如何让一个button 的 backgroundimage 等比例缩放 background imageview 并不完美

 UIButton *button= [[UIButton alloc] initWithFrame:CGRectMake(0,0,70,70)];
 button.buttonType = UIButtonTypeCustom;
 UIImage *buttonImage = [UIImage imageNamed:@"image.png"];
 UIImage *stretchableButtonImage = [buttonImage stretchableImageWithLeftCapWidth:12 topCapHeight:0]; 
 [button setBackgroundImage:stretchableButtonImage forState:UIControlStateNormal];