问题
优化微博国际版时候，发现控制台打出了这样的异常：
Main Thread Checker: UI API called on a background thread: -[UIApplication delegate]
PID: 5433, TID: 4812574, Thread name: (none), Queue name: com.apple.root.user-initiated-qos, QoS: 25
Backtrace:
4 WeiboOverseas 0x00000001014bef6c -[*** layoutSpecThatFits:] + 3788
5 WeiboOverseas 0x00000001022063d0 -[ASDisplayNode(ASLayoutSpec) _locked_layoutElementThatFits:] + 456
6 WeiboOverseas 0x000000010221bcf0 __27+[ASDisplayNode initialize]_block_invoke + 192
7 WeiboOverseas 0x0000000102205810 -[ASDisplayNode(ASLayoutSpec) calculateLayoutLayoutSpec:] + 588
8 WeiboOverseas 0x0000000102222bcc -[ASDisplayNode calculateLayoutThatFits:] + 608
9 WeiboOverseas 0x0000000102222830 -[ASDisplayNode calculateLayoutThatFits:restrictedToSize:relativeToParentSize:] + 364
10 WeiboOverseas 0x00000001021fc988 -[ASDisplayNode(ASLayoutElement) layoutThatFits:parentSize:] + 1408
11 WeiboOverseas 0x00000001021fc3fc -[ASDisplayNode(ASLayoutElement) layoutThatFits:] + 104
12 WeiboOverseas 0x00000001021e75a4 -[ASDataController _layoutNode:withConstrainedSize:] + 576
13 WeiboOverseas 0x00000001021e724c __47-[ASDataController _allocateNodesFromElements:]_block_invoke + 692
14 WeiboOverseas 0x00000001021f6374 __ASDispatchApply_block_invoke + 144
15 libdispatch.dylib 0x000000010575a338 _dispatch_call_block_and_release + 24
16 libdispatch.dylib 0x000000010575b730 _dispatch_client_callout + 16
17 libdispatch.dylib 0x000000010576cff8 _dispatch_root_queue_drain + 1020
18 libdispatch.dylib 0x000000010576d698 _dispatch_worker_thread2 + 152
19 libsystem_pthread.dylib 0x0000000187447b38 _pthread_wqthread + 212
20 libsystem_pthread.dylib 0x000000018744a740 start_wqthread + 8
2021-03-12 12:06:18.059579+0800 WeiboOverseas[5433:4812574] [reports] Main Thread Checker: UI API called on a background thread: -[UIApplication delegate]
PID: 5433, TID: 4812574, Thread name: (none), Queue name: com.apple.root.user-initiated-qos, QoS: 25
Backtrace:
4 WeiboOverseas 0x00000001014bef6c -[***** layoutSpecThatFits:] + 3788
5 WeiboOverseas 0x00000001022063d0 -[ASDisplayNode(ASLayoutSpec) _locked_layoutElementThatFits:] + 456
6 WeiboOverseas 0x000000010221bcf0 __27+[ASDisplayNode initialize]_block_invoke + 192
7 WeiboOverseas 0x0000000102205810 -[ASDisplayNode(ASLayoutSpec) calculateLayoutLayoutSpec:] + 588
8 WeiboOverseas 0x0000000102222bcc -[ASDisplayNode calculateLayoutThatFits:] + 608
9 WeiboOverseas 0x0000000102222830 -[ASDisplayNode calculateLayoutThatFits:restrictedToSize:relativeToParentSize:] + 364
10 WeiboOverseas 0x00000001021fc988 -[ASDisplayNode(ASLayoutElement) layoutThatFits:parentSize:] + 1408
11 WeiboOverseas 0x00000001021fc3fc -[ASDisplayNode(ASLayoutElement) layoutThatFits:] + 104
12 WeiboOverseas 0x00000001021e75a4 -[ASDataController _layoutNode:withConstrainedSize:] + 576
13 WeiboOverseas 0x00000001021e724c __47-[ASDataController _allocateNodesFromElements:]_block_invoke + 692
14 WeiboOverseas 0x00000001021f6374 __ASDispatchApply_block_invoke + 144
15 libdispatch.dylib 0x000000010575a338 _dispatch_call_block_and_release + 24
16 libdispatch.dylib 0x000000010575b730 _dispatch_client_callout + 16
17 libdispatch.dylib 0x000000010576cff8 _dispatch_root_queue_drain + 1020
18 libdispatch.dylib 0x000000010576d698 _dispatch_worker_thread2 + 152
19 libsystem_pthread.dylib 0x0000000187447b38 _pthread_wqthread + 212
20 libsystem_pthread.dylib 0x000000018744a740 start_wqthread + 8
2021-03-12 12:06:32.497324+0800 WeiboOverseas[5433:4812368] Could not signal service com.apple.WebKit.Networking: 113: Could not find specified service
=================================================================
Main Thread Checker: UI API called on a background thread: -[UIWindow safeAreaInsets]
PID: 5433, TID: 4812574, Thread name: (none), Queue name: com.apple.root.user-initiated-qos, QoS: 25
Backtrace:
4 WeiboOverseas 0x00000001014befa4 -[**** layoutSpecThatFits:] + 3844
5 WeiboOverseas 0x00000001022063d0 -[ASDisplayNode(ASLayoutSpec) _locked_layoutElementThatFits:] + 456
6 WeiboOverseas 0x000000010221bcf0 __27+[ASDisplayNode initialize]_block_invoke + 192
7 WeiboOverseas 0x0000000102205810 -[ASDisplayNode(ASLayoutSpec) calculateLayoutLayoutSpec:] + 588
8 WeiboOverseas 0x0000000102222bcc -[ASDisplayNode calculateLayoutThatFits:] + 608
9 WeiboOverseas 0x0000000102222830 -[ASDisplayNode calculateLayoutThatFits:restrictedToSize:relativeToParentSize:] + 364
10 WeiboOverseas 0x00000001021fc988 -[ASDisplayNode(ASLayoutElement) layoutThatFits:parentSize:] + 1408
11 WeiboOverseas 0x00000001021fc3fc -[ASDisplayNode(ASLayoutElement) layoutThatFits:] + 104
12 WeiboOverseas 0x00000001021e75a4 -[ASDataController _layoutNode:withConstrainedSize:] + 576
13 WeiboOverseas 0x00000001021e724c __47-[ASDataController _allocateNodesFromElements:]_block_invoke + 692
14 WeiboOverseas 0x00000001021f6374 __ASDispatchApply_block_invoke + 144
15 libdispatch.dylib 0x000000010575a338 _dispatch_call_block_and_release + 24
16 libdispatch.dylib 0x000000010575b730 _dispatch_client_callout + 16
17 libdispatch.dylib 0x000000010576cff8 _dispatch_root_queue_drain + 1020
18 libdispatch.dylib 0x000000010576d698 _dispatch_worker_thread2 + 152
19 libsystem_pthread.dylib 0x0000000187447b38 _pthread_wqthread + 212
20 libsystem_pthread.dylib 0x000000018744a740 start_wqthread + 8
2021-03-12 12:06:32.542614+0800 WeiboOverseas[5433:4812574] [reports] Main Thread Checker: UI API called on a background thread: -[UIWindow safeAreaInsets]
PID: 5433, TID: 4812574, Thread name: (none), Queue name: com.apple.root.user-initiated-qos, QoS: 25
Backtrace:
4 WeiboOverseas 0x00000001014befa4 -[**** layoutSpecThatFits:] + 3844
5 WeiboOverseas 0x00000001022063d0 -[ASDisplayNode(ASLayoutSpec) _locked_layoutElementThatFits:] + 456
6 WeiboOverseas 0x000000010221bcf0 __27+[ASDisplayNode initialize]_block_invoke + 192
7 WeiboOverseas 0x0000000102205810 -[ASDisplayNode(ASLayoutSpec) calculateLayoutLayoutSpec:] + 588
8 WeiboOverseas 0x0000000102222bcc -[ASDisplayNode calculateLayoutThatFits:] + 608
9 WeiboOverseas 0x0000000102222830 -[ASDisplayNode calculateLayoutThatFits:restrictedToSize:relativeToParentSize:] + 364
10 WeiboOverseas 0x00000001021fc988 -[ASDisplayNode(ASLayoutElement) layoutThatFits:parentSize:] + 1408
11 WeiboOverseas 0x00000001021fc3fc -[ASDisplayNode(ASLayoutElement) layoutThatFits:] + 104
12 WeiboOverseas 0x00000001021e75a4 -[ASDataController _layoutNode:withConstrainedSize:] + 576
13 WeiboOverseas 0x00000001021e724c __47-[ASDataController _allocateNodesFromElements:]_block_invoke + 692
14 WeiboOverseas 0x00000001021f6374 __ASDispatchApply_block_invoke + 144
15 libdispatch.dylib 0x000000010575a338 _dispatch_call_block_and_release + 24
16 libdispatch.dylib 0x000000010575b730 _dispatch_client_callout + 16
17 libdispatch.dylib 0x000000010576cff8 _dispatch_root_queue_drain + 1020
18 libdispatch.dylib 0x000000010576d698 _dispatch_worker_thread2 + 152
19 libsystem_pthread.dylib 0x0000000187447b38 _pthread_wqthread + 212
20 libsystem_pthread.dylib 0x000000018744a740 start_wqthread + 8
具体也是在这个时候，会有严重掉帧现象，通过调用栈看了一圈，找了一遍，整个人有点懵。。。
问题解决
- 其实异常第一行就告诉我们是哪里问题了,问题是子线程调用主线程的东东了，还告诉你调用的就是 -[UIApplication delegate] 和 -[UIWindow safeAreaInsets]，这里我大约已经意识到问题在哪里了。
- 然后继续看调用栈 [**** layoutSpecThatFits:] + 3844 ，这里说明是在我们的layoutSpecThatFits方法里。 好了去找吧。
Main Thread Checker: UI API called on a background thread: -[UIApplication delegate]
Main Thread Checker: UI API called on a background thread: -[UIWindow safeAreaInsets]
最后发现，是因为这地方布局时候我们有做是否刘海屏的区分，刘海屏的判断代码我之前是这么写的：
var isIPhoneX: Bool {
get {
if #available(iOS 11.0, *), let window = UIApplication.shared.delegate?.window, let bottom = window?.safeAreaInsets.bottom {
return bottom > 0
}
return false
}
}
而这个计算属性用到的其他方法并不能保证任意线程调用 emmmm
这里我改成了static var， 然后再在app开始时 didFinishLaunchingWithOptions进行赋值，就可以解决这个问题了。
