Accessing Foundation Framework from Go with cgo

foundation.go

// How to build: "CC=clang go build"

package main

import (
	"fmt"
	"net/url"
	"strconv"
	"unsafe"
)

//#cgo CFLAGS: -x objective-c
//#cgo LDFLAGS: -framework Foundation
//#include "foundation.h"
import "C"

// NSString -> C string
func cstring(s *C.NSString) *C.char { return C.nsstring2cstring(s) }

// NSString -> Go string
func gostring(s *C.NSString) string { return C.GoString(cstring(s)) }

// NSNumber -> Go int
func goint(i *C.NSNumber) int { return int(C.nsnumber2int(i)) }

// NSArray length
func nsarraylen(arr *C.NSArray) uint { return uint(C.nsarraylen(arr)) }

// NSArray item
func nsarrayitem(arr *C.NSArray, i uint) unsafe.Pointer {
	return C.nsarrayitem(arr, C.ulong(i))
}

// NSURL -> Go url.URL
func gourl(nsurlptr *C.NSURL) *url.URL {
	nsurl := *C.nsurldata(nsurlptr)

	userInfo := url.UserPassword(
		gostring(nsurl.user),
		gostring(nsurl.password),
	)

	host := gostring(nsurl.host)

	if nsurl.port != nil {
		port := goint(nsurl.port)
		host = host + ":" + strconv.FormatInt(int64(port), 10)
	}

	return &url.URL{
		Scheme:   gostring(nsurl.scheme),
		User:     userInfo, // username and password information
		Host:     host,     // host or host:port
		Path:     gostring(nsurl.path),
		RawQuery: gostring(nsurl.query),    // encoded query values, without '?'
		Fragment: gostring(nsurl.fragment), // fragment for references, without '#'
	}
}

// NSArray<NSURL> -> Go []url.URL
func gourls(arr *C.NSArray) []url.URL {
	var result []url.URL
	length := nsarraylen(arr)

	for i := uint(0); i < length; i++ {
		nsurl := (*C.NSURL)(nsarrayitem(arr, i))
		u := gourl(nsurl)
		result = append(result, *u)
	}

	return result
}

func UserApplicationSupportDirectories() []url.URL {
	return gourls(C.UserApplicationSupportDirectories())
}

func main() {
	fmt.Printf("%#+v\n", UserApplicationSupportDirectories())
}

foundation.h

#import <Foundation/Foundation.h>

typedef struct _NSURLdata {
    NSString *scheme;
    NSString *user;
    NSString *password;
    NSString *host;
    NSNumber *port;
    NSString *path;
    NSString *query;
    NSString *fragment;
} NSURLdata;

const char* nsstring2cstring(NSString*);
int nsnumber2int(NSNumber*);
unsigned long nsarraylen(NSArray*);
const void* nsarrayitem(NSArray*, unsigned long);
const NSURLdata* nsurldata(NSURL*);
const NSArray* UserApplicationSupportDirectories();

foundation.m

#import "foundation.h"

const char*
nsstring2cstring(NSString *s) {
    if (s == NULL) { return NULL; }

    const char *cstr = [s UTF8String];
    return cstr;
}

int
nsnumber2int(NSNumber *i) {
    if (i == NULL) { return 0; }
    return i.intValue;
}

unsigned long
nsarraylen(NSArray *arr) {
    if (arr == NULL) { return 0; }
    return arr.count;
}

const void*
nsarrayitem(NSArray *arr, unsigned long i) {
	if (arr == NULL) { return NULL; }
	return [arr objectAtIndex:i];
}

const NSURLdata*
nsurldata(NSURL *url) {
    NSURLdata *urldata = malloc(sizeof(NSURLdata));
    urldata->scheme = url.scheme;
    urldata->user = url.user;
    urldata->password = url.password;
    urldata->host = url.host;
    urldata->port = url.port;
    urldata->path = url.path;
    urldata->query = url.query;
    urldata->fragment = url.fragment;
    return urldata;
}

const NSArray*
UserApplicationSupportDirectories() {
    NSFileManager *manager = [NSFileManager defaultManager];
    return [manager URLsForDirectory: NSApplicationSupportDirectory
                           inDomains: NSUserDomainMask];
}

This code

package main

/*
#cgo CFLAGS: -x objective-c
#cgo LDFLAGS: -framework Foundation
#import <Foundation/Foundation.h>
void hello() {
    NSLog(@"Hello World");
}
*/
import "C"

func main() {
    C.hello()
}

from your Coderwall post doesn't work on macOS 10.12 beta (16A254g). If you install Xcode v7.3.1 (7D1014) and run CC=clang go build this will fail to compile with errors like these.

In file included from /System/Library/Frameworks/Foundation.framework/Headers/Foundation.h:10:
In file included from /System/Library/Frameworks/Foundation.framework/Headers/NSArray.h:5:
/System/Library/Frameworks/Foundation.framework/Headers/NSObject.h:44:12: error: unknown property attribute 'class'
@property (class, readonly) BOOL supportsSecureCoding;

Just a heads up! :)