AutoLayout: removeFromSuperview / removeConstraints throws exception and crashes hard

Question

We use auto layout constraints selectively, primarily to position labels in relation to editable field elements (UITextView, UITextField, typically). However, since impleme

Answer 1

I just stumbled across the same error under OSX Mavericks with an OSX app I'm developing, but unlike the other answers given, I definitely don't have any other threads interacting with the UI objects, and the view hierarchy in question is definitely visible too. I'm not using blocks either. Bizarrely the problem went away when I removed a vertical constraint on an NSTextField.

FWIW the problematic view whose removal from its superview causes the "internal error. Cannot find an outgoing row head for incoming head" error is one of many side-panel controls which together present the properties of objects in the main view which can be cut, copied, created etc. This means that the user can be pasting new objects into the main view quite rapidly, meaning the side-panel's controls are being destroyed and new ones created very rapidly, too. Of course, with everything in the main thread, this should not make a difference, but it seems to.

The exact constraint causing problems was

[self addConstraint: [NSLayoutConstraint constraintWithItem:control attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:other attribute:NSLayoutAttributeHeight multiplier:1.4 constant:0.0]];

where control is the (editable) NSTextField causing problems, and 'other' is another (non-editable) NSTextField label.