import threading
import cv2


buffer = []


def worker():
    global buffer
    while True:
        cv2.waitKey(50)
        try:
            if len(buffer) > 0:
                task = buffer.pop(0)
                if task is None:
                    cv2.destroyAllWindows()
                else:
                    flag, img, title = task
                    cv2.imshow(flag, img)
                    cv2.setWindowTitle(flag, title)
                    cv2.setWindowProperty(flag, cv2.WND_PROP_TOPMOST, 1)
        except Exception as e:
            print(e)
    pass


def show_preview(flag, img, title='preview'):
    buffer.append((flag, img[..., ::-1].copy(), title))


def close_all_preview():
    buffer.append(None)


threading.Thread(target=worker, daemon=True).start()