当应用程序进入前台时，Webthread锁定主线程

Question

我使用UIWebView在我的iOS应用程序中嵌入YouTube HTML播放器，有时当应用程序从后台进入前台时(视频正在播放)，应用程序刚刚冻结，我得到0x000000008badf00d崩溃。

OS Version:          iOS 7.1.1 (11D201)
Report Version:      104
Exception Type:  00000020
Exception Codes: 0x000000008badf00d
Highlighted Thread:  0
Application Specific Information:
com.myapp.myapp failed to resume in time
Elapsed total CPU time (seconds): 6.460 (user 6.460, system 0.000), 27% CPU 
Elapsed application CPU time (seconds): 3.069, 13% CPU
Thread 0:
0   libsystem_kernel.dylib          0x39911f9c __psynch_mutexwait + 24
1   libsystem_pthread.dylib         0x3997b298 _pthread_mutex_lock_contended + 48
2   libsystem_pthread.dylib         0x39977d36 _pthread_mutex_lock + 258
3   WebCore                         0x36e2f190 _WebTryThreadLock(bool) + 40
4   WebCore                         0x36e2ffb8 WebThreadLock + 76
5   WebCore                         0x36e6b962 -[TileLayer layoutSublayers] + 14
6   QuartzCore                      0x3106db3a CA::Layer::layout_if_needed(CA::Transaction*) + 346
7   QuartzCore                      0x3106d9cc CA::Layer::layout_and_display_if_needed(CA::Transaction*) + 12
8   QuartzCore                      0x3106d3e0 CA::Context::commit_transaction(CA::Transaction*) + 224
9   QuartzCore                      0x3106d1f2 CA::Transaction::commit() + 310
10  AVFoundation                    0x2da4fb60 -[AVPlayerLayer _updateSubtitles:forceRender:] + 696
11  AVFoundation                    0x2da500a8 -[AVPlayerLayer layerDidBecomeVisible:] + 200
12  QuartzCore                      0x3106bbf4 CA::Layer::mark_visible(CA::Transaction*, bool) + 80
13  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
14  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
15  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
16  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
17  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
18  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
19  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
20  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
21  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
22  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
23  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
24  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
25  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
26  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
27  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
28  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
29  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
30  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
31  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
32  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
33  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
34  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
35  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
36  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
37  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
38  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
39  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
40  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
41  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
42  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
43  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
44  QuartzCore                      0x3106bc30 CA::Layer::mark_visible(CA::Transaction*, bool) + 140
45  QuartzCore                      0x3106cb82 CA::Layer::set_visible(unsigned int) + 346
46  QuartzCore                      0x3106c9e6 CA::Context::set_layer(void const*) + 134
47  UIKit                           0x313feab2 -[UIWindow _createContext] + 1138
48  UIKit                           0x316572a4 _UIWindowUpdateVisibleContextOrder + 172
49  UIKit                           0x31657158 +[UIWindow _prepareWindowsPassingTestForAppResume:] +         12
50  UIKit                           0x315fc534 -[UIApplication _handleApplicationResumeEvent:] + 72
51  UIKit                           0x313fb60e -[UIApplication handleEvent:withNewEvent:] + 1878
52  UIKit                           0x313fadf4 -[UIApplication sendEvent:] + 68
53  UIKit                           0x3145f400 _UIApplicationHandleEvent + 612
54  GraphicsServices                0x33a68b52 _PurpleEventCallback + 606
55  GraphicsServices                0x33a6873a PurpleEventCallback + 30
56  CoreFoundation                  0x2eb91844     __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 32
57  CoreFoundation                  0x2eb917de __CFRunLoopDoSource1 + 342
58  CoreFoundation                  0x2eb8ffaa __CFRunLoopRun + 1402
59  CoreFoundation                  0x2eafa764 CFRunLoopRunSpecific + 520
60  CoreFoundation                  0x2eafa546 CFRunLoopRunInMode + 102
61  GraphicsServices                0x33a676ce GSEventRunModal + 134
62  UIKit                           0x3145988c UIApplicationMain + 1132
63  myapp                           0x0003ba0e main (main.m:14)
64  myapp                           0x0003b9c4 start + 36

基本上我使用UIWebView加载一个HTML文件与YouTube HTML播放器，并使用[webView stringByEvaluatingJavaScriptFromString: jsString]来控制它(如loadVideoById(), play(), pause()等)。还有一些机制，使应用程序可以在后台播放视频，大多数时候它工作得很好，这个崩溃只发生在它在后台播放几个视频，并试图进入前台，但很难重现。

似乎是webthread锁住了主线程，所以iOS杀死了我的应用，我已经研究了几个星期这个问题，但没有运气，欢迎任何建议

Answer 1

经过大量的实验，这个问题可以避免通过隐藏html播放器div标签与CSS当应用程序进入后台，然后再次显示它后，收到appDidBecomeActive事件。

我认为YouTube html播放器试图做一些UI相关的应用程序变得活跃，所以它可能会锁定主线程，得到0x000000008badf00d崩溃。

Answer 2

我在iOS 7上遇到过这个问题，只有当尝试在后台线程中使用包含YouTube iFrame播放器的HTML字符串调用loadHTMLString:baseURL:时。将该调用与我所有其他UI调用一起移动到主线程似乎解决了这个问题。

我应该注意的是，我无法在iOS 8和9中重现这个问题，所以这也可能是一个WebKit错误，在后来的iOS更新中得到了修复。