ios 图片拉伸（不变形）处理及图片初始化

 +(instancetype)imageWithName:(NSString *)imageName{    // 1.定义返回值    UIImage *resultImage = nil;    // 2.判断当前是否是ios7    if (isIOS7) {        // 注意, 不是所有的图片都有_os7结尾的图片        NSString *newImageName = [imageName stringByAppendingString:@"_os7"];        resultImage = [UIImage imageNamed:newImageName];        // 判断是否有ios7的图片    }    // 判断是否有ios7的图片    if (resultImage == nil) {        resultImage = [UIImage imageNamed:imageName];    }    // 4.返回结果    return resultImage;}+(instancetype)resizableImageWithName:(NSString *)imageName{    /*    // 1.创建图片    UIImage *image = [UIImage imageWithName:imageName];    // 2.处理图片    image =  [image stretchableImageWithLeftCapWidth:0.5 topCapHeight:0.5];    // 3.返回图片    return image;     */        return [self resizableImageWithName:imageName leftRatio:0.5 topRatio:0.5];        }+(instancetype)resizableImageWithName:(NSString *)imageName leftRatio:(CGFloat)leftRatio topRatio:(CGFloat)topRatio{    // 1.创建图片    UIImage *image = [UIImage imageWithName:imageName];    // 2.处理图片    CGFloat left = image.size.width * leftRatio;    CGFloat top = image.size.height * topRatio;        image =  [image stretchableImageWithLeftCapWidth:left topCapHeight:top];    // 3.返回图片    return image;}