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- (void)URLSession:(NSURLSession *)session
didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge
 completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential *credential))completionHandler
{
    //首先选择默认的处理方式
    NSURLSessionAuthChallengeDisposition disposition = NSURLSessionAuthChallengePerformDefaultHandling;

    //用于保存服务端返回的证书
    __block NSURLCredential *credential = nil;

    if (self.sessionDidReceiveAuthenticationChallenge) {
        //返回自己的处理方式
        disposition = self.sessionDidReceiveAuthenticationChallenge(session, challenge, &credential);
    } else {

        //如果对保护空间的验证方式不是 NSURLAuthenticationMethodServerTrust 则选择 NSURLSessionAuthChallengePerformDefaultHandling 处理方式
        if ([challenge.protectionSpace.authenticationMethod isEqualToString:NSURLAuthenticationMethodServerTrust]) {

            //如果校验返回的证书不通过，那么选择 NSURLSessionAuthChallengeCancelAuthenticationChallenge ，即取消会话
            if ([self.securityPolicy evaluateServerTrust:challenge.protectionSpace.serverTrust forDomain:challenge.protectionSpace.host]) {
                credential = [NSURLCredential credentialForTrust:challenge.protectionSpace.serverTrust];
                if (credential) {
                    disposition = NSURLSessionAuthChallengeUseCredential;
                } else {
                    disposition = NSURLSessionAuthChallengePerformDefaultHandling;
                }
            } else {
                disposition = NSURLSessionAuthChallengeCancelAuthenticationChallenge;
            }
        } else {
            disposition = NSURLSessionAuthChallengePerformDefaultHandling;
        }
    }

    if (completionHandler) {
        completionHandler(disposition, credential);
    }
}