UIImage+Dummy 提供方法来创建虚拟映像。
UIImage *image1 = [UIImage imageNamed:@"dummy.150x200"]; UIImage *image2 = [UIImage imageNamed:@"dummy.150x200.#407EFF"]; UIImage *image3 = [UIImage imageNamed:@"dummy.150x200.blue"]; UIImage *image4 = [UIImage imageNamed:@"dummy.150x200.blue..image.png"]; //如果 'image.png' 存在, 'image.png' UIImage 对象已经创建好了; //如果不存在，则创建虚拟 UIImage 对象.
