拖线拿到按钮，只设置一次就好，awakeFromNib,点属性currentBackgroundImage，获取到当前的背景图片，处理拉伸，重新给button设置处理后的拉伸图片

被拉伸，注意四个角

处理后

-(void)awakeFromNib

{

[super awakeFromNib];

UIImage *image = _loginRegisterButton.currentBackgroundImage;

image = [image stretchableImageWithLeftCapWidth:image.size.width * 0.5 topCapHeight:image.size.height * 0.5];

[_loginRegisterButton setBackgroundImage:image forState:UIControlStateNormal];

}