ios 给出一张图片，返回一个指定大小的图片

/*--------------------------------------------------

 *  根据size截取图片中间矩形区域的图片这里的size是正方形

 ---------------------------------------------------*/

-(UIImage *)cutCenterImage:(UIImage *)image size:(CGSize)size

{

    CGSize imageSize = image.size;

    CGRect rect;

    //根据图片的大小计算出图片中间矩形区域的位置与大小

    if (imageSize.width > imageSize.height)

    {

        float leftMargin = (imageSize.width - imageSize.height) *0.5;

        rect = CGRectMake(leftMargin, 0, imageSize.height, imageSize.height);

    }

    else

    {

        float topMargin = (imageSize.height - imageSize.width) *0.5;

        rect = CGRectMake(0, topMargin, imageSize.width, imageSize.width);

    }

    

    CGImageRef imageRef = image.CGImage;

    //截取中间区域矩形图片

    CGImageRef imageRefRect = CGImageCreateWithImageInRect(imageRef, rect);

    

    UIImage *tmp = [[UIImagealloc]initWithCGImage:imageRefRect];

    CGImageRelease(imageRefRect);

    

    UIGraphicsBeginImageContext(size);

    CGRect rectDraw = CGRectMake(0, 0, size.width, size.height);

    [tmp drawInRect:rectDraw];

    //从当前context中创建一个改变大小后的图片

    tmp = UIGraphicsGetImageFromCurrentImageContext();

    // 使当前的context出堆栈

    UIGraphicsEndImageContext();

    

    return tmp;

}