Why choose drop companion_tag instead of just simply convert it? Are there some reasons I don't consider?

[Linxuanhua1]

lofty-rs/lofty/src/tag/mod.rs

Lines 242 to 250 in f3fcfd0

	pub fn re_map(&mut self, tag_type: TagType) {
	if let Some(companion_tag) = self.companion_tag.take() {
	log::warn!("Discarding format-specific items due to remap");
	drop(companion_tag);
	}
	self.retain(|i| i.re_map(tag_type));
	self.tag_type = tag_type
	}

For example, just simply convert ----:com.apple.iTunes:ARTISTS -> TXXX:ARTISTS

Because most of softwares do the same thing, such as mp3tag、musicbrainz picard and foobar

[Serial-ATA]

The companion tag only holds the items that don't have a generic conversion. It even logs a warning saying that the tag will no longer hold those format-specific items:

log::warn!("Discarding format-specific items due to remap"); 

If you start with an ID3v2 tag and convert to another format, it wouldn't make sense to keep those ID3v2-specific items around.

[Linxuanhua1]

If you start with an ID3v2 tag and convert to another format, it wouldn't make sense to keep those ID3v2-specific items around.

Could you make an example？

[Serial-ATA]

Imagine you have foo.mp3 with an ID3v2 tag. If you do:

let file = lofty::read_from_path("foo.mp3")?;

Behind the scenes, Lofty is reading an MpegFile which gets converted to a TaggedFile.

Notice that MpegFile contains concrete tag types like Id3v2Tag, while TaggedFile only contains Tags. When you do that conversion, most but not all items make it into that Tag. To prevent losing all those items, we have an immutable "companion tag" that gets stored in the Tag:

lofty-rs/lofty/src/id3/v2/tag.rs

Lines 1291 to 1301 in f3fcfd0

	impl From<Id3v2Tag> for Tag {
	fn from(input: Id3v2Tag) -> Self {
	let (remainder, mut tag) = input.split_tag();
	if unsafe { global_options().preserve_format_specific_items } && remainder.0.len() > 0 {
	tag.companion_tag = Some(CompanionTag::Id3v2(remainder.0));
	}
	tag
	}
	}

Then extracted if you convert it back:

lofty-rs/lofty/src/id3/v2/tag.rs

Lines 1303 to 1313 in f3fcfd0

	impl From<Tag> for Id3v2Tag {
	fn from(mut input: Tag) -> Self {
	if unsafe { global_options().preserve_format_specific_items } {
	if let Some(companion) = input.companion_tag.take().and_then(CompanionTag::id3v2) {
	return SplitTagRemainder(companion).merge_tag(input);
	}
	}
	SplitTagRemainder::default().merge_tag(input)
	}
	}

So you can do:

let mut file = lofty::read_from_path("foo.mp3")?;
let tag = file.remove(TagType::Id3v2).unwrap();

// Convert the tag back to an ID3v2 tag
let id3v2: Id3v2Tag = tag.into();

// Then back to a generic tag
let tag: Tag = id3v2.into();

And in that process, you don't lose any items.

[Linxuanhua1]

And in that process, you don't lose any items.

Sorry, maybe I don't express clearly. I mean I want to know what kind of tag need this processing. More exactly is what kind of field.

[Serial-ATA]

Do you mean what fields would be in the companion tag? That's any field that doesn't have an ItemKey, like RelativeVolumeAdjustmentFrame.

[Linxuanhua1]

Do you mean what fields would be in the companion tag? That's any field that doesn't have an ItemKey, like RelativeVolumeAdjustmentFrame.

Yes, thank you!