- (BOOL)evaluateServerTrust:(SecTrustRef)serverTrust
                  forDomain:(NSString *)domain
{
    //当使用自建证书验证域名时，需要使用AFSSLPinningModePublicKey或者AFSSLPinningModeCertificate
    if (domain && self.allowInvalidCertificates && self.validatesDomainName && (self.SSLPinningMode == AFSSLPinningModeNone || [self.pinnedCertificates count] == 0)) {
        // https://developer.apple.com/library/mac/documentation/NetworkingInternet/Conceptual/NetworkingTopics/Articles/OverridingSSLChainValidationCorrectly.html
        //  According to the docs, you should only trust your provided certs for evaluation.
        //  Pinned certificates are added to the trust. Without pinned certificates,
        //  there is nothing to evaluate against.
        //
        //  From Apple Docs:
        //          "Do not implicitly trust self-signed certificates as anchors (kSecTrustOptionImplicitAnchors).
        //           Instead, add your own (self-signed) CA certificate to the list of trusted anchors."
        NSLog(@"In order to validate a domain name for self signed certificates, you MUST use pinning.");
        return NO;
    }

    //需要验证域名时，需要添加一个验证域名的策略
    NSMutableArray *policies = [NSMutableArray array];
    if (self.validatesDomainName) {
        [policies addObject:(__bridge_transfer id)SecPolicyCreateSSL(true, (__bridge CFStringRef)domain)];
    } else {
        [policies addObject:(__bridge_transfer id)SecPolicyCreateBasicX509()];
    }

    //设置验证的策略，可以是多个
    SecTrustSetPolicies(serverTrust, (__bridge CFArrayRef)policies);

    if (self.SSLPinningMode == AFSSLPinningModeNone) {
        return self.allowInvalidCertificates || AFServerTrustIsValid(serverTrust);
    } else if (!AFServerTrustIsValid(serverTrust) && !self.allowInvalidCertificates) {
        return NO;
    }

    //获取服务器证书的内容
    switch (self.SSLPinningMode) {
        case AFSSLPinningModeNone:
        default:
            return NO;
        case AFSSLPinningModeCertificate: {
            //AFSSLPinningModeCertificate是直接将本地的证书设置为信任的根证书，然后来进行判断，并且比较本地证书的内容和服务器证书内容是否相同，如果有一个相同则返回YES
            NSMutableArray *pinnedCertificates = [NSMutableArray array];
            for (NSData *certificateData in self.pinnedCertificates) {
                [pinnedCertificates addObject:(__bridge_transfer id)SecCertificateCreateWithData(NULL, (__bridge CFDataRef)certificateData)];
            }
            //设置本地的证书为服务器的根证书
            SecTrustSetAnchorCertificates(serverTrust, (__bridge CFArrayRef)pinnedCertificates);

            //通过本地的证书来判断服务器证书是否可信，不可信，则验证不通过
            if (!AFServerTrustIsValid(serverTrust)) {
                return NO;
            }

            // obtain the chain after being validated, which *should* contain the pinned certificate in the last position (if it's the Root CA)
            NSArray *serverCertificates = AFCertificateTrustChainForServerTrust(serverTrust);
            //客户端是否完全包含服务端证书
            for (NSData *trustChainCertificate in [serverCertificates reverseObjectEnumerator]) {
                if ([self.pinnedCertificates containsObject:trustChainCertificate]) {
                    return YES;
                }
            }
            
            return NO;
        }
        case AFSSLPinningModePublicKey: {
            //AFSSLPinningModePublicKey是通过比较证书当中公钥(PublicKey)部分来进行验证，通过SecTrustCopyPublicKey方法获取本地证书和服务器证书，然后进行比较，如果有一个相同，则通过验证
            NSUInteger trustedPublicKeyCount = 0;
            //服务端证书
            NSArray *publicKeys = AFPublicKeyTrustChainForServerTrust(serverTrust);

            //判断服务器证书的公钥与本地的证书公钥是否相同，相同则客户端认证通过
            for (id trustChainPublicKey in publicKeys) {
                //遍历客户端证书
                for (id pinnedPublicKey in self.pinnedPublicKeys) {
                    //对比两者的证书是否相同
                    if (AFSecKeyIsEqualToKey((__bridge SecKeyRef)trustChainPublicKey, (__bridge SecKeyRef)pinnedPublicKey)) {
                        trustedPublicKeyCount += 1;
                    }
                }
            }
            return trustedPublicKeyCount > 0;
        }
    }
    
    return NO;
}