Set a pac url to com.apple.CommCenter (ip1), i.e. cellular data

pac_add.c

//
//  pac_add.m
//  pac_add
//
// https://gist.github.com/esterTion/8bdeb78b35c172a4485f6d9e29476336/edit
//
//  Created by esterTion on 2019/4/19.
//  xcrun -sdk iphoneos clang -isysroot $THEOS/sdks/iPhoneOS11.2.sdk -arch arm64 -I$THEOS/include -framework Foundation -framework SystemConfiguration -L$THEOS/lib -lactivator main.m -o pac_add
//

#import <Foundation/Foundation.h>
#import <stdio.h>
#import "GetBSDProcessList.h"
#import <libactivator/libactivator.h>
#import <SystemConfiguration/SystemConfiguration.h>


static void killWifid() {
	kinfo_proc *results = NULL;
	size_t procCount=0;
	GetBSDProcessList(&results, &procCount);
	kinfo_proc* current_process = results;
	for (int i=0; i<procCount && i<max_processes; i++)
	{
		if (strcmp(current_process->kp_proc.p_comm, "wifid") == 0) {
			kill(current_process->kp_proc.p_pid, 9);
			break;
		}
		current_process += 1;
	}
	free(results);
	results = NULL;
}

// https://stackoverflow.com/a/40305705
#import <ifaddrs.h>
#import <arpa/inet.h>

static BOOL CheckWiFi() {

    struct ifaddrs *interfaces = NULL;
    struct ifaddrs *temp_addr = NULL;

    BOOL hasWifi = NO;

    int err = getifaddrs(&interfaces);
    if(err == 0) {

        temp_addr = interfaces;

        while(temp_addr) {

            if(temp_addr->ifa_addr->sa_family == AF_INET) {

                struct sockaddr_in *addr = (struct sockaddr_in *)temp_addr->ifa_addr;

                if(memcmp(temp_addr->ifa_name, "en", 2) == 0) {
                    hasWifi = YES;
                    break;
                }
            }

            temp_addr = temp_addr->ifa_next;
        }
    }

    freeifaddrs(interfaces);
    return hasWifi;
}

int main (int argc, const char * argv[])
{
    @autoreleasepool
    {
        if (argc < 2) exit(1);
        NSString *ip1Id = NULL;
        NSString *filePath = @"/var/preferences/SystemConfiguration/preferences.plist";
        bool restartWifi = false;
        bool restartHotspot = false;
        for (int i=2; i<argc; i++) {
          if (strcmp(argv[i], "-wifi") == 0) restartWifi = true;
          else if (strcmp(argv[i], "-hotspot") == 0) restartHotspot = true;
        }
        while (1) {
            NSMutableDictionary* preferences = [NSMutableDictionary dictionaryWithContentsOfFile:filePath];
            if (!ip1Id) {
                for (NSString *serviceId in preferences[@"NetworkServices"]) {
                    if ([preferences[@"NetworkServices"][serviceId][@"UserDefinedName"] isEqualToString:@"com.apple.CommCenter (ip1)"]) {
                        ip1Id = serviceId;
                        break;
                    }
                }
                printf("%s\n", [ip1Id cStringUsingEncoding:NSUTF8StringEncoding]);
            }
            
            if (!ip1Id) exit(2);
            //printf("check\n");
            if (!preferences[@"NetworkServices"][ip1Id][@"Proxies"]) {
                if (CheckWiFi()) {
                    //printf("current in wifi\n");
                } else {

                    SCDynamicStoreRef sc = SCDynamicStoreCreate(NULL, CFSTR("com.apple.wirelessmodemsettings.MISManager"), NULL, NULL);
                    NSDictionary* info = (__bridge_transfer NSDictionary*)SCDynamicStoreCopyValue(sc, CFSTR("com.apple.MobileInternetSharing"));
                    CFRelease(sc);
                    bool hotspotOn = [info[@"ExternalInterfaces"] count] > 0;
                    //printf("hotspot state: %d\n", hotspotOn);
                    [info release];

                    preferences[@"NetworkServices"][ip1Id][@"Proxies"] = @{
                                                                        @"ProxyAutoConfigEnable": @1,
                                                                        @"ProxyAutoConfigURLString": [[NSString alloc] initWithCString:argv[1] encoding:NSUTF8StringEncoding]
                                                                        };
                    bool success = [preferences writeToFile:filePath atomically:true];
                    killWifid(); // first kill wifid to let system reload cellular proxy
                    LAEvent *event;
                    sleep(1);    // sleep 1 sec, and then toggle wifi twice, to reset wifi state
                    if (restartHotspot && hotspotOn) {
                        printf("toggle hotspot\n");
                        event = [LAEvent eventWithName:@"switch-flip.com.a3tweaks.switch.hotspot" mode:[[LAActivator sharedInstance] currentEventMode]];
                        [[LAActivator sharedInstance] sendEvent:event toListenerWithName:@"switch-flip.com.a3tweaks.switch.hotspot"];
                    }
                    sleep(1);
                    if (restartWifi) {
                        printf("toggle wifi\n");
                        event = [LAEvent eventWithName:@"switch-flip.com.a3tweaks.switch.wifi" mode:[[LAActivator sharedInstance] currentEventMode]];
                        [[LAActivator sharedInstance] sendEvent:event toListenerWithName:@"switch-flip.com.a3tweaks.switch.wifi"];
                    }
                        sleep(3);
                    if (restartWifi) {
                        printf("toggle wifi\n");
                        event = [LAEvent eventWithName:@"switch-flip.com.a3tweaks.switch.wifi" mode:[[LAActivator sharedInstance] currentEventMode]];
                        [[LAActivator sharedInstance] sendEvent:event toListenerWithName:@"switch-flip.com.a3tweaks.switch.wifi"];
                    }

                    if (restartHotspot && hotspotOn) {
                        printf("toggle hotspot\n");
                        sleep(3);
                        event = [LAEvent eventWithName:@"switch-flip.com.a3tweaks.switch.hotspot" mode:[[LAActivator sharedInstance] currentEventMode]];
                        [[LAActivator sharedInstance] sendEvent:event toListenerWithName:@"switch-flip.com.a3tweaks.switch.hotspot"];
                    }
                    //printf("write: %d\n", success);
                }
            }
            [preferences release];
            sleep(5);
        }
    }
    return 0;
}