add get DisplaysNum support

robotgo.go

@@ -239,6 +239,10 @@ func displayIdx(id ...int) int {
 	return display
 }
 
+func getNumDisplays() int {
+	return int(C.get_num_displays())
+}
+
 // SysScale get the sys scale
 func SysScale(displayId ...int) float64 {
 	display := displayIdx(displayId...)

robotgo_mac_win.go

@@ -50,3 +50,8 @@ func ActivePID(pid int32, args ...int) error {
 	internalActive(pid, hwnd)
 	return nil
 }
+
+// DisplaysNum get the count of displays
+func DisplaysNum() int {
+	return getNumDisplays()
+}

robotgo_x11.go

@@ -17,6 +17,7 @@ import (
 	"errors"
 	"log"
 
+	"github.com/robotn/xgb/xinerama"
 	"github.com/robotn/xgb/xproto"
 	"github.com/robotn/xgbutil"
 	"github.com/robotn/xgbutil/ewmh"
@@ -149,3 +150,24 @@ func GetXidFromPid(xu *xgbutil.XUtil, pid int32) (xproto.Window, error) {
 
 	return 0, errors.New("failed to find a window with a matching pid.")
 }
+
+// DisplaysNum get the count of displays
+func DisplaysNum() int {
+	c, err := xgb.NewConn()
+	if err != nil {
+		return 0
+	}
+	defer c.Close()
+
+	err = xinerama.Init(c)
+	if err != nil {
+		return 0
+	}
+
+	reply, err := xinerama.QueryScreens(c).Reply()
+	if err != nil {
+		return 0
+	}
+
+	return int(reply.Number)
+}

screen/goScreen.h

@@ -96,6 +96,33 @@ char* get_XDisplay_name(){
 	#endif
 }
 
+#if defined(IS_WINDOWS)
+bool CALLBACK MonitorEnumProc(HMONITOR hMonitor, HDC hdcMonitor, LPRECT lprcMonitor, LPARAM dwData) {
+    uint32_t *count = (uint32_t*)dwData;
+    (*count)++;
+    return true;
+}
+#endif
+
+uint32_t get_num_displays() {
+#if defined(IS_MACOSX)
+	uint32_t count = 0;
+	if (CGGetActiveDisplayList(0, nil, &count) == kCGErrorSuccess) {
+		return count;
+	}
+	return 0;
+#elif defined(USE_X11)
+	return 0;
+#elif defined(IS_WINDOWS)
+    uint32_t count = 0;
+    if (EnumDisplayMonitors(NULL, NULL, MonitorEnumProc, (LPARAM)&count)) {
+        return count;
+	}
+	return 0;
+#endif
+}
+
+
 void bitmap_dealloc(MMBitmapRef bitmap){
 	if (bitmap != NULL) {
 		destroyMMBitmap(bitmap);