问题：

ajax跨域访问是一个老问题了，解决方法很多，比较常用的是JSONP方法，JSONP方法是一种非官方方法，而且这种方法只支持GET方式，不如POST方式安全。
即使使用jquery的jsonp方法，type设为POST，也会自动变为GET。
如果跨域使用POST方式，可以使用创建一个隐藏的iframe来实现，与ajax上传图片原理一样，但这样会比较麻烦。
因此，通过设置Access-Control-Allow-Origin来实现跨域访问

//
//  JWURLProtocol.h
//  NSURLProtocolExample
//
//  Created by yangjw on 16/8/19.
//  Copyright © 2016年 lujb. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface JWURLProtocol : NSURLProtocol

@end

//
//  JWURLProtocol.m
//  NSURLProtocolExample
//
//  Created by yangjw on 16/8/19.
//  Copyright © 2016年 lujb. All rights reserved.
//

#import "JWURLProtocol.h"

static NSString * const JWURLProtocolHandledKey = @"JWURLProtocolHandledKey";

@implementation JWURLProtocol

+ (BOOL)canInitWithRequest:(NSURLRequest *)request
{
    //只处理http和https请求
    NSString *scheme = [[request URL] scheme];
    if ( ([scheme caseInsensitiveCompare:@"http"] == NSOrderedSame ||
          [scheme caseInsensitiveCompare:@"https"] == NSOrderedSame))
    {
        //看看是否已经处理过了，防止无限循环
        if ([NSURLProtocol propertyForKey:JWURLProtocolHandledKey inRequest:request]) {
            return NO;
        }
        
        return YES;
    }
    return NO;
}

+ (NSURLRequest *) canonicalRequestForRequest:(NSURLRequest *)request {
    /** 可以在此处添加头等信息  */
    //    NSMutableURLRequest *mutableReqeust = [request mutableCopy];
    //    mutableReqeust = [self redirectHostInRequset:mutableReqeust];
    //    return mutableReqeust;
    return request;
}

+ (BOOL)requestIsCacheEquivalent:(NSURLRequest *)a toRequest:(NSURLRequest *)b
{
    return [super requestIsCacheEquivalent:a toRequest:b];
}

- (void)startLoading
{
    NSMutableURLRequest *mutableReqeust = [[self request] mutableCopy];
    //打标签，防止无限循环
    [NSURLProtocol setProperty:@YES forKey:JWURLProtocolHandledKey inRequest:mutableReqeust];
    //异步
        NSOperationQueue *queue = [[NSOperationQueue alloc] init];
    
        [NSURLConnection sendAsynchronousRequest:mutableReqeust queue:queue completionHandler:^(NSURLResponse *response, NSData *data, NSError *error) {
            [self mockRequest:mutableReqeust data:data];
        }];
    
    //    同步
    //    NSHTTPURLResponse* urlResponse = nil;
    //    NSError *error = [[NSError alloc] init];
    //    NSData *data =[NSURLConnection sendSynchronousRequest:mutableRequest returningResponse:&urlResponse error:&error];
    //    [self mockRequest:mutableRequest data:data];

}

#pragma mark - Mock responses

-(void) mockRequest:(NSURLRequest*)request data:(NSData*)data {
    id client = [self client];
    
    //    问题来自于webkit块因为起源于跨域请求的响应。因为我们我们必须迫使Access-Control-Allow-Origin模拟响应，然后我们还需要强迫响应的内容类型。
    //    设置为*则所域可以用ajax跨域获取数据，设置为指定的域名只能指定的域名用ajax跨域获取到数据。
    NSDictionary *headers = @{@"Access-Control-Allow-Origin" : @"*", @"Access-Control-Allow-Headers" : @"Content-Type"};
    NSHTTPURLResponse *response = [[NSHTTPURLResponse alloc] initWithURL:request.URL statusCode:200 HTTPVersion:@"1.0" headerFields:headers];
    
    [client URLProtocol:self didReceiveResponse:response
     cacheStoragePolicy:NSURLCacheStorageNotAllowed];
    [client URLProtocol:self didLoadData:data];
    [client URLProtocolDidFinishLoading:self];
}
- (void)stopLoading
{
    
}
@end

局部使用

- (void)viewDidLoad 
{
    [super viewDidLoad];
    /** 注册 网络请求拦截 */
    [NSURLProtocol registerClass:[JWURLProtocol class]];
}

- (void)viewWillDisappear:(BOOL)animated
{
    [super viewWillDisappear:animated];
    /** 取消注册网路请求拦截 */
    [NSURLProtocol unregisterClass:[JWURLProtocol class]];
}

全局使用

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Override point for customization after application launch.
    
    //注册protocol
    [NSURLProtocol registerClass:[JWURLProtocol class]];
    return YES;
}

** 在UIWebView中使用会出现 js、css、图片重定向后无法访问的问题，如果想解决此问题可以使用此方法解决

//TODO: 重定向
- (NSURLRequest *)connection:(NSURLConnection *)connection willSendRequest:(NSURLRequest *)request redirectResponse:(NSURLResponse *)response
{
   if (response != nil)
   {
       [[self client] URLProtocol:self wasRedirectedToRequest:request redirectResponse:response];
   }
   return request;
}

苹果现在提倡使用NSURLSession

下面给一个完整的NSURLSession的封装类

#import <Foundation/Foundation.h>

@interface JWURLSessionProtocol : NSURLProtocol

@end

//
//  JWURLSessionProtocol.m
//  NSURLProtocolExample
//
//  Created by yangjw on 16/8/22.
//  Copyright © 2016年 lujb. All rights reserved.
//

#import "JWURLSessionProtocol.h"

static NSString * const JWURLProtocolHandledKey = @"JWURLProtocolHandledKey";

@interface JWURLSessionProtocol ()<NSURLSessionDataDelegate>

@property (atomic, strong, readwrite) NSURLSessionDataTask *task;
@property (nonatomic, strong        ) NSURLSession         *session;
@end

@implementation JWURLSessionProtocol
+ (BOOL)canInitWithRequest:(NSURLRequest *)request
{
    //只处理http和https请求
    NSString *scheme = [[request URL] scheme];
    if ( ([scheme caseInsensitiveCompare:@"http"] == NSOrderedSame ||
          [scheme caseInsensitiveCompare:@"https"] == NSOrderedSame))
    {
//        NSLog(@"====>%@",request.URL);
        
        //看看是否已经处理过了，防止无限循环
        if ([NSURLProtocol propertyForKey:JWURLProtocolHandledKey inRequest:request]) {
            return NO;
        }
        
        return YES;
    }
    return NO;
}

+ (NSURLRequest *) canonicalRequestForRequest:(NSURLRequest *)request {
    /** 可以在此处添加头等信息  */
    NSMutableURLRequest *mutableReqeust = [request mutableCopy];
    return mutableReqeust;
}
- (void)startLoading
{
    NSMutableURLRequest *mutableReqeust = [[self request] mutableCopy];
    //打标签，防止无限循环
    [NSURLProtocol setProperty:@YES forKey:JWURLProtocolHandledKey inRequest:mutableReqeust];
    
    NSURLSessionConfiguration *configure = [NSURLSessionConfiguration defaultSessionConfiguration];
    
      NSOperationQueue *queue = [[NSOperationQueue alloc] init];
    
    self.session  = [NSURLSession sessionWithConfiguration:configure delegate:self delegateQueue:queue];
    self.task = [self.session dataTaskWithRequest:mutableReqeust];
    [self.task resume];
}

- (void)stopLoading
{
    [self.session invalidateAndCancel];
    self.session = nil;
}

- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error
{
    if (error != nil) {
        [self.client URLProtocol:self didFailWithError:error];
    }else
    {
        [self.client URLProtocolDidFinishLoading:self];
    }
}

- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask
didReceiveResponse:(NSURLResponse *)response
 completionHandler:(void (^)(NSURLSessionResponseDisposition disposition))completionHandler
{
    [self.client URLProtocol:self didReceiveResponse:response cacheStoragePolicy:NSURLCacheStorageNotAllowed];
    
    completionHandler(NSURLSessionResponseAllow);
}

- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data
{
    [self.client URLProtocol:self didLoadData:data];
}

- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask willCacheResponse:(NSCachedURLResponse *)proposedResponse completionHandler:(void (^)(NSCachedURLResponse * _Nullable))completionHandler
{
    completionHandler(proposedResponse);
}

//TODO: 重定向
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task willPerformHTTPRedirection:(NSHTTPURLResponse *)response newRequest:(NSURLRequest *)newRequest completionHandler:(void (^)(NSURLRequest *))completionHandler
{
    NSMutableURLRequest *    redirectRequest;
    redirectRequest = [newRequest mutableCopy];
    [[self class] removePropertyForKey:JWURLProtocolHandledKey inRequest:redirectRequest];
    [[self client] URLProtocol:self wasRedirectedToRequest:redirectRequest redirectResponse:response];
    
    [self.task cancel];
    [[self client] URLProtocol:self didFailWithError:[NSError errorWithDomain:NSCocoaErrorDomain code:NSUserCancelledError userInfo:nil]];
}

- (instancetype)initWithRequest:(NSURLRequest *)request cachedResponse:(NSCachedURLResponse *)cachedResponse client:(id<NSURLProtocolClient>)client
{
    self = [super initWithRequest:request cachedResponse:cachedResponse client:client];
    if (self) {
        
        // Some stuff
    }
    return self;
}

@end