图片缩小

//如果图片大于maxSize，将图片高宽等比例缩放至maxSize

- (UIImage *)imageWithConstrainSize:(CGSize)maxSize

{

    if(self.size.width <= maxSize.width && self.size.height <= maxSize.height) return self;

    

    CGFloat scale = MAX(maxSize.width/self.size.width, maxSize.height/self.size.height);

    CGSize scaleSize = CGSizeMake(scale * self.size.width,  scale * self.size.height);

    

    UIGraphicsBeginImageContext(scaleSize);

    // 绘制改变大小的图片

    [self drawInRect:CGRectMake(0, 0, scaleSize.width, scaleSize.height)];

    // 从当前context中创建一个改变大小后的图片

    UIImage* scaledImage = UIGraphicsGetImageFromCurrentImageContext();

    // 使当前的context出堆栈

    UIGraphicsEndImageContext();

    // 返回新的改变大小后的图片

    return scaledImage;

}