ActiveWindowPID_windows

package main

import (
	"fmt"
	"log"
	"os/exec"
	"syscall"
	"time"
	"unsafe"
)

const (
	//Activates and displays the window.
	//If the window is minimized or maximized,
	//the system restores it to its original size and position
	swRestore = 9
)

var (
	user32                       = syscall.MustLoadDLL("user32.dll")
	procEnumWindows              = user32.MustFindProc("EnumWindows")
	procSetForegroundWindow      = user32.MustFindProc("SetForegroundWindow")
	procShowWindow               = user32.MustFindProc("ShowWindow")
	procGetWindowThreadProcessId = user32.MustFindProc("GetWindowThreadProcessId")
)

func EnumWindows(enumFunc uintptr, lparam uintptr) (err error) {
	r1, _, e1 := syscall.Syscall(procEnumWindows.Addr(), 2, uintptr(enumFunc), uintptr(lparam), 0)
	if r1 == 0 {
		if e1 != 0 {
			err = error(e1)
		} else {
			err = syscall.EINVAL
		}
	}
	return
}

func FindWindowByPID(pid int) (syscall.Handle, error) {
	var hwnd syscall.Handle
	cb := syscall.NewCallback(func(h syscall.Handle, p uintptr) uintptr {
		var i uintptr
		procGetWindowThreadProcessId.Call(uintptr(h), uintptr(unsafe.Pointer(&i)))
		if uintptr(pid) == i {
			hwnd = h
			return 0
		}
		return 1
	})
	EnumWindows(cb, 0)
	if hwnd == 0 {
		return 0, fmt.Errorf("no window with pid found with pid %d", pid)
	}
	return hwnd, nil
}

func main() {
	cmd := exec.Command(`notepad`)
	cmd.Start()

	pid := cmd.Process.Pid
	log.Println(pid)

	time.Sleep(5 * time.Second)

	h, err := FindWindowByPID(pid)
	if err != nil {
		log.Fatal(err)
	}
	procShowWindow.Call(uintptr(h), swRestore)
	procSetForegroundWindow.Call(uintptr(h))
}