iOS 使用UIImageWriteToSavedPhotosAlbum()保存到相册图片被压缩的问题

起源：前两天有人说使用SDWebImage下载图片后保存到相册发现图片被压缩了，原图2.2M，保存到相册后传到Mac上显示图片大小只有500K左右。于是尝试了一下，代码如下：

    NSURL *imgUrl = [NSURL URLWithString:@"https://xxxx.png"];

    [[SDWebImageDownloader sharedDownloader] downloadImageWithURL:imgUrl completed:^(UIImage * _Nullable image, NSData * _Nullable data, NSError * _Nullable error, BOOL finished) {
        if (image) {
            UIImageWriteToSavedPhotosAlbum(image, self, @selector(image:didFinishSavingWithError:contextInfo:), nil);
            //清除sd缓存，防止图片被sdwebimage缓存
//            [[SDImageCache sharedImageCache] clearDiskOnCompletion:nil];
//            [[SDImageCache sharedImageCache] clearMemory];
        }
    }];

然后到相册里一查还真是变成了500多k、首先查看data.length，大小和原图一致，确定了不是SDWebImage 框架的问题。那么一定是这个函数对图片进行了压缩：

// Adds a photo to the saved photos album.  The optional completionSelector should have the form:
//  - (void)image:(UIImage *)image didFinishSavingWithError:(NSError *)error contextInfo:(void *)contextInfo;
UIKIT_EXTERN void UIImageWriteToSavedPhotosAlbum(UIImage *image, __nullable id completionTarget, __nullable SEL completionSelector, void * __nullable contextInfo) __TVOS_PROHIBITED;

于是乎使用Photos框架中的PHPhotoLibrary来存储图片以解决这个问题，代码如下：

[[SDWebImageDownloader sharedDownloader] downloadImageWithURL:imgUrl completed:^(UIImage * _Nullable image, NSData * _Nullable data, NSError * _Nullable error, BOOL finished)  {
        
        NSLog(@"dataLen:%lu",data.length);
        if (finished)
        {
            UIImage * imgsave = image;
            NSString * path = NSHomeDirectory();
            //图片存储的沙盒路径
            NSString * Pathimg = [path stringByAppendingString:@"/Documents/test.png"];
            [UIImagePNGRepresentation(imgsave) writeToFile:Pathimg atomically:YES];
            
            //使用url存储到相册
            NSURL *imagePathUrl = [NSURL fileURLWithPath:Pathimg];
            [PHPhotoLibrary requestAuthorization:^(PHAuthorizationStatus status) {
                if (status == PHAuthorizationStatusAuthorized) {//权限判断
                    PHPhotoLibrary *library = [PHPhotoLibrary sharedPhotoLibrary];
                    [library performChanges:^{
                        [PHAssetChangeRequest creationRequestForAssetFromImageAtFileURL:imagePathUrl];
                    } completionHandler:^(BOOL success, NSError * _Nullable error) {
                        if (success) NSLog(@"保存成功");
                        else NSLog(@"保存失败");
                    }];
                }
            }];
        }
    }];