CustomHTTPProtocol shows how to use an NSURLProtocol subclass to intercept the HTTP/HTTPS requests made by a high-level subsystem that does not otherwise expose its network connections.  

+ (NSURLRequest *)canonicalRequestForRequest:(NSURLRequest *)request;

- (instancetype)initWithRequest:(NSURLRequest *)request cachedResponse:(nullable NSCachedURLResponse *)cachedResponse client:(nullable id <NSURLProtocolClient>)client NS_DESIGNATED_INITIALIZER;

/*! 
    @method registerClass:
    @abstract This method registers a protocol class, making it visible
    to several other NSURLProtocol class methods.
    @discussion When the URL loading system begins to load a request,
    each protocol class that has been registered is consulted in turn to
    see if it can be initialized with a given request. The first
    protocol handler class to provide a YES answer to
    <tt>+canInitWithRequest:</tt> "wins" and that protocol
    implementation is used to perform the URL load. There is no
    guarantee that all registered protocol classes will be consulted.
    Hence, it should be noted that registering a class places it first
    on the list of classes that will be consulted in calls to
    <tt>+canInitWithRequest:</tt>, moving it in front of all classes
    that had been registered previously.
    <p>A similar design governs the process to create the canonical form
    of a request with the <tt>+canonicalRequestForRequest:</tt> class
    method.
    @param protocolClass the class to register.
    @result YES if the protocol was registered successfully, NO if not.
    The only way that failure can occur is if the given class is not a
    subclass of NSURLProtocol.
*/
+ (BOOL)registerClass:(Class)protocolClass;

@protocol CustomHTTPProtocolDelegate;

/*! An NSURLProtocol subclass that overrides the built-in HTTP/HTTPS protocol to intercept 
 *  authentication challenges for subsystems, ilke UIWebView, that don't otherwise allow it.  
 *  To use this class you should set up your delegate (+setDelegate:) and then call +start. 
 *  If you don't call +start the class is completely benign.
 *
 *  The really tricky stuff here is related to the authentication challenge delegate 
 *  callbacks; see the docs for CustomHTTPProtocolDelegate for the details.
 */

@interface CustomHTTPProtocol : NSURLProtocol

/*! Call this to start the module.  Prior to this the module is just dormant, and 
 *  all HTTP requests proceed as normal.  After this all HTTP and HTTPS requests 
 *  go through this module.
 */

+ (void)start;

+ (void)start
{
    [NSURLProtocol registerClass:self];
}

+ (NSURLRequest *)canonicalRequestForRequest:(NSURLRequest *)request
{
    NSURLRequest *      result;
    
    assert(request != nil);
    // can be called on any thread
    
    // Canonicalising a request is quite complex, so all the heavy lifting has 
    // been shuffled off to a separate module.
    
    result = CanonicalRequestForRequest(request);

    [self customHTTPProtocol:nil logWithFormat:@"canonicalized %@ to %@", [request URL], [result URL]];
    
    return result;
}

- (BOOL)application:(UIApplication *)application willFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    #pragma unused(application)
    #pragma unused(launchOptions)
    WebViewController *   webViewController;
    
    assert(self.window != nil);
    
    sAppStartTime = [NSDate timeIntervalSinceReferenceDate];
    
    self.credentialsManager = [[CredentialsManager alloc] init];

    // Prepare the globals needed by our logging code.  The call to -threadInfoForCurrentThread 
    // sets up the main thread's thread info record and ensures it has a thread number of 0.

    self.threadInfoByThreadID = [[NSMutableDictionary alloc] init];
    (void) [self threadInfoForCurrentThread];
    
    // Start up the core code.  Change the if expression to NO to disable the CustomHTTPProtocol for 
    // comparative testing and so on.
    
    [CustomHTTPProtocol setDelegate:self];
    if (YES) {
        [CustomHTTPProtocol start];
    }
    
    // Create the web view controller and set up the UI.  We do this after setting 
    // up the core code in case this triggers any HTTP requests.
    // 
    // By default the Test button is not shown because this sample is focused on UIWebView.  
    // If you want to runs tests with NSURL{Session,Connection}, change the if expression to 
    // show the Test button and then configure the test by changing the code in -testAction:.
    
    webViewController = [[UIStoryboard storyboardWithName:@"Main" bundle:[NSBundle bundleForClass:[self class]]] instantiateViewControllerWithIdentifier:@"webView"];
    assert(webViewController != nil);
    webViewController.delegate = self;
    if (NO) {
        webViewController.navigationItem.rightBarButtonItem = [[UIBarButtonItem alloc] initWithTitle:@"Test" style:UIBarButtonItemStyleBordered target:self action:@selector(testAction:)];
    }
    [((UINavigationController *) self.window.rootViewController) pushViewController:webViewController animated:NO];

	[self.window makeKeyAndVisible];
    
    return YES;
}

extern NSMutableURLRequest * CanonicalRequestForRequest(NSURLRequest *request)

extern NSMutableURLRequest * CanonicalRequestForRequest(NSURLRequest *request)

static void CanonicaliseHeaders(NSMutableURLRequest * request)

### 注意

>* 1、NSURLProtocol 只能拦截 UIURLConnection、NSURLSession 和 UIWebView 中的请求;
对于 WKWebView 中发出的网络请求也无能为力，如果真的要拦截来自 WKWebView 中的请求，还是需要实现 WKWebView 对应的 WKNavigationDelegate，并在代理方法中获取请求。

>* 2、[OHHTTPStubs](https://github.com/zhangkn/OHHTTPStubs) 就是基于NSURLProtocol基础上实现的