按大小裁剪为指定的尺寸并解决图片裁剪之后显示不正确的问题

+ (UIImage *)cutCenterImageSize:(CGSize)size iMg:(UIImage *)img {

    CGFloat scale = [UIScreen mainScreen].scale;

    size.width = size.width*scale;

    size.height = size.height *scale;

    CGSize imageSize = img.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);

    }
//记录旋转方向
    CGAffineTransform transform = CGAffineTransformIdentity;
    switch (img.imageOrientation) {
        case UIImageOrientationDown:
        case UIImageOrientationDownMirrored:
            transform = CGAffineTransformTranslate(transform, img.size.width, img.size.height);
            transform = CGAffineTransformRotate(transform,M_PI);
            break;
            
        case UIImageOrientationLeft:
        case UIImageOrientationLeftMirrored:
            transform = CGAffineTransformTranslate(transform, img.size.width,0);
            transform = CGAffineTransformRotate(transform,M_PI_2);
            break;
            
        case UIImageOrientationRight:
        case UIImageOrientationRightMirrored:
            transform = CGAffineTransformTranslate(transform,0, img.size.height);
            transform = CGAffineTransformRotate(transform, -M_PI_2);
            break;
        default:
            break;
    }
    

    

    CGImageRef imageRef = img.CGImage;

    //截取中间区域矩形图片

    CGImageRef imageRefRect =CGImageCreateWithImageInRect(imageRef, rect);

    

    UIImage *tmp = [[UIImage alloc] 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();
    // 旋转为正确的方向
    CGContextRef ctx = CGBitmapContextCreate(NULL, tmp.size.width, tmp.size.height,
                                                 CGImageGetBitsPerComponent(tmp.CGImage),0,
                                                 CGImageGetColorSpace(tmp.CGImage),
                                                 CGImageGetBitmapInfo(tmp.CGImage));
    CGContextConcatCTM(ctx, transform);
    

    return tmp;

}