一个通用的给UIButton的background的图片做拉伸的委托

UIButton+Stretchable.m

@implementation UIButton (StretchableUIButton)
/* Automatically set cap insets for the background image. This assumes that
 the image is a standard slice size with a 1 px stretchable interior */
- (void)setBackgroundImageStretchableForState:(UIControlState)controlState
{
    UIImage *image = [self backgroundImageForState:controlState];
    if (image)
    {
        CGFloat capWidth =  floorf(image.size.width / 2);
        CGFloat capHeight =  floorf(image.size.height / 2);
        UIImage *capImage = [image resizableImageWithCapInsets:
                             UIEdgeInsetsMake(capHeight, capWidth, capHeight, capWidth)];
        
        [self setBackgroundImage:capImage forState:controlState];
    }
}