将所有POST参数（sign除外）进行字典排序，组成字符串:
比如:(根据首字母排序,参数间用&相连,字典转成json格式)

ap_id=20140723007148&biz_conte={"outTradeNo":201503201",toalAmount":
"8.8",discountAmount":8.",unDiscountAmount":80",subject": 扫码支付",godsDetail": [{"godsI":
"aple-01",godsName":ipad",godsCategory":78230",price":8.8",quantiy":1"}],"operatorId":
"op01",storeId":pudong01",terminalId":t_01",timeExpire":
"20150329101"}&charset=utf-8&method=alipay.trade.precate×tamp=2017-0724 03:750

解决办法:
1.首先传入的是参数字典.
2.获取字典的所有key
3.创建一个可变字符串
4.将key和value对应用’=’相连起来
5.其中value是字典的话,就转成json格式,再相连
6.如果有空格,换行等等的格式字符,就将其替换掉

示例:

NSMutableString *contentString  =[NSMutableString string];
    NSDictionary* dict = [self convertToDict];
    NSArray *keys = [dict allKeys];
    //按字母顺序排序
    NSArray *sortedArray = [keys sortedArrayUsingComparator:^NSComparisonResult(id obj1, id obj2) {
        return [obj1 compare:obj2 options:NSNumericSearch];
    }];
    //拼接字符串
    for (NSString *categoryId in sortedArray) {
        if (![categoryId isEqualToString:@"sign"] && ![categoryId isEqualToString:@"timestamp"]){
            if([categoryId isEqualToString:@"biz_content"]){
                NSError *error = nil;
                NSDictionary* bizDict = [dict objectForKey:@"biz_content"];
                NSData *jsonData = [NSJSONSerialization dataWithJSONObject:bizDict options:NSJSONWritingPrettyPrinted error: &error];
                NSMutableData *tempJsonData = [NSMutableData dataWithData:jsonData];
                NSString* jsonString1 = [[NSString alloc] initWithData:tempJsonData encoding:NSUTF8StringEncoding];
                NSString *jsonString2 = [jsonString1 stringByReplacingOccurrencesOfString:@" : " withString:@":"];
                [contentString appendFormat:@"biz_content=%@&",jsonString2];
            }else{
                [contentString appendFormat:@"%@=%@&", categoryId, [dict valueForKey:categoryId]];
            }


        }
    }
    //添加key字段
    [contentString appendFormat:@"timestamp=%@", [dict objectForKey:@"timestamp"]];
    NSString *strUrl1 = [contentString stringByReplacingOccurrencesOfString:@"  " withString:@""];
//    NSString *strUrl2 = [strUrl1 stringByReplacingOccurrencesOfString:@"\t" withString:@""];
    NSString *strUrl3 = [strUrl1 stringByReplacingOccurrencesOfString:@"\n" withString:@""];

上述代码中,timestamp是最后的一个key,是不用用&相连的,所以拿出来单独拼接.biz_content是一个字典,所以也单独进行判断,转成json格式进行拼接,这样就完成待签名字符串的整理了.