目的：利用NSURLSession获取token.（NSURLConnection被弃用）
融云获取token:

    //实例化
    NSURLSession *session = [NSURLSession sharedSession];

    //定义NSMutableURLRequest
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:@"https://api.cn.ronghub.com/user/getToken.json"]];
    request.HTTPMethod = @"POST";
    
    NSString *nonce = [NSString stringWithFormat:@"%d", arc4random()];
    NSString *date = [NSString stringWithFormat:@"%f",[[[NSDate alloc] init] timeIntervalSince1970]];
    NSString *timestamp = [self sha1:[NSString stringWithFormat:@"%@%@%@", @"bSFbDeDXB9tBvC", nonce, date]];
    

    [request setValue:@"pgyu6atqylj5u" forHTTPHeaderField:@"App-Key"];
    [request setValue:nonce forHTTPHeaderField:@"Nonce"];
    [request setValue:date forHTTPHeaderField:@"Timestamp"];
    [request setValue:timestamp forHTTPHeaderField:@"Signature"];
    
    NSString *body = [NSString stringWithFormat:@"userId=%@&name=%@&portraitUri=%@",@"panda",@"wx",@"http%3A%2F%2Fabc.com%2Fmyportrait.jpg"];
    
    request.HTTPBody = [body dataUsingEncoding:NSUTF8StringEncoding];
    
    //实例NSURLSessionTask
    NSURLSessionTask *task = [session dataTaskWithRequest:request completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
        
        if (error) {
            NSLog(@"error %@", error);
        } else {
            
            NSString *stringH = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
        NSLog(@"response %@   \n%@", response, stringH);
            
        }
    }];
  //请求开始
    [task resume];

sha1:

//别忘了导入加密库
#import <CommonCrypto/CommonDigest.h>

- (NSString*)sha1:(NSString *)hashString
{
    const char *cstr = [hashString cStringUsingEncoding:NSUTF8StringEncoding];
    NSData *data = [NSData dataWithBytes:cstr length:hashString.length];
    
    uint8_t digest[CC_SHA1_DIGEST_LENGTH];
    
    CC_SHA1(data.bytes, (CC_LONG)data.length, digest);
    
    NSMutableString* output = [NSMutableString stringWithCapacity:CC_SHA1_DIGEST_LENGTH * 2];
    
    for(int i = 0; i < CC_SHA1_DIGEST_LENGTH; i++)
        [output appendFormat:@"%02x", digest[i]];
    
    return output;
}