UINavigationController方法setToolbarHidden在Xcode 9中的bug：自动布局约束的无限计算导致OOM
问题描述：
我有一个嵌套在UITabBarController中的UINavigationController实例。我使用导航控制器访问某个视图控制器（选项卡栏仍然可见），从此我继续到第二个视图控制器（选项卡栏不再可见）。UINavigationController方法setToolbarHidden在Xcode 9中的bug：自动布局约束的无限计算导致OOM
在第二个视图控制器，只要我打个电话给： [self.navigationController setToolbarHidden:NO] 应用程序冻结和内存增长，直到OOM异常崩溃了。
我承认，嵌套的标签栏里面的导航控制器，不建议，但这种设置似乎正常工作，直到iOS的11
编辑：停止执行的时候，我看到了很多电话给：
UIView(UIConstraintBasedLayout)
UIView(AdditionalLayerSupport)
NSLayoutConstraint
下面是完整的堆栈跟踪
* thread #1, queue = 'com.apple.main-thread', stop reason = signal SIGSTOP * frame #0: 0x0000000106dd895c libobjc.A.dylib`objc_msgSend
+ 28
frame #1: 0x00000001067b6b9b Foundation`-[NSConcreteMapTable removeObjectForKey:] + 138
frame #2: 0x00000001069e6019 Foundation`_substituteOutAllOccurencesOfBodyVar + 1282
frame #3: 0x00000001067f3c5b Foundation`-[NSISEngine tryAddingDirectly:] + 144
frame #4: 0x00000001067f332f Foundation`-[NSISEngine tryToAddConstraintWithMarker:expression:integralizationAdjustment:mutuallyExclusiveConstraints:]
+ 440
frame #5: 0x00000001069f2067 Foundation`-[NSLayoutConstraint _addLoweredExpression:toEngine:integralizationAdjustment:lastLoweredConstantWasRounded:mutuallyExclusiveConstraints:]
+ 273
frame #6: 0x00000001067ea601 Foundation`-[NSLayoutConstraint _addToEngine:integralizationAdjustment:mutuallyExclusiveConstraints:] + 240
frame #7: 0x0000000109c9488d UIKit`__57-[UIView(AdditionalLayoutSupport)
_switchToLayoutEngine:]_block_invoke_2 + 452
frame #8: 0x00000001067f0de1 Foundation`-[NSISEngine withBehaviors:performModifications:] + 131
frame #9: 0x0000000109c946a2 UIKit`__57-[UIView(AdditionalLayoutSupport)
_switchToLayoutEngine:]_block_invoke + 604
frame #10: 0x0000000109c9441e UIKit`-[UIView(AdditionalLayoutSupport) _switchToLayoutEngine:] + 223
frame #11: 0x00000001091ed84f UIKit`__45-[UIView(Hierarchy) _postMovedFromSuperview:]_block_invoke + 112
frame #12: 0x00000001067f0de1 Foundation`-[NSISEngine withBehaviors:performModifications:] + 131
frame #13: 0x00000001091ed778 UIKit`-[UIView(Hierarchy) _postMovedFromSuperview:] + 855
frame #14: 0x00000001091fe031 UIKit`-[UIView(Internal) _addSubview:positioned:relativeTo:] + 1927
frame #15: 0x0000000109b507e1 UIKit`-[_UILayoutArrangement insertItem:atIndex:] + 502
frame #16: 0x0000000109ca1b4d UIKit`__50-[_UIOrderedLayoutArrangement insertItem:atIndex:]_block_invoke + 50
frame #17: 0x0000000109ca18df UIKit`-[_UIOrderedLayoutArrangement _trackChangesAffectingExternalBaselineConstraints:] + 320
frame #18: 0x0000000109ca1aea UIKit`-[_UIOrderedLayoutArrangement insertItem:atIndex:] + 478
frame #19: 0x000000010982edea UIKit`-[UIStackView insertArrangedSubview:atIndex:] + 283
frame #20: 0x0000000109b29972 UIKit`-[_UIButtonBar _layoutBar] + 3639
frame #21: 0x0000000109b2bb44 UIKit`-[_UIButtonBarStackView updateConstraints] + 48
frame #22: 0x0000000109c958b6 UIKit`-[UIView(AdditionalLayoutSupport)
_sendUpdateConstraintsIfNecessaryForSecondPass:] + 161
frame #23: 0x0000000109c95ed2 UIKit`-[UIView(AdditionalLayoutSupport)
_updateConstraintsIfNeededCollectingViews:forSecondPass:] + 1296
frame #24: 0x0000000109c95d51 UIKit`-[UIView(AdditionalLayoutSupport)
_updateConstraintsIfNeededCollectingViews:forSecondPass:] + 911
frame #25: 0x0000000109c95d51 UIKit`-[UIView(AdditionalLayoutSupport)
_updateConstraintsIfNeededCollectingViews:forSecondPass:] + 911
frame #26: 0x0000000109c95d51 UIKit`-[UIView(AdditionalLayoutSupport)
_updateConstraintsIfNeededCollectingViews:forSecondPass:] + 911
frame #27: 0x00000001067f0de1 Foundation`-[NSISEngine withBehaviors:performModifications:] + 131
frame #28: 0x0000000109c96703 UIKit`__100-[UIView(AdditionalLayoutSupport)
_updateConstraintsIfNeededWithViewForVariableChangeNotifications:]_block_invoke
+ 90
frame #29: 0x0000000109c94f61 UIKit`-[UIView(AdditionalLayoutSupport)
_withUnsatisfiableConstraintsLoggingSuspendedIfEngineDelegateExists:] + 104
frame #30: 0x0000000109c96272 UIKit`-[UIView(AdditionalLayoutSupport)
_updateConstraintsIfNeededWithViewForVariableChangeNotifications:] + 160
frame #31: 0x0000000109c9738c UIKit`-[UIView(AdditionalLayoutSupport)
_updateConstraintsAtEngineLevelIfNeededWithViewForVariableChangeNotifications:]
+ 401
frame #32: 0x00000001091ef1b6 UIKit`-[UIView(Hierarchy) layoutBelowIfNeeded] + 1517
frame #33: 0x000000010957b35e UIKit`-[_UIButtonBarButton willMoveToWindow:] + 63
frame #34: 0x00000001091ec996 UIKit`-[UIView(Hierarchy) _willMoveToWindow:] + 861
frame #35: 0x00000001091eb493 UIKit`__UIViewWillBeRemovedFromSuperview + 484
frame #36: 0x00000001091eb0ea UIKit`-[UIView(Hierarchy) removeFromSuperview] + 95
frame #37: 0x0000000109b295d3 UIKit`-[_UIButtonBar _layoutBar] + 2712
frame #38: 0x0000000109b2bb44 UIKit`-[_UIButtonBarStackView updateConstraints] + 48
frame #39: 0x0000000109c958b6 UIKit`-[UIView(AdditionalLayoutSupport)
_sendUpdateConstraintsIfNecessaryForSecondPass:] + 161
frame #40: 0x0000000109c95ed2 UIKit`-[UIView(AdditionalLayoutSupport)
_updateConstraintsIfNeededCollectingViews:forSecondPass:] + 1296
frame #41: 0x0000000109c95d51 UIKit`-[UIView(AdditionalLayoutSupport)
_updateConstraintsIfNeededCollectingViews:forSecondPass:] + 911
frame #42: 0x0000000109c95d51 UIKit`-[UIView(AdditionalLayoutSupport)
_updateConstraintsIfNeededCollectingViews:forSecondPass:] + 911
frame #43: 0x0000000109c95d51 UIKit`-[UIView(AdditionalLayoutSupport)
_updateConstraintsIfNeededCollectingViews:forSecondPass:] + 911
frame #44: 0x00000001067f0de1 Foundation`-[NSISEngine withBehaviors:performModifications:] + 131
frame #45: 0x0000000109c96703 UIKit`__100-[UIView(AdditionalLayoutSupport)
_updateConstraintsIfNeededWithViewForVariableChangeNotifications:]_block_invoke
+ 90
frame #46: 0x0000000109c94f61 UIKit`-[UIView(AdditionalLayoutSupport)
_withUnsatisfiableConstraintsLoggingSuspendedIfEngineDelegateExists:] + 104
frame #47: 0x0000000109c96272 UIKit`-[UIView(AdditionalLayoutSupport)
_updateConstraintsIfNeededWithViewForVariableChangeNotifications:] + 160
frame #48: 0x0000000109c9738c UIKit`-[UIView(AdditionalLayoutSupport)
_updateConstraintsAtEngineLevelIfNeededWithViewForVariableChangeNotifications:]
+ 401
frame #49: 0x00000001091efa5b UIKit`-[UIView(Hierarchy) _updateConstraintsAsNecessaryAndApplyLayoutFromEngine] + 159
frame #50: 0x00000001095742d5 UIKit`-[UILayoutContainerView layoutSubviews] + 270
frame #51: 0x0000000109204551 UIKit`-[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 1331
frame #52: 0x00000001064db4ba QuartzCore`-[CALayer layoutSublayers] + 153
frame #53: 0x00000001064df5a9 QuartzCore`CA::Layer::layout_if_needed(CA::Transaction*) + 401
frame #54: 0x00000001064681cd QuartzCore`CA::Context::commit_transaction(CA::Transaction*) + 365
frame #55: 0x0000000106493ae4 QuartzCore`CA::Transaction::commit() + 500
frame #56: 0x0000000109160687 UIKit`_afterCACommitHandler + 272
frame #57: 0x00000001080f8db7 CoreFoundation`__CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__
+ 23
frame #58: 0x00000001080f8d0e CoreFoundation`__CFRunLoopDoObservers + 430
frame #59: 0x00000001080dd324 CoreFoundation`__CFRunLoopRun + 1572
frame #60: 0x00000001080dca89 CoreFoundation`CFRunLoopRunSpecific + 409
frame #61: 0x000000010dc429c6 GraphicsServices`GSEventRunModal + 62
frame #62: 0x0000000109135d30 UIKit`UIApplicationMain + 159
frame #63: 0x0000000101ff6bf9 MyAppName`main(argc=1, argv=0x00007fff5de3e0a8) at main.m:23
frame #64: 0x000000010f453d81 libdyld.dylib`start + 1
frame #65: 0x000000010f453d81 libdyld.dylib`start + 1
答
这是由同一个团队OP是从开发人员的答案。
一个更圆的研究后，我们发现，问题是站在我们这一边：
- (NSArray *)toolbarItems
{
return [self toolbarItemsWithRunningAdditionalAnimation:NO];
}
原开发商提出的方法toolbarItemsWithRunningAdditionalAnimation到每一个方法被调用时将返回新对象。当去这个控制器时，iOS会调用toolbarItems至少3次，所以每次我们让iOS感到困惑时都会给它一个新的对象，所以它会在无限循环中重新计算自动布局约束。我们下面也正在原来修复，当我们开始总是返回类的物品的同一阵列但它变得过时：
- (NSArray *)toolbarItems {
if (_cachedToolbarItems) { return _cachedToolbarItems; }
_cachedToolbarItems = [self toolbarItemsWithRunningAdditionalAnimation:NO];
return _cachedToolbarItems'
}
我们正在做我们的应用程序下面的（伪）：
UIToolbar *toolbar = self.navigationController.toolbar;
NSArray <UIBarButtonItem *> *items = @[
flexibleSpace,
share,
flexibleSpace,
play,
flexibleSpace,
stats,
flexibleSpace
];
[toolbar setItems:items animated:animated];
[self.navigationController setToolbarHidden:NO animated:animated];
造成自动布局约束无限计算的问题项为share和stats。它们都有共同之处 - 它们都是使用-[UIBarButtonItem initWithImage:style:target:action:]初始化器创建的。当我们开始使用另一个初始化程序时：
UIButton *customShareButton = ... // we create button ourselves.
UIBarButtonItem *shareItem = [[UIBarButtonItem alloc] initWithCustomView: customShareButton];
问题就消失了。
答
除了被接受的答案，我们发现我们的问题与这样的事实有关：在我们班，我们覆盖了方法 - (NSArray *)toolbarItems。
每次调用我们的方法时，都会创建一组新的toolbarItems。 好像来自iOS 11，每次都有新的工具栏项目被返回到这个方法，UINavigationController:layoutIfNeeded被再次调用，而这又调用toolbarItems由于我们的实现返回新项目。这会导致无限循环。
如果遇到此问题，请检查您是否覆盖- (NSArray *)toolbarItems