When we use https to make network requests, we will receive the certificates from the server. These certificates are divided into agency certificates and certificates issued by ourselves. In ios, if we are going to request an organization certified to send an https request, we don't need to process it. But if it is a self signed certificate, we have to process it, otherwise we won't get the data. So we need to deal with it in a proxy method

//Create Session first   
    NSURLSession * session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration] 
delegate:self delegateQueue:[NSOperationQueue mainQueue]];
    //Build task
    NSURLSessionDataTask * task = [session dataTaskWithURL:[NSURL URLWithString:@"https://www.apple.com"] completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
        NSLog(@"%@",[[NSString alloc]initWithData:data encoding:NSUTF8StringEncoding]);
    }];
    
    //Perform task
    [task resume];

In the following proxy method, the following challenge.protectionSpace represents a secure space.

-(void)URLSession:(NSURLSession *)session didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge completionHandler:
(void (^)(NSURLSessionAuthChallengeDisposition, NSURLCredential * _Nullable))completionHandler
{
    //1. Judge whether the authentication method adopted by the server is: trust the server certificate
    if ([challenge.protectionSpace.authenticationMethod isEqualToString:NSURLAuthenticationMethodServerTrust]) {
       
        //2. Create authentication certificate
        NSURLCredential * credential = [NSURLCredential credentialForTrust:challenge.protectionSpace.serverTrust];
        //3. Handling
       if(completionHandler)
       {   //UseCredential represents the certificate sent back by the server
           completionHandler(NSURLSessionAuthChallengeUseCredential,credential);
       }
        
    }
    
}

Where about nsurlsessionautchallengedisposition is an enumeration with the following enumeration values

 NSURLSessionAuthChallengeDisposition (Management):
 NSURLSessionAuthChallengeUseCredential                     
        -  Use server to send back certificate(Save in challenge inside)
 NSURLSessionAuthChallengePerformDefaultHandling
        -  Default handling,Certificate ignored
 NSURLSessionAuthChallengeCancelAuthenticationChallenge
        -  Cancel entire request,Ignore certificate
 NSURLSessionAuthChallengeRejectProtectionSpace
        -  This rejection,try again