18aae320 h=--& v=--& TapSectionalTableViewCell:0x18a3d270.midX == + 352> Marker:0x18aae320.marker
<NSAutoresizingMaskLayoutConstraint:0x18aae410 h=--& v=--& H:[TapSectionalTableViewCell:0x18a3d270(704)]> Marker:0x18aae410.marker
<NSAutoresizingMaskLayoutConstraint:0x18aae450 h=--& v=--& TapSectionalTableViewCell:0x18a3d270.midY == + 144> Marker:0x18aae450.marker

........ EXPLETIVES DELETED .........

<NSAutoresizingMaskLayoutConstraint:0xc2de2f0 h=--& v=--& TapGenericCollectionCell:0xc2ac500.midX == + 499> Marker:0xc2de2f0.marker
<NSAutoresizingMaskLayoutConstraint:0xc2de3b0 h=--& v=--& V:[TapGenericCollectionCell:0xc2ac500(34)]> Marker:0xc2de3b0.marker
<NSAutoresizingMaskLayoutConstraint:0xc2de430 h=-&- v=-&- UIView:0x18953f80.height == UIView:0xc2acb20.height> Marker:0xc2de430.marker
<NSAutoresizingMaskLayoutConstraint:0xc2de520 h=-&- v=-&- UIView:0x18923af0.height == UIView:0xc2ae570.height> Marker:0xc2de520.marker
<NSAutoresizingMaskLayoutConstraint:0xc2de560 h=--& v=--& H:[TapGenericCollectionCell:0xc2ac500(280)]> Marker:0xc2de560.marker

........ EXPLETIVES DELETED .........

<NSContentSizeLayoutConstraint:0xc2f5730 H:[_UIBaselineLayoutStrut:0x18994a30(0)] Hug:250 CompressionResistance:750> Marker:0xc2f5730.posErrorMarker
<NSContentSizeLayoutConstraint:0xc2f5730 H:[_UIBaselineLayoutStrut:0x18994a30(0)] Hug:250 CompressionResistance:750> Marker:0xc2f5730.posErrorMarker
<NSContentSizeLayoutConstraint:0xc2f5770 V:[_UIBaselineLayoutStrut:0x18994a30(18)] Hug:250 CompressionResistance:750> Marker:0xc2f5770.posErrorMarker

internal error. Cannot find an outgoing row head for incoming head UIView:0x189712b0.Width, which should never happen.'


/**** BEGIN Individual Field Controller - This code is from the base individual field controller used in our editable form collection *****/

- (void)viewDidLoad {
[super viewDidLoad];
self.view.clipsToBounds = YES;
self.view.opaque = YES;

CGRect viewFrame = self.view.frame;
viewFrame.size = [self defaultFieldSize];
self.view.frame = viewFrame;

if (self.backgroundColor) {
self.view.backgroundColor = self.backgroundColor;
}
else {
self.view.backgroundColor = [UIColor whiteColor];
}
[self createLabelAndField];

[self setLabelAndFieldContraints];

[self.view addConstraints:self.labelValueConstraints];
[self.view setNeedsUpdateConstraints];
}

- (void)createLabelAndField {
[self removeLabelAndField];

UILabel *label = [[UILabel alloc] init];
label.font = self.labelFont;
label.textColor = self.labelColor;
label.lineBreakMode = NSLineBreakByWordWrapping;
label.textAlignment = NSTextAlignmentLeft;
label.adjustsFontSizeToFitWidth = NO;
label.numberOfLines = 0;

if (self.backgroundColor) {
label.backgroundColor = self.backgroundColor;
}
else {
label.backgroundColor = [UIColor whiteColor];
}

[self.view addSubview:label];

self.label = label;


/// EXAMPLE valueView initialization from a subclass that handles long text

TapEditableTextView *textView = [[TapEditableTextView alloc] init];
if (self.hasLabelOverValue) {
textView.shouldMimicTextField = NO;
}
else {
textView.shouldMimicTextField = YES;
}
textView.delegate = self;
textView.keyboardType = UIKeyboardTypeDefault;
textView.font = self.valueFont;
textView.textColor = self.valueColor;
textView.textAlignment = NSTextAlignmentLeft;
textView.normalBackgroundColor = self.backgroundColor;
textView.editable = NO;
textView.textLines = self.textLines;

self.valueTextView = textView;
self.valueView = textView;
[self.view addSubview:textView];
}

- (void)removeLabelAndField {
[self clearConstraints];

if (self.label) {
[self.label removeFromSuperview];
self.label = nil;
}
if (self.valueView) {
[self.valueView removeFromSuperview];
self.valueView = nil;
}
}

- (void)clearConstraints {
if (self.isViewLoaded && self.labelValueConstraints) {
[self.view removeConstraints:self.labelValueConstraints];
}
self.labelValueConstraints = nil;
self.labelToValueHorizConstraint = nil;
self.valueWidthConstraint = nil;
}

// This is called in our field's viewDidLoad, after we've created our label and valueView (UITextField, UITextView, etc)
- (void)setLabelAndFieldContraints {
[self clearConstraints];

self.labelValueConstraints = [NSMutableArray array];

self.label.translatesAutoresizingMaskIntoConstraints = NO;
self.valueView.translatesAutoresizingMaskIntoConstraints = NO;

NSLayoutConstraint *constraint = nil;

constraint = [NSLayoutConstraint
constraintWithItem:self.label attribute:NSLayoutAttributeLeft
relatedBy:NSLayoutRelationEqual
toItem:self.view attribute:NSLayoutAttributeLeft
multiplier:1.0f constant:self.labelValueGap];
constraint.priority = UILayoutPriorityRequired;
[self.labelValueConstraints addObject:constraint];


constraint = [NSLayoutConstraint
constraintWithItem:self.label attribute:NSLayoutAttributeTop
relatedBy:NSLayoutRelationEqual
toItem:self.view attribute:NSLayoutAttributeTop
multiplier:1.0f constant:0];
constraint.priority = 550;
[self.labelValueConstraints addObject:constraint];


constraint = [NSLayoutConstraint
constraintWithItem:self.label attribute:NSLayoutAttributeBottom
relatedBy:NSLayoutRelationEqual
toItem:self.view attribute:NSLayoutAttributeBottom
multiplier:1.0f constant:0];
constraint.priority = 400;
[self.labelValueConstraints addObject:constraint];


constraint = [NSLayoutConstraint
constraintWithItem:self.valueView attribute:NSLayoutAttributeTop
relatedBy:NSLayoutRelationEqual
toItem:self.view attribute:NSLayoutAttributeTop
multiplier:1.0f constant:0];
constraint.priority = UILayoutPriorityRequired;
[self.labelValueConstraints addObject:constraint];


constraint = [NSLayoutConstraint
constraintWithItem:self.valueView attribute:NSLayoutAttributeBottom
relatedBy:NSLayoutRelationEqual
toItem:self.view attribute:NSLayoutAttributeBottom
multiplier:1.0f constant:0];
constraint.priority = 499;
[self.labelValueConstraints addObject:constraint];


constraint = [NSLayoutConstraint
constraintWithItem:self.valueView attribute:NSLayoutAttributeRight
relatedBy:NSLayoutRelationEqual
toItem:self.view attribute:NSLayoutAttributeRight
multiplier:1.0f constant: -(kDisclosureWidth + self.labelValueGap) ];
constraint.priority = 901;
[self.labelValueConstraints addObject:constraint];


constraint = [NSLayoutConstraint
constraintWithItem:self.valueView attribute:NSLayoutAttributeLeading
relatedBy:NSLayoutRelationGreaterThanOrEqual
toItem:self.label attribute:NSLayoutAttributeTrailing
multiplier:1.0f constant:self.labelValueGap];
constraint.priority = UILayoutPriorityDefaultHigh + 1;
[self.labelValueConstraints addObject:constraint];
self.labelToValueHorizConstraint = constraint;


constraint = [NSLayoutConstraint
constraintWithItem:self.label attribute:NSLayoutAttributeBaseline
relatedBy:NSLayoutRelationEqual
toItem:self.valueView attribute:NSLayoutAttributeBaseline
multiplier:1.0f constant:0.f];
constraint.priority = 600;
[self.labelValueConstraints addObject:constraint];


constraint = [NSLayoutConstraint
constraintWithItem:self.valueView attribute:NSLayoutAttributeWidth
relatedBy:NSLayoutRelationEqual
toItem:self.view attribute:NSLayoutAttributeWidth
multiplier:(1.f - self.labelWidthPercentage) constant:0];
constraint.priority = 305;
[self.labelValueConstraints addObject:constraint];
self.valueWidthConstraint = constraint;


[self setCompressionAndHuggingForLabelView:self.label];
[self setCompressionAndHuggingForValueView:self.valueView];
}

- (void)setCompressionAndHuggingForLabelView:(UILabel *)labelView {
if (!labelView) {
return;
}
[labelView setContentCompressionResistancePriority:510 forAxis:UILayoutConstraintAxisHorizontal];
[labelView setContentCompressionResistancePriority:UILayoutPriorityDefaultHigh forAxis:UILayoutConstraintAxisVertical];
[labelView setContentHuggingPriority:450 forAxis:UILayoutConstraintAxisHorizontal];
[labelView setContentHuggingPriority:UILayoutPriorityDefaultHigh forAxis:UILayoutConstraintAxisVertical];
}

- (void)setCompressionAndHuggingForValueView:(UIView *)valueView {
if (!valueView) {
return;
}
[valueView setContentCompressionResistancePriority:509 forAxis:UILayoutConstraintAxisHorizontal];
[valueView setContentCompressionResistancePriority:UILayoutPriorityDefaultHigh forAxis:UILayoutConstraintAxisVertical];
[valueView setContentHuggingPriority:300 forAxis:UILayoutConstraintAxisHorizontal];
[valueView setContentHuggingPriority:650 forAxis:UILayoutConstraintAxisVertical];
}

/****** END Individual Field Controller ******/

Answer

I had an (extensive) conversation with an Apple engineer about this crash.

Here are the two most probable causes:

  1. You have an invalid constraint, such as view1.left = view2.left + 20 where view2 is unexpectedly nil, or has a multiplier of 0. Be sure to double (and triple) check your constraints to make sure that they are correct. Here are 2 examples of problematic constraints:

    // The first constraint would be a problem if view2 were nil
    [NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:view2 attribute:NSLayoutAttributeBottom multiplier:1 constant:20];
    // The second constraint is a problem because the 0 multiplier causes view2 to be "lost"
    [NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:view2 attribute:NSLayoutAttributeBottom multiplier:0 constant:5];
    
  2. You're hitting a bug in the internal Foundation auto layout engine related to accumulated loss of floating point precision. When you've crashed, the way you can know that this is the case is to search through the (large) exception log in the console for a very small (nearly zero) floating point number such as this:

<505:-7.45058e-08>*PWPlotLegendEntryView:0x600000582be0.Height{id: 34609} +

(Search for e- in the console output to find small numbers like this.) This number (-7.45058e-08 in this case) represents the coefficient at this particular point in time while the internal engine is solving constraints. In this case, the number is supposed to be exactly 0, but due to the way the auto layout engine does calculations with floating point numbers it has become an extremely tiny negative number instead which blows everything up. If you can find such a number in the output, you know that you've hit this bug.

How can you work around this issue?

Changing the order that you add (activate) constraints can end up changing the order of calculations in the internal engine, which as a result can cause this issue to disappear as the math is done without any problematic loss of precision.

This issue seems to come up more frequently when you have changed the content compression resistance or content hugging priorities for views, so try commenting out any code that does that to see if it's causing this bug to happen, or re-ordering it to happen earlier or later in your constraint setup code.

More details about my specific case:

I ran into this crash on iOS. The steps to reproduce it were quite interesting:

  1. A view controller containing a table view was pushed on screen (in a navigation controller).
  2. The table view had to contain enough cells so they didn't all fit in the visible area, then it had to be scrolled to the last cell and then back up a bit (presumably, this was causing cells to be reused, which was triggering this issue).
  3. Then, when the view controller containing the table view was popped off the navigation stack, immediately after the pop animation completed the app would crash at the point where the view controller's view was removed from the view hierarchy.

After a lot of trial and error I was able to isolate the issue to one specific thing: setting the content compression resistance & hugging priorities for a UIImageView in each of the table view cells. In this case, the image view is being positioned using Auto Layout inside the cell, and to achieve the correct layout the image view needs to be exactly its intrinsic content size (the size of its image).

This was the problematic code:

// Inside of the UITableViewCell's updateConstraints method...

[self.imageView setContentCompressionResistancePriority:​UILayoutPriorityRequired forAxis:​UILayoutConstraintAxisHorizontal];        
[self.imageView setContentCompressionResistancePriority:​UILayoutPriorityRequired forAxis:UILayoutConstraintAxisVertical];       
[self.imageView setContentHuggingPriority:​UILayoutPriorityRequired forAxis:​UILayoutConstraintAxisHorizontal];      
[self.imageView setContentHuggingPriority:​UILayoutPriorityRequired forAxis:UILayoutConstraintAxisVertical];

Removing the above code and replacing it with 2 constraints (at Required priority) to fix the width & height of the image view to the image's size achieved the same result, but avoided the crash. Here's the replacement code (using PureLayout):

[self.imageView autoSetDimensionsToSize:self.imageView.image.size];

I also found that just moving the problematic 4 lines to a different place in my constraint setup code resolved the issue, presumably because this changed the order of calculations sufficiently to prevent the problematic loss of precision.