iOS开发-stringByEvaluatingJavaScriptFromString致使崩溃

只能在主线程调用

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);  
dispatch_async(queue, ^{  
    [webView stringByEvaluatingJavaScriptFromString:@"aaa"];
});
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//崩溃信息
Tried to obtain the web lock from a thread other than the main thread or the web thread. 
This may be a result of calling to UIKit from a secondary thread. Crashing now...
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崩溃产生的缘由是你在主线程之外的线程调用了UIKit，系统在执行stringByEvaluatingJavaScriptFromString的时候调用了UIKit里的一些方法，因此不容许在主线程以外的线程去调用这个方法。

解决方法也有不少能够用

[webView performSelectorOnMainThread:]
//或者
dispatch_async(dispatch_get_main_queue(), ^{  
    [webView stringByEvaluatingJavaScriptFromString:@"aaa"];
});        
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