c++opencv调用setmousecallback时运行顺序问题

问题遇到的现象和发生背景 c++opencv调用setmousecallback时运行顺序不正确，就是代码中先输出1，再出现图片

问题相关代码，请勿粘贴截图

#include
#include
using namespace std;
using namespace cv;
#define WINDOW "原图"
Mat g_srcImage;
Point previousPoint;
void On_mouse(int event, int x, int y, int flags, void*);
int main()
{
    g_srcImage = imread("test.png", 1);
    imshow(WINDOW, g_srcImage);
    setMouseCallback(WINDOW, On_mouse, 0);
    cout << 1;
    waitKey(0);
    return 0;
}
void On_mouse(int event, int x, int y, int flags, void*)
{

    if (event == EVENT_LBUTTONDOWN)
    {
        previousPoint = Point(x, y);
    }
    else if (event == EVENT_MOUSEMOVE && (flags & EVENT_FLAG_LBUTTON))
    {
        Point pt(x, y);
        line(g_srcImage, previousPoint, pt, Scalar(0, 0, 255), 2, 5, 0);
        previousPoint = pt;
        imshow(WINDOW, g_srcImage);
    }
}

这明显是setmousecallback是开启了一个子线程监听，所以才会先输出1