// 1.1 创建UIImage对象
    UIImage *image = [UIImage imageNamed:@"buttongreen"];
    
    // 1.2 拿到image的尺寸
    CGFloat imageWidth = image.size.width;
    CGFloat imageHeight = image.size.height;
    
    // 1.3 返回一张受保护而且拉伸的图片--->CapInsets:哪些地方要保护
    // 方式一:
    (1)UIImage *resizableImage = [image resizableImageWithCapInsets:UIEdgeInsetsMake(imageHeight * 0.5, imageWidth * 0.5, imageHeight * 0.5 - 1, imageWidth * 0.5 - 1)];
     /*
     UIImageResizingModeTile,  平铺
     UIImageResizingModeStretch,  拉伸(伸缩)
     */
    (2)UIImage *resizableImage = [image resizableImageWithCapInsets:UIEdgeInsetsMake(imageHeight * 0.5, imageWidth * 0.5, imageHeight * 0.5 - 1, imageWidth * 0.5 - 1) resizingMode:UIImageResizingModeTile];

    // 方式二:
       // 右边需要被保护的区域 = 图片的width - leftCapWidth - 1
       // 底部需要被保护的区域(bottom cap) = height - topCapHeight - 1
    UIImage *resizableImage = [image stretchableImageWithLeftCapWidth:imageWidth * 0.5 topCapHeight:imageHeight * 0.5];

    // 2.把图片设置到按钮上
    [self.button setBackgroundImage:resizableImage forState:UIControlStateNormal];

由于可能在项目中很多地方要用到图片拉伸方法，所以把它定义成为一个类扩展

#import <UIKit/UIKit.h>
@interface UIImage (SJMExtention)
/**
 *  返回一张受保护的图片(被拉伸的)
 */
+ (instancetype)resizableImageWithLocalImageName:(NSString *)localImageName;
@end

#import "UIImage+SJMExtention.h"
@implementation UIImage (SJMExtention)

+ (instancetype)resizableImageWithLocalImageName:(NSString *)localImageName {
    // 创建图片对象
    UIImage *image = [UIImage imageNamed:localImageName];
    
    // 获取图片的尺寸
    CGFloat imageWidth = image.size.width;
    CGFloat imageHeight = image.size.height;
    
    return [image stretchableImageWithLeftCapWidth:imageWidth * 0.5 topCapHeight:imageHeight * 0.5];
}
@end

不用代码设置: 选中图片，选择水平和垂直拉伸，修改受保护的区域(宽/2, 高/2)