问FLTK Fl_Box不会更改其图像()

EN

static Fl_RGB_Image *greenRgb;
static Fl_RGB_Image *redRgb;
static Fl_RGB_Image *blackRgb;

Fl_Box *makeTristate(char const *name) {
    Fl_Box *ret = new Fl_Box(0, 0, 300, 32, name);
    ret->labelsize(24);
    ret->align(FL_ALIGN_RIGHT | FL_ALIGN_IMAGE_NEXT_TO_TEXT);
    ret->image(whiteRgb);
    return ret;
}

void setTristate(Fl_Box *tris, int state) {
    Fl_RGB_Image *wanted;
    if (state < 0) {
        wanted = redRgb;
    } else if (state > 0) {
        wanted = greenRgb;
    } else {
        wanted = blackRgb;
    }
    if (wanted != tris->image()) {
        LOG_PRINTF("changing %p (%s) from %p to %p", tris, tris->label(), tris->image(), wanted);
        tris->image(wanted);
        tris->redraw();
    }
}