iOS动画块调用layoutIfNeeded()触发EXC_BAD_ACCESS崩溃排查

问题背景 #

崩溃发生在动画块内调用layoutIfNeeded()方法的位置，已提供完整相关函数实现与崩溃报告，初步判断崩溃与内存问题相关，暂未定位具体根因。

问题代码 #

private func showWarningView() {
    DispatchQueue.main.async { [weak self] in
        guard let strongSelf = self else { return }
        if let safeAreaHeight = strongSelf.superview?.safeAreaLayoutGuide.layoutFrame.size.height {
            let visibleBannerHeight: CGFloat = strongSelf.isBannerVisible ? strongSelf.bannerHeight : 0
            let newConstraintConstant = (safeAreaHeight - strongSelf.externalBottomOffset - visibleBannerHeight) - strongSelf.warningBannerHeight
            strongSelf.addInternalBottomOffset(with: strongSelf.warningBannerHeight)
            strongSelf.warningViewTopConstraint.constant = newConstraintConstant
        }
        strongSelf.isWarningViewVisible = true
        let showWarningView = UIViewPropertyAnimator(duration: 0.25, curve: .easeIn, animations: {
            self?.layoutIfNeeded() // <---- 崩溃发生在此处
        })
        showWarningView.startAnimation()
    }
}

崩溃基础信息 #

Exception Type:  EXC_BAD_ACCESS (SIGSEGV)
Exception Subtype: KERN_INVALID_ADDRESS at 0x0000000000000010
Exception Codes: 0x0000000000000001, 0x0000000000000010
VM Region Info: 0x10 is not in any region.  Bytes before following region: 4337303536
      REGION TYPE                 START - END      [ VSIZE] PRT/MAX SHRMOD  REGION DETAIL
      UNUSED SPACE AT START
--->  
      __TEXT                   102860000-102864000 [   16K] r-x/r-x SM=COW  ...
Exception Note:  EXC_CORPSE_NOTIFY
Termination Reason: SIGNAL 11 Segmentation fault: 11
Terminating Process: exc handler [265]

• 崩溃触发线程：Thread 0（主线程）
• 内核诊断信息：VM - Compressor failed a blocking pager_get

崩溃调用栈（Thread 0） #

0   libobjc.A.dylib                 0x00000001d5d22f40 objc_msgSend + 32
1   UIKitCore                       0x00000001bf72d010 -[UIView(AdditionalLayoutSupport) _nsis_center:bounds:inEngine:forLayoutGuide:] + 112 (NSLayoutConstraint_UIKitAdditions.m:3513)
2   UIKitCore                       0x00000001bf7455b0 -[UILayoutGuide _updateLayoutFrameInOwningView:fromEngine:] + 80 (UILayoutGuide.m:109)
3   UIKitCore                       0x00000001bf6c8350 -[UIView(AdditionalLayoutSupport) _is_layout] + 320 (NSLayoutConstraint_UIKitAdditions.m:3173)
4   UIKitCore                       0x00000001bf5b1a7c -[UIView _updateConstraintsAsNecessaryAndApplyLayoutFromEngine] + 1028 (UIView.m:12933)
5   UIKitCore                       0x00000001bf5a9fb4 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 2592 (UIView.m:18426)
6   QuartzCore                      0x00000001c0d20cd0 CA::Layer::layout_if_needed(CA::Transaction*) + 532 (CALayer.mm:10116)
7   UIKitCore                       0x00000001bf60dd90 -[UIView(Hierarchy) layoutBelowIfNeeded] + 584 (UIView.m:12868)
8        ******             0x0000000102eb59d4 closure #1 in closure #1 in BottomDrawView.showWarningView() + 52 (BottomDrawView.swift:410)
9     ******                    0x0000000102eb59d4 partial apply for closure #1 in closure #1 in BottomDrawView.showWarningView() + 68 (:0)
10    ******                    0x0000000102f33378 thunk for @escaping @callee_guaranteed () -> () + 20 (:0)
11  UIKitCore                       0x00000001bf8ee77c -[UIViewPropertyAnimator _runAnimations] + 212 (UIViewPropertyAnimator.m:1912)
12  UIKitCore                       0x00000001bf78c6a4 __49-[UIViewPropertyAnimator startAnimationAsPaused:]_block_invoke_2 + 108 (UIViewPropertyAnimator.m:3279)
13  UIKitCore                       0x00000001bf637fec __49-[UIViewPropertyAnimator startAnimationAsPaused:]_block_invoke + 128 (UIViewPropertyAnimator.m:3267)
14  UIKitCore                       0x00000001bfb6eba8 __49-[UIViewPropertyAnimator startAnimationAsPaused:]_block_invoke.1142 + 284 (UIViewPropertyAnimator.m:3276)
15  UIKitCore                       0x00000001bf95fa20 -[UIViewPropertyAnimator _setupAnimationTracking:] + 132 (UIViewPropertyAnimator.m:3230)
16  UIKitCore                       0x00000001bf4c146c -[UIViewPropertyAnimator startAnimationAsPaused:] + 1048 (UIViewPropertyAnimator.m:3334)
17    ******                    0x0000000102eb4868 closure #1 in BottomDrawView.showWarningView() + 460 (BottomDrawView.swift:412)
18    ******                    0x0000000102f33378 thunk for @escaping @callee_guaranteed () -> () + 20 (:0)
19  libdispatch.dylib               0x00000001bcca2e6c _dispatch_call_block_and_release + 32 (init.c:1517)
20  libdispatch.dylib               0x00000001bcca4a30 _dispatch_client_callout + 20 (object.m:560)
21  libdispatch.dylib               0x00000001bccb2f48 _dispatch_main_queue_drain + 928 (inline_internal.h:2622)
22  libdispatch.dylib               0x00000001bccb2b98 _dispatch_main_queue_callback_4CF + 44 (queue.c:7770)
23  CoreFoundation                  0x00000001bcff5800 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 16 (CFRunLoop.c:1795)
24  CoreFoundation                  0x00000001bcfaf704 __CFRunLoopRun + 2532 (CFRunLoop.c:3144)
25  CoreFoundation                  0x00000001bcfc2bc8 CFRunLoopRunSpecific + 600 (CFRunLoop.c:3268)
26  GraphicsServices                0x00000001d90f6374 GSEventRunModal + 164 (GSEvent.c:2200)
27  UIKitCore                       0x00000001bf932648 -[UIApplication _run] + 1100 (UIApplication.m:3511)
28  UIKitCore                       0x00000001bf6b3d90 UIApplicationMain + 364 (UIApplication.m:5064)
29    ******                    0x0000000102864a08 main + 68 (AppDelegate.swift:22)
30  dyld                            0x0000000104505ce4 start + 520 (dyldMain.cpp:879)

根因定位 #

这是野指针访问触发的EXC_BAD_ACCESS崩溃，崩溃地址0x0000000000000010是典型的空指针偏移16位访问特征，对应objc_msgSend向已释放对象发送消息的场景，结合调用栈布局阶段访问UILayoutGuide的逻辑，核心问题是闭包捕获的引用失效：

• 外层GCD主队列闭包虽然用了[weak self]，也通过strongSelf做了入栈时的空值校验，但创建UIViewPropertyAnimator时，动画闭包没有显式声明弱引用捕获，直接捕获了self?。UIViewPropertyAnimator会被UIKit动画核心强持有直到动画执行完成，如果动画运行过程中BottomDrawView被从视图层级移除、完成释放，动画闭包里捕获的self不会被自动置空，会变成悬垂野指针。
• 从调用栈看，崩溃发生在Auto Layout遍历UILayoutGuide更新布局帧的阶段：BottomDrawView释放时，它绑定的布局向导（包括安全区布局向导、自定义布局约束关联对象）已经被回收，此时执行layoutIfNeeded触发布局遍历，就会访问到已释放的内存块，触发段错误。

修复方案 #

修改动画闭包的捕获列表，对self做弱引用捕获，动画执行前重新做一次空值校验：

let showWarningView = UIViewPropertyAnimator(duration: 0.25, curve: .easeIn, animations: { [weak self] in
    guard let self = self else { return }
    self.layoutIfNeeded()
})

额外优化点：

• 执行layoutIfNeeded前可增加校验，判断self.window != nil时再触发布局，避免视图不在可见层级时执行无效动画
• 在BottomDrawView的deinit方法中终止正在运行的该动画器，避免动画对象持有无效引用：showWarningView.stopAnimation(true)

内容的提问来源于stack exchange，提问作者DaVinci