To ensure compatibility of mobile terminals
urlsession that cannot use ios
The curl Library of C language can be used to implement the download operation.
Compilation of curl in ios environment
Because I compiled openssl before, I took it directly and used it.
openssl is used to increase https support
After compilation is passed, it is very simple to use.

Use code

//
//  ViewController.m
//  libcurlHTTPS
//
//  Created by kai_leedarson on 2018/10/29.
//  Copyright 2018 maple_leedarson. All rights reserved.
//

#import "ViewController.h"
#include <curl/curl.h>
#import "JsonDictExchange.h"
@interface ViewController ()
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    curl_version_info_data *data=  curl_version_info(CURLVERSION_NOW);
    printf("\nopenssl version %s\n",data->ssl_version);
}
static int ProgressCallback(void *clientp, double dltotal, double dlnow, double ultotal, double ulnow)
{
    static int pre_percent =0;
    if ( dltotal > -0.1 && dltotal < 0.1 )
        return 0;
    int nPos = (int) ( (dlnow/dltotal)*100 );
    pre_percent = nPos;
    NSLog(@"%d%%",pre_percent);
    return 0;
}

bool getUrl(const char *filename,char *url)
{
    CURL *curl;
    CURLcode res;
    FILE *fp;
    if ((fp = fopen(filename, "wt+")) == NULL){
        // Return results are stored in files
        return false;
    }
    struct curl_slist *headers = NULL;
    //Add HTTP header
//    headers = curl_slist_append(headers, "Accept:application/json");
//    headers = curl_slist_append(headers, "Content-Type:application/json");
 //   headers = curl_slist_append(headers, "charset:utf-8");
    curl = curl_easy_init();    // Initialization 
    if (curl)
    {
//        curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers); //change protocol header
        curl_easy_setopt(curl, CURLOPT_VERBOSE, 0L);
        curl_easy_setopt(curl, CURLOPT_URL,url);
        curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 0L);
        curl_easy_setopt(curl, CURLOPT_SSL_VERIFYHOST, 0L);
        curl_easy_setopt(curl, CURLOPT_SSL_ENABLE_ALPN, 0L);
        curl_easy_setopt(curl, CURLOPT_WRITEDATA, fp);
        curl_easy_setopt(curl, CURLOPT_NOPROGRESS, 0);
        curl_easy_setopt(curl, CURLOPT_PROGRESSFUNCTION, ProgressCallback);
        NSLog(@"request");
        res = curl_easy_perform(curl);   // implement
        NSLog(@"The request return value is%i",res);
        curl_slist_free_all(headers);
        curl_easy_cleanup(curl);
    }
    fclose(fp);
    return true;
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    [self testWeb];
}

- (void)testWeb {
    NSString * path0 = [[NSBundle bundleForClass:[self class]] pathForResource:[NSString stringWithUTF8String:"testCloud.json"]  ofType:nil];
    NSData *receiveData = [[NSData alloc] initWithContentsOfFile:path0];
    //The obtained json is first converted into a string
    NSString *receiveStr = [[NSString alloc]initWithData:receiveData encoding:NSUTF8StringEncoding];
    //Re analysis
    NSDictionary *jsonDict = [JsonDictExchange convertJsonWith:receiveStr];
    NSMutableArray  *dataArray = [jsonDict[@"data"] mutableCopy];
    for (NSInteger i=0; i<dataArray.count; i++) {
        NSMutableDictionary *info = [dataArray[i] mutableCopy];
        NSLog(@"Start downloading[%ld]url\n%@",(long)i,info[@"url"]);
        NSString *downloadPath = info[@"url"];
        NSArray *array = [info[@"url"] componentsSeparatedByString:@"/"];
        NSString *fullPath = [[NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject] stringByAppendingPathComponent:array.lastObject];
        //6.3 Perform clipping
        char *charPath1 = (char *)[downloadPath UTF8String];
        char *charPath2 = (char *)[fullPath UTF8String];
        getUrl(charPath2,charPath1);
    }
}


@end

Project structure, remember to remove bitcode as no and add a libz Library

Download Effect Pictures

Lazy link

I only compiled the real-time versions of armv7 and arm64, and I had to compile the rest myself.

At present, there is no problem of using multithreaded download to speed up the process.