NSURLProtocol + UIWebView + 某些域 = app UI 已冻结

Question

我们正在为 iOS 构建浏览器。我们决定尝试使用自定义 NSURLProtocol 子类来实现我们自己的缓存方案并执行用户代理欺骗。它很好地完成了这两件事......问题是，导航到某些站点（msn.com 是最糟糕的）将导致 整个应用程序的 UI 到最多冻结十五秒。显然有什么东西阻塞了主线程，但它不在我们的代码中。

此问题仅在 UIWebView 和自定义协议结合使用时出现。如果我们换成 WKWebView（由于各种原因我们不能使用），问题就消失了。类似地，如果我们不注册协议以使其永远不会被使用，问题就会消失。

协议的作用似乎也无关紧要；我们编写了一个简单的虚拟协议，它除了转发响应外什么都不做（post 的底部）。我们将该协议放入没有任何其他代码的基本测试浏览器中——结果相同。我们还尝试使用其他人的 (RNCachingURLProtocol) 并观察到相同的结果。似乎这两个组件与某些页面的简单组合导致了冻结。我不知所措，无法尝试解决（甚至调查）这个问题，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

我刚刚检查了 NSURLProtocol 与 msn.com 的行为，发现在某些时候 startLoading 方法在 WebCoreSynchronousLoaderRunLoopMode 模式下被调用。这会导致主线程阻塞。

浏览 CustomHTTPProtocol Apple sample code，我发现了描述此问题的评论。修复通过以下方式实现：

@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 Apple 工程师很有幽默感。

Exciting huh?

详情见full apple sample。

问题无法用 WKWebView 重现，因为 NSURLProtocol 不适用于它。有关详细信息，请参阅 next question。

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