iOS This application is modifying the autolayout engine from a background thread after the engine was accessed from the main thread. This can lead to engine corruption and weird crashes.

This application is modifying the autolayout engine from a background thread after the engine was accessed from the main thread. This can lead to engine corruption and weird crashes.关于这个问题网上有好多，但是解决办法好像给的并不明确。告诉我该在回到主线程的时候刷新UI，然而我并不知道这个问题出现的地方在哪里，也不知道哪里需要回到主线程后刷新UI。

• 首先下载PSPDFUIKitMainThreadGuard（它本身就只有一个.m，你没有搞错！）

• 接着在PSPDFAssertIfNotMainThread方法上打个断点

60AF00CE-9D7D-4A28-84D2-DDAC5C81CA91.png

• 报错后你就可以找到错误根源了

  
  
  F10B43E3-1631-4FBB-AAD1-776C85F6A1A5.png

• 最后，就可以把该放进GCD执行的代码放进去就OK了！

dispatch_async(dispatch_get_main_queue(), ^{  
    // Some UIKit call that had timing issues but works fine   
    // in the next runloop.  
    [self updateUI];  
});