Swift: Crash on removeFromSuperview()

Question

Problem: App crashes when I attempt to remove subview.

Context: I have a UICollectionViewCell that gets updated with a model object. I've tried putting these methods in prepareForReuse and/or the update(model: viewModel) method.

Notes: The line That view's superview: NO SUPERVIEW sticks out. I went as far as adding the subview back into before calling removeFromSuperView() to ensure there was some in a superView and it would still crash.

I later re-add newly generated instances of the class timelineView once all the old ones are cleared out.

It only crashes after scrolling through about ~50-150 cells

Update: Thank you for your feedback in the comments. I'm not removing all the views from the cell. Just one stackview with a variable number of components. But your right about removing subviews vs nil'ing out. I'm more curious how things like stackView.subviews.first?.removeFromSuperview() can fail, since it seems like a relatively safe call (or I'm dead wrong about this).

First Try

for subview in stackView.subviews {
    subview.removeFromSuperview() // Crash
}

Second Try

let subviews = stackView.subviews
for subview in subviews {
    if stackView != nil && subview.superview != nil && subview.responds(to: #selector(removeFromSuperview)) && stackView.superview != nil {
            stackView.subviews.first?.removeFromSuperview() // Crash
    }
}

Third Try

let subviews = stackView.subviews
for subview in subviews {
    if stackView != nil && subview.superview != nil && subview.responds(to:#selector(removeFromSuperview)) && stackView.superview != nil {
        if var firstSubview = stackView.subviews.first {
            for constraint in firstSubview.constraints {
                constraint.isActive = false
            }
            stackView.removeArrangedSubview(firstSubview)
            firstSubview.removeFromSuperview()  // Crash
        }
    }
}

Exception:

    2016-12-09 00:56:26.792168 appName[59081:4139912] [LayoutConstraints] View hierarchy unprepared for constraint.
    Constraint: <NSLayoutConstraint:0x618000490cc0 'UISV-canvas-connection' UIStackView:0x7fe25cd25dd0.bottom == appName.timeLineView:0x7fe25e007890.bottom   (active)>
    Container hierarchy: 
    <UIStackView: 0x7fe25cd25dd0; frame = (0 0; 718 40); opaque = NO; autoresize = RM+BM; layer = <CATransformLayer: 0x6080002295a0>>
    View not found in container hierarchy: <appName.timeLineView: 0x7fe25e007890; frame = (0 0; 718 40); autoresize = W+H; layer = <CALayer: 0x610000835c40>>
    That view's superview: NO SUPERVIEW
2016-12-09 00:58:38.655 appName[59081:4139912] *** Terminating app due to uncaught exception 'NSGenericException', reason: 'Unable to install constraint on view.  Does the constraint reference something from outside the subtree of the view?  That's illegal. constraint:<NSLayoutConstraint:0x618000490cc0 'UISV-canvas-connection' UIStackView:0x7fe25cd25dd0.bottom == appName.timeLineView:0x7fe25e007890.bottom   (active)> view:<UIStackView: 0x7fe25cd25dd0; frame = (0 0; 718 40); opaque = NO; autoresize = RM+BM; layer = <CATransformLayer: 0x6080002295a0>>'
*** First throw call stack:
(
    0   CoreFoundation                      0x000000010df0434b __exceptionPreprocess + 171
    1   libobjc.A.dylib                     0x000000011215121e objc_exception_throw + 48
    2   CoreFoundation                      0x000000010df6d265 +[NSException raise:format:] + 197
    3   Foundation                          0x000000010f097865 -[NSLayoutConstraint _addToEngine:integralizationAdjustment:mutuallyExclusiveConstraints:] + 200
    4   Foundation                          0x000000010f27c6ed __36-[NSISEngine rebuildFromConstraints]_block_invoke + 208
    5   Foundation                          0x000000010f09dbd0 -[NSISEngine withBehaviors:performModifications:] + 155
    6   Foundation                          0x000000010f27c610 -[NSISEngine rebuildFromConstraints] + 488
    7   Foundation                          0x000000010f0a3e1a -[NSISEngine optimize] + 121
    8   Foundation                          0x000000010f27c883 -[NSISEngine _optimizeIfNotDisabled] + 57
    9   Foundation                          0x000000010f0a4ec3 -[NSISEngine removeConstraintWithMarker:] + 799
    10  Foundation                          0x000000010f09bca8 -[NSLayoutConstraint _removeFromEngine:] + 229
    11  UIKit                               0x00000001112314a0 -[UIView(UIConstraintBasedLayout) _layoutEngine_willRemoveLayoutConstraint:] + 105
    12  UIKit                               0x0000000111231bb1 -[UIView(UIConstraintBasedLayout) nsli_removeConstraint:] + 93
    13  UIKit                               0x0000000111243cef _UIViewRemoveConstraintsMadeDanglyByChangingSuperview + 1016
    14  UIKit                               0x000000011091965a __45-[UIView(Hierarchy) _postMovedFromSuperview:]_block_invoke + 61
    15  UIKit                               0x00000001109195a2 -[UIView(Hierarchy) _postMovedFromSuperview:] + 857
    16  UIKit                               0x000000011091721c __UIViewWasRemovedFromSuperview + 172
    17  UIKit                               0x0000000110916d13 -[UIView(Hierarchy) removeFromSuperview] + 564
    18  appName                          0x0000000109aad884 _TFC10appName22TestCollectionViewCell15prepareForReusefT_T_ + 3380
    19  appName                          0x0000000109aada42 _TToFC10appName22TestCollectionViewCell15prepareForReusefT_T_ + 34
    20  UIKit                               0x00000001111bee14 -[UICollectionView _dequeueReusableViewOfKind:withIdentifier:forIndexPath:viewCategory:] + 561
    21  UIKit                               0x00000001111bf834 -[UICollectionView dequeueReusableCellWithReuseIdentifier:forIndexPath:] + 169
    22  appName                          0x0000000109a83623 _TFC10appName30FlightsViewController14collectionViewfTCSo16UICollectionView13cellForItemAtV10Foundation9IndexPath_CSo20UICollectionViewCell + 339
    23  appName                          0x0000000109a83ca7 _TToFC10appName30FlightsViewController14collectionViewfTCSo16UICollectionView13cellForItemAtV10Foundation9IndexPath_CSo20UICollectionViewCell + 87
    24  UIKit                               0x00000001111aa980 -[UICollectionView _createPreparedCellForItemAtIndexPath:withLayoutAttributes:applyAttributes:isFocused:notify:] + 467
    25  UIKit                               0x00000001111aa7a7 -[UICollectionView _createPreparedCellForItemAtIndexPath:withLayoutAttributes:applyAttributes:] + 35
    26  UIKit                               0x00000001111afc7f -[UICollectionView _updateVisibleCellsNow:] + 4803
    27  UIKit                               0x00000001111b5913 -[UICollectionView layoutSubviews] + 313
    28  UIKit                               0x000000011092cf50 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 1237
    29  QuartzCore                          0x00000001102c9cc4 -[CALayer layoutSublayers] + 146
    30  QuartzCore                          0x00000001102bd788 _ZN2CA5Layer16layout_if_neededEPNS_11TransactionE + 366
    31  QuartzCore                          0x00000001102bd606 _ZN2CA5Layer28layout_and_display_if_neededEPNS_11TransactionE + 24
    32  QuartzCore                          0x000000011024b680 _ZN2CA7Context18commit_transactionEPNS_11TransactionE + 280
    33  QuartzCore                          0x0000000110278767 _ZN2CA11Transaction6commitEv + 475
    34  QuartzCore                          0x00000001101d3b97 _ZN2CA7Display11DisplayLink14dispatch_itemsEyyy + 611
    35  CoreFoundation                      0x000000010de96964 __CFRUNLOOP_IS_CALLING_OUT_TO_A_TIMER_CALLBACK_FUNCTION__ + 20
    36  CoreFoundation                      0x000000010de965f3 __CFRunLoopDoTimer + 1075
    37  CoreFoundation                      0x000000010de9617a __CFRunLoopDoTimers + 250
    38  CoreFoundation                      0x000000010de8df01 __CFRunLoopRun + 2065
    39  CoreFoundation                      0x000000010de8d494 CFRunLoopRunSpecific + 420
    40  GraphicsServices                    0x00000001144e9a6f GSEventRunModal + 161
    41  UIKit                               0x0000000110868964 UIApplicationMain + 159
    42  appName                          0x0000000109a2ba9f main + 111
    43  libdyld.dylib                       0x00000001133b468d start + 1
)

Thank you

Answer 1

Had a similar issue, and I think I've solved it just now by being very careful when removing items from the stackview - here's an extension that sums it up:

extension UIStackView {

    func safelyRemoveArrangedSubviews() {

        // Remove all the arranged subviews and save them to an array
        let removedSubviews = arrangedSubviews.reduce([]) { (sum, next) -> [UIView] in
            self.removeArrangedSubview(next)
            return sum + [next]
        }

        // Deactive all constraints at once
        NSLayoutConstraint.deactivate(removedSubviews.flatMap({ $0.constraints }))

        // Remove the views from self
        removedSubviews.forEach({ $0.removeFromSuperview() })
    }
}

Answer 2

The problem is linked to your view LayoutConstraints.

It seems one constraint named UISV-canvas-connection is based on the view you are removing:

2016-12-09 00:56:26.792168 appName[59081:4139912] [LayoutConstraints] View hierarchy unprepared for constraint.
Constraint: <NSLayoutConstraint:0x618000490cc0 'UISV-canvas-connection' UIStackView:0x7fe25cd25dd0.bottom == appName.timeLineView:0x7fe25e007890.bottom   (active)>
Container hierarchy: 
<UIStackView: 0x7fe25cd25dd0; frame = (0 0; 718 40); opaque = NO; autoresize = RM+BM; layer = <CATransformLayer: 0x6080002295a0>>
View not found in container hierarchy: <appName.timeLineView: 0x7fe25e007890; frame = (0 0; 718 40); autoresize = W+H; layer = <CALayer: 0x610000835c40>>
That view's superview: NO SUPERVIEW

This constraint is base on a UIStackView that is no more in the view hierarchy (thus the message: That view's superview: NO SUPERVIEW).

So the constraint can't be evaluated based on this view after it was removed from superview.