fix empty indexPath crash shouldInvalidateLayout

[SNagappan]

Details

shouldInvalidateLayout crashes if IndexPath is empty

Related Issue

#77

Types of changes

• Docs change / refactoring / dependency upgrade
• [x ] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[bryankeller]

Hi @SNagappan - thanks for finding this bug and making a fix. Before I approve, I just want to understand a bit more about how this crash is possible. Do you have a repro case? I'm trying to figure out why shouldInvalidateLayout is getting called with an empty index path in the first place.

[bryankeller]

Ah, I see you emailed me some additional context. Thanks for that! So weird that this is happening. Looks like it might be related to voiceover / UIAccessibility, which would explain why it's so rare in your project.

Crashed: com.apple.main-thread
0  libswiftUIKit.dylib            0x1c285a0c8 IndexPath.section.getter + 168
1  MagazineLayout                 0x1037be1e8 itemModelHeightModeDuringPreferredAttributesCheck #1 (at:sectionModels:) in ModelState.itemModelHeightModeDuringPreferredAttributesCheck(at:) + 118 (ModelState.swift:118)
2  MagazineLayout                 0x1037afe38 MagazineLayout.shouldInvalidateLayout(forPreferredLayoutAttributes:withOriginalAttributes:) + 132 (ModelState.swift:132)
3  MagazineLayout                 0x1037b020c @objc MagazineLayout.shouldInvalidateLayout(forPreferredLayoutAttributes:withOriginalAttributes:) + 4379738636 (<compiler-generated>:4379738636)
4  UIKitCore                      0x1b1c352a8 -[UICollectionView _checkForPreferredAttributesInView:originalAttributes:] + 656
5  UIKitCore                      0x1b1c36838 -[UICollectionView _createPreparedSupplementaryViewForElementOfKind:atIndexPath:withLayoutAttributes:applyAttributes:] + 880
6  UIKitCore                      0x1b1c5c0f4 -[UICollectionView preferredFocusedView] + 2404
7  UIKit                          0x1f578b9fc -[UICollectionViewAccessibility preferredFocusedView] + 56
8  UIKitCore                      0x1b1f3bd28 __UIFocusEnvironmentPreferredFocusedView + 64
9  UIKitCore                      0x1b29929e0 -[UIView preferredFocusEnvironments] + 48
10 UIKit                          0x1f5815858 -[UIViewAccessibility preferredFocusEnvironments] + 68
11 UIKitCore                      0x1b1f3bdb0 _UIFocusEnvironmentEffectivePreferredFocusEnvironments + 72
12 UIKitCore                      0x1b1f4cce4 -[_UIFocusEnvironmentPreferenceEnumerationContext _resolvePreferredFocusEnvironments] + 224
13 UIKitCore                      0x1b1f4cbc4 -[_UIFocusEnvironmentPreferenceEnumerationContext prefersNothingFocused] + 40
14 UIKitCore                      0x1b1f4d8ec _enumeratePreferredFocusEnvironments + 184
15 UIKitCore                      0x1b1f4d74c -[_UIFocusEnvironmentPreferenceEnumerator enumeratePreferencesForEnvironment:usingBlock:] + 200
16 UIKitCore                      0x1b1f4dfe8 -[_UIDeepestPreferredEnvironmentSearch deepestPreferredFocusableItemForEnvironment:withRequest:] + 592
17 UIKitCore                      0x1b1f92484 -[UIFocusUpdateContext _updateDestinationItemIfNeeded] + 232
18 UIKitCore                      0x1b1f9227c -[UIFocusUpdateContext _destinationItemInfo] + 28
19 UIKitCore                      0x1b1f92158 -[UIFocusUpdateContext nextFocusedItem] + 24
20 UIKitCore                      0x1b1f81bf8 -[UIFocusSystem updateFocusIfNeeded] + 192
21 UIKitCore                      0x1b1c3d0cc __43-[UICollectionView _updateVisibleCellsNow:]_block_invoke.1183 + 184
22 UIKitCore                      0x1b29a4d0c +[UIView(Animation) performWithoutAnimation:] + 96
23 UIKitCore                      0x1b1c3b6f0 -[UICollectionView _updateVisibleCellsNow:] + 9220
24 UIKitCore                      0x1b1c3ed38 -[UICollectionView layoutSubviews] + 320
25 UIKitCore                      0x1b29b1954 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 2464
26 QuartzCore                     0x1b2eb19e8 -[CALayer layoutSublayers] + 288
27 QuartzCore                     0x1b2eb7eb4 CA::Layer::layout_if_needed(CA::Transaction*) + 520
28 QuartzCore                     0x1b2ec31d0 CA::Layer::layout_and_display_if_needed(CA::Transaction*) + 140
29 QuartzCore                     0x1b2e0f500 CA::Context::commit_transaction(CA::Transaction*, double, double*) + 416
30 QuartzCore                     0x1b2e39698 CA::Transaction::commit() + 728
31 QuartzCore                     0x1b2e3a948 CA::Transaction::observer_callback(__CFRunLoopObserver*, unsigned long, void*) + 92
32 CoreFoundation                 0x1afbebecc __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 32
33 CoreFoundation                 0x1afbe65b0 __CFRunLoopDoObservers + 604
34 CoreFoundation                 0x1afbe6af8 __CFRunLoopRun + 960
35 CoreFoundation                 0x1afbe6200 CFRunLoopRunSpecific + 572
36 GraphicsServices               0x1c5ce1598 GSEventRunModal + 160
37 UIKitCore                      0x1b24ac004 -[UIApplication _run] + 1052
38 UIKitCore                      0x1b24b15d8 UIApplicationMain + 164

[bryankeller]

LGTM, thanks for the details over email and for putting up a fix!