在某些场景下，我们需要监测系统中网络接入情况的变化，据此作出相应的处理。
例如，用户成功接入了某一个网络，移动场景下从一个WiFi网络切换到另一个网络，网络断开等。

下面基于Go语言，在Mac平台实现了一个检测网络变化的例子。再这个示例中，使用到了cgo进行C和Go混合编程。Go语言负责处理业务逻辑，具体的操作系统事件的监测由C语言的接口完成。

在循环中监测到变化，调用C语言中定义的callback函数，具体的处理动作再调用Go中定义的networkchange()函数处理。

package main

/*
#cgo LDFLAGS: -framework SystemConfiguration -framework CoreFoundation
#include <SystemConfiguration/SystemConfiguration.h>
#include <stdlib.h>

void callback(SCDynamicStoreRef store, CFArrayRef changedKeys, void *info) {
    char **services = (char **)info;
    CFIndex count = CFArrayGetCount(changedKeys);
    for (CFIndex i = 0; i < count; i++) {
        CFStringRef key = CFArrayGetValueAtIndex(changedKeys, i);
        const char *cKey = CFStringGetCStringPtr(key, kCFStringEncodingUTF8);
        if (cKey) {
            services[i] = strdup(cKey);
        }
    }
}

    extern void networkchange();
    void callback(SCDynamicStoreRef store, CFArrayRef changedKeys, void *info) {
        printf("network event detected\n");
        networkchange();
    }
*/
import "C"
import (
    "fmt"
    "time"
    "unsafe"
)

func main() {
    storeName := C.CString("NetworkChangeDetector")
    defer C.free(unsafe.Pointer(storeName))

    cfString := C.CFStringCreateWithCString(C.kCFAllocatorDefault, storeName, C.kCFStringEncodingUTF8)
    defer C.CFRelease(C.CFTypeRef(cfString)) // 释放 CFStringRef

    store := C.SCDynamicStoreCreate(nil, storeName, C.SCDynamicStoreCallBack(C.callback), nil)
    if *(*C.uintptr_t)(unsafe.Pointer(&store)) == 0 {
        fmt.Println("Failed to create SCDynamicStore")
        return
    }

    keys := C.CFArrayCreateMutable(nil, 0, &C.kCFTypeArrayCallBacks)
    defer C.CFRelease(C.CFTypeRef(keys))

    key1 := C.CFStringCreateWithCString(C.kCFAllocatorDefault, C.CString("State:/Network/Interface"), C.kCFStringEncodingUTF8)
    defer C.CFRelease(C.CFTypeRef(key1))
    C.CFArrayAppendValue(keys, unsafe.Pointer(key1))

    key2 := C.CFStringCreateWithCString(C.kCFAllocatorDefault, C.CString("State:/Network/Global/IPv4"), C.kCFStringEncodingUTF8)
    defer C.CFRelease(C.CFTypeRef(key2))
    C.CFArrayAppendValue(keys, unsafe.Pointer(key2))


    key3 := C.CFStringCreateWithCString(C.kCFAllocatorDefault, C.CString("State:/Network/Global/IPv6"), C.kCFStringEncodingUTF8)
    defer C.CFRelease(C.CFTypeRef(key1))
    C.CFArrayAppendValue(keys, unsafe.Pointer(key3))

    var context C.SCDynamicStoreContext
    context.version = 0
    context.info = nil
    context.retain = nil
    context.release = nil
    context.copyDescription = nil

    emptyArray := C.CreateEmptyCFArray()
    C.SCDynamicStoreSetNotificationKeys(store, keys, emptyArray)

    rl := C.CFRunLoopGetCurrent()
    src := C.SCDynamicStoreCreateRunLoopSource(nil, store, 0)
    C.CFRunLoopAddSource(rl, src, C.kCFRunLoopDefaultMode)
    C.CFRunLoopRun()
}


//export onNetworkChange
func onNetworkChange(count C.int, keys **C.char, info unsafe.Pointer) {
    ctx := (*Context)(info)
    fmt.Printf("Network services changed: %s\n", ctx.message)
    for i := 0; i < int(count); i++ {
        key := C.GoString(*(**C.char)(unsafe.Pointer(uintptr(unsafe.Pointer(keys)) + uintptr(i)*unsafe.Sizeof(uintptr(0)))))
        if key != "" {
            fmt.Println(key)
        }
    }