根据size截取屏幕中间矩形区域

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    UIImageView *imageView=[[UIImageView alloc]initWithFrame:CGRectMake(50, 100, 200, 200)];
    imageView.backgroundColor=[UIColor redColor];
    //显示截取的图
    imageView.image=[self cutCenterImage:[UIImage imageNamed:@"1.jpg"] size:CGSizeMake(200, 200)];
    [self.view addSubview:imageView];
    
    
}
#pragma mark 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 = [[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();
    
    return tmp;
}