2 Conversion and Downcast

The following example shows how you can read the tag in an mp3 file, convert it into an mp4 tag, and write it to an m4a file.

2.1 Converting from a Box<dyn AudioTag to Another

use audiotags::{Config, Tag, TagType};

fn main() {
    // we have an mp3 and an m4a file
    const MP3_FILE: &'static str = "assets/a.mp3";
    const M4A_FILE: &'static str = "assets/a.m4a";
    // read tag from the mp3 file. Using `default()` so that the
    // type of tag is guessed from the file extension
    let mut mp3tag = Tag::default().read_from_path(MP3_FILE).unwrap();
    // set the title
    mp3tag.set_title("title from mp3 file");
    // we can convert it to an mp4 tag and save it to an m4a file.
    let mut mp4tag = mp3tag.to_dyn_tag(TagType::Mp4);
    mp4tag.write_to_path(M4A_FILE).unwrap();

    // reload the tag from the m4a file; this time specifying the
    // tag type (you can also use `default()`)
    let mut mp4tag = Tag::new()
        .with_tag_type(TagType::Mp4)
        .read_from_path(M4A_FILE)
        .unwrap();
    // the tag originated from an mp3 file is successfully written
    // to an m4a file!
    assert_eq!(mp4tag.title(), Some("title from mp3 file"));
    // multiple artists
    mp4tag.add_artist("artist1 of mp4");
    mp4tag.add_artist("artist2 of mp4");
    assert_eq!(
        mp4tag.artists(),
        Some(vec!["artist1 of mp4", "artist2 of mp4"])
    );
    // convert to id3 tag, which does not support multiple artists
    mp4tag.set_config(Config::default().sep_artist("/"));
    // separator is by default `;` but we can customise it
    let mp3tag = mp4tag.to_dyn_tag(TagType::Id3v2);
    assert_eq!(mp3tag.artist(), Some("artist1 of mp4/artist2 of mp4"));
}

2.2 Converting into a Concrete Type (Downcasting)

Can I convert into a concrete type?

Yes, you can directly convert .into() it (this is technically known as a “downcast”):

use audiotags::{FlacTag, Tag};

fn main() {
    let id3v2tag = Tag::default().read_from_path("assets/a.mp3").unwrap();
    let _flactag: FlacTag = id3v2tag.into();
    // of course, you can `let id3v2tag_concrete: Id3v2Tag = id3v2tag.into();`
}

You can even convert .into() the ‘backend’ tag type:

use audiotags::Tag;

fn main() {
    let mp3tag = Tag::default().read_from_path("assets/a.mp3").unwrap();
    let flactag: metaflac::Tag = mp3tag.into(); // into the 'backend' tag
    // then you can use methods specific to metaflac
    let _ = flactag.get_streaminfo();
}

This is useful when you really need to use the methods not provided by audiotags::traits::*.

You can also downcast the concrete audiotags::FlacTag to metaflac::Tag and so no.

2.3 Upcasting

Since you’re allowed to downcast, naturally you can also upcast:

use audiotags::*;

fn main() {
    let mut innertag = metaflac::Tag::default();
    innertag
        .vorbis_comments_mut()
        .set_title(vec!["title from metaflac::Tag"]);
    let tag: FlacTag = innertag.into();
    let _id3tag = tag.to_dyn_tag(TagType::Id3v2);
    // in this case the "title" metadata will be
    // losslessly written into the id3tag.
    // However, if you have FLAC-specific fields,
    // they will be lost upon conversion
}