Autolayout: Removefromsuperview/Removeconstraints Throws Exception and Crashes Hard

AutoLayout: removeFromSuperview / removeConstraints throws exception and crashes hard

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.

NSGenericException', reason: 'Unable to install constraint on view

You need to install the constraint on the "higher" of the two views. A good, general way to do this is like this:

NSLayoutConstraint* constraint = ...;
NSView* firstView = constraint.firstItem;
NSView* secondView = constraint.secondItem;    
[[firstView ancestorSharedWithView: secondView] addConstraint: constraint];

Just a word of caution: It's good to remember here that constraint attributes are evaluated in the context of the view on which they are added. So for instance, the value of NSLayoutAttributeLeft of viewA, for a constraint installed on viewB, is interpreted in the coordinate space of viewB. For constraints that only reference sibling views or their superview, that fact is largely irrelevant, but there's no restriction that constraints can't reference two views that aren't siblings or direct parents.

Cannot find an outgoing row head for incoming head UIButton

Apparently the offending UIButton is in the ViewController we were going out of (!) and not the one being created in the transition.

Not sure why....

Anyway - the problematic constraint was a 4:3 ratio constraint...

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