设置CAMetalLayer.presentsWithTransaction=true后后台线程崩溃问题

问题场景 #

当设置metalLayer.presentsWithTransaction = true，且在后台渲染线程调用nextDrawable.present()时，应用偶尔崩溃，抛出如下异常：

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Modifications to the layout engine must not be performed from a background thread after it has been accessed from the main thread.'

预期结果：应用不会崩溃，渲染可正常进行。

崩溃堆栈信息 #

Fatal Exception: NSInternalInconsistencyException
Modifications to the layout engine must not be performed from a background thread after it has been accessed from the main thread.

Fatal Exception: NSInternalInconsistencyException
0  CoreFoundation                 0x1aa30a5ac __exceptionPreprocess
1  libobjc.A.dylib                0x1be38442c objc_exception_throw
2  CoreAutoLayout                 0x1be60ad4c -[NSISEngine tryToOptimizeReturningMutuallyExclusiveConstraints]
3  CoreAutoLayout                 0x1be60aa20 -[NSISEngine _optimizeWithoutRebuilding]
4  CoreAutoLayout                 0x1be60a4f8 -[NSISEngine optimize]
5  CoreAutoLayout                 0x1be605b34 -[NSISEngine performPendingChangeNotifications]
6  UIKitCore                      0x1ad13eef4 -[UIView(Hierarchy) layoutSubviews]
7  UIKitCore                      0x1ad151dd4 -[UIView(CALayerDelegate) layoutSublayersOfLayer:]
8  QuartzCore                     0x1ad65b280 -[CALayer layoutSublayers]
9  QuartzCore                     0x1ad65b73c CA::Layer::layout_if_needed(CA::Transaction*)
10 QuartzCore                     0x1ad66fb64 CA::Layer::layout_and_display_if_needed(CA::Transaction*)
11 QuartzCore                     0x1ad5b7114 CA::Context::commit_transaction(CA::Transaction*, double, double*)
12 QuartzCore                     0x1ad5e2418 CA::Transaction::commit()
13 Flutter                        0x1032e4778 (Missing)
14 Flutter                        0x1035b85ec (Missing)
15 Flutter                        0x1035b9094 (Missing)
16 Flutter                        0x1035b88b0 (Missing)
17 Flutter                        0x1035bedb0 (Missing)
18 Flutter                        0x1032ef46c (Missing)
19 Flutter                        0x1032f13e0 (Missing)
20 CoreFoundation                 0x1aa288050 __CFRUNLOOP_IS_CALLING_OUT_TO_A_TIMER_CALLBACK_FUNCTION__
21 CoreFoundation                 0x1aa287c50 __CFRunLoopDoTimer
22 CoreFoundation                 0x1aa2870c4 __CFRunLoopDoTimers
23 CoreFoundation                 0x1aa281178 __CFRunLoopRun
24 CoreFoundation                 0x1aa2804bc CFRunLoopRunSpecific
25 Flutter                        0x1032f12bc (Missing)
26 Flutter                        0x1032f0c44 (Missing)
27 libsystem_pthread.dylib        0x1f185aca8 _pthread_start
28 libsystem_pthread.dylib        0x1f1863788 thread_start

解决方案 #

• 将present()移至主线程执行：
  当presentsWithTransaction = true时，present()操作会触发CA事务提交，而该过程可能间接触发UIKit布局引擎的操作，这类操作必须在主线程完成。可以将present调用包装到主线程队列中：

  DispatchQueue.main.async {
      drawable.present()
  }
  

• 关闭presentsWithTransaction：
  如果业务场景不需要依赖CA事务的提交时机，直接设置metalLayer.presentsWithTransaction = false，这样present操作不会绑定到CA事务，避免触发主线程相关的布局逻辑。注意该调整可能影响渲染同步时机，需测试验证是否符合业务需求。

• 确保所有布局操作仅在主线程执行：
  排查代码中是否存在后台线程触发UI布局的逻辑，比如修改UIView的frame、约束等操作必须严格放在主线程处理，即使是Metal渲染相关的UI层调整也不例外。

内容的提问来源于stack exchange，提问作者luckysmg