NSURLProtocol + UIWebView +某些域=应用程序UI冻结

Question

我们正在为iOS构建浏览器。我们决定尝试使用自定义的NSURLProtocol子类来实现我们自己的缓存方案并执行用户代理欺骗。它能够完成这两件事情......问题在于，导航到某些网站（msn.com是最糟糕的）会导致整个应用程序的用户界面冻结达15秒。显然有些东西阻塞了主线程，但它不在我们的代码中。

此问题只出现在UIWebView和自定义协议的组合中。如果我们换一个WKWebView（我们因各种原因不能使用），问题就消失了。同样，如果我们没有注册协议，以至于它没有被利用，问题就会消失。

它也似乎没有太大的问题什么协议;我们写了一个毫无意义的虚拟协议，只是转发响应（帖子的底部）。我们将该协议放入没有任何其他代码的裸机测试浏览器中 - 结果相同。我们也尝试使用别人的（RNCachingURLProtocol）并观察到相同的结果。看起来，这两个组件与某些页面的简单组合会导致冻结。我无法尝试解决（甚至调查）此问题，并且非常感谢任何指导或提示。谢谢！

import UIKit 

private let KEY_REQUEST_HANDLED = "REQUEST_HANDLED" 

final class CustomURLProtocol: NSURLProtocol { 
    var connection: NSURLConnection! 

    override class func canInitWithRequest(request: NSURLRequest) -> Bool { 
     return NSURLProtocol.propertyForKey(KEY_REQUEST_HANDLED, inRequest: request) == nil 
    } 

    override class func canonicalRequestForRequest(request: NSURLRequest) -> NSURLRequest { 
     return request 
    } 

    override class func requestIsCacheEquivalent(aRequest: NSURLRequest, toRequest bRequest: NSURLRequest) -> Bool { 
     return super.requestIsCacheEquivalent(aRequest, toRequest:bRequest) 
    } 

    override func startLoading() { 
     var newRequest = self.request.mutableCopy() as! NSMutableURLRequest 
     NSURLProtocol.setProperty(true, forKey: KEY_REQUEST_HANDLED, inRequest: newRequest) 
     self.connection = NSURLConnection(request: newRequest, delegate: self) 
    } 

    override func stopLoading() { 
     connection?.cancel() 
     connection = nil 
    } 

    func connection(connection: NSURLConnection!, didReceiveResponse response: NSURLResponse!) { 
     self.client!.URLProtocol(self, didReceiveResponse: response, cacheStoragePolicy: .NotAllowed) 
    } 

    func connection(connection: NSURLConnection!, didReceiveData data: NSData!) { 
     self.client!.URLProtocol(self, didLoadData: data) 
    } 

    func connectionDidFinishLoading(connection: NSURLConnection!) { 
     self.client!.URLProtocolDidFinishLoading(self) 
    } 

    func connection(connection: NSURLConnection!, didFailWithError error: NSError!) { 
     self.client!.URLProtocol(self, didFailWithError: error) 
    } 
} 

Answer 1

我刚刚检查NSURLProtocol行为与msn.com，发现在某些时候在WebCoreSynchronousLoaderRunLoopMode模式称为startLoading方法。这会导致主线程阻塞。

翻阅CustomHTTPProtocol Apple sample code，我发现了描述这个问题的评论。修复是在下一步实施：

@interface CustomHTTPProtocol() <NSURLSessionDataDelegate> 

@property (atomic, strong, readwrite) NSThread * clientThread; ///< The thread on which we should call the client. 

/*! The run loop modes in which to call the client. 
* \details The concurrency control here is complex. It's set up on the client 
* thread in -startLoading and then never modified. It is, however, read by code 
* running on other threads (specifically the main thread), so we deallocate it in 
* -dealloc rather than in -stopLoading. We can be sure that it's not read before 
* it's set up because the main thread code that reads it can only be called after 
* -startLoading has started the connection running. 
*/ 
@property (atomic, copy, readwrite) NSArray * modes; 

- (void)startLoading 
{ 
    NSMutableArray *calculatedModes; 
    NSString *currentMode; 

    // At this point we kick off the process of loading the URL via NSURLSession. 
    // The thread that calls this method becomes the client thread. 

    assert(self.clientThread == nil); // you can't call -startLoading twice 

    // Calculate our effective run loop modes. In some circumstances (yes I'm looking at 
    // you UIWebView!) we can be called from a non-standard thread which then runs a 
    // non-standard run loop mode waiting for the request to finish. We detect this 
    // non-standard mode and add it to the list of run loop modes we use when scheduling 
    // our callbacks. Exciting huh? 
    // 
    // For debugging purposes the non-standard mode is "WebCoreSynchronousLoaderRunLoopMode" 
    // but it's better not to hard-code that here. 

    assert(self.modes == nil); 
    calculatedModes = [NSMutableArray array]; 
    [calculatedModes addObject:NSDefaultRunLoopMode]; 
    currentMode = [[NSRunLoop currentRunLoop] currentMode]; 
    if ((currentMode != nil) && ! [currentMode isEqual:NSDefaultRunLoopMode]) { 
     [calculatedModes addObject:currentMode]; 
    } 
    self.modes = calculatedModes; 
    assert([self.modes count] > 0); 

    // Create new request that's a clone of the request we were initialised with, 
    // except that it has our 'recursive request flag' property set on it. 

    // ... 

    // Latch the thread we were called on, primarily for debugging purposes. 

    self.clientThread = [NSThread currentThread]; 

    // Once everything is ready to go, create a data task with the new request. 

    self.task = [[[self class] sharedDemux] dataTaskWithRequest:recursiveRequest delegate:self modes:self.modes]; 
    assert(self.task != nil); 

    [self.task resume]; 
} 

一些苹果工程师有良好的幽默感。

令人兴奋的吧？

有关详细信息，请参阅full apple sample。

问题与WKWebView不兼容，因为NSURLProtocol不适用于它。详情请参阅next question。