让提示界面始终保持在页面的最前端（页面置顶）

UIView *parentView = nil;
        NSArray* windows = [UIApplication sharedApplication].windows;
        UIView* window = [windows objectAtIndex:0];
        if(window.subviews.count > 0)
        {
            parentView = [window.subviews objectAtIndex:0];
           [parentView addSubview:m_loadingImageView];
        }