TagExt: Add TagExt::contains

src/ape/tag/mod.rs

@@ -211,6 +211,11 @@ impl Accessor for ApeTag {
 
 impl TagExt for ApeTag {
 	type Err = LoftyError;
+	type RefKey<'a> = &'a str;
+
+	fn contains<'a>(&'a self, key: Self::RefKey<'a>) -> bool {
+		self.items.iter().any(|i| i.key().eq_ignore_ascii_case(key))
+	}
 
 	fn is_empty(&self) -> bool {
 		self.items.is_empty()

src/id3/v1/tag.rs

@@ -168,6 +168,20 @@ impl Accessor for ID3v1Tag {
 
 impl TagExt for ID3v1Tag {
 	type Err = LoftyError;
+	type RefKey<'a> = &'a ItemKey;
+
+	fn contains<'a>(&'a self, key: Self::RefKey<'a>) -> bool {
+		match key {
+			ItemKey::TrackTitle => self.title.is_some(),
+			ItemKey::AlbumTitle => self.album.is_some(),
+			ItemKey::TrackArtist => self.artist.is_some(),
+			ItemKey::TrackNumber => self.track_number.is_some(),
+			ItemKey::Year => self.year.is_some(),
+			ItemKey::Genre => self.genre.is_some(),
+			ItemKey::Comment => self.comment.is_some(),
+			_ => false,
+		}
+	}
 
 	fn is_empty(&self) -> bool {
 		self.title.is_none()

src/id3/v2/tag.rs

@@ -420,6 +420,11 @@ impl Accessor for ID3v2Tag {
 
 impl TagExt for ID3v2Tag {
 	type Err = LoftyError;
+	type RefKey<'a> = &'a FrameID;
+
+	fn contains<'a>(&'a self, key: Self::RefKey<'a>) -> bool {
+		self.frames.iter().any(|frame| &frame.id == key)
+	}
 
 	fn is_empty(&self) -> bool {
 		self.frames.is_empty()

src/iff/aiff/tag.rs

@@ -137,6 +137,17 @@ impl AIFFTextChunks {
 
 impl TagExt for AIFFTextChunks {
 	type Err = LoftyError;
+	type RefKey<'a> = &'a ItemKey;
+
+	fn contains<'a>(&'a self, key: Self::RefKey<'a>) -> bool {
+		match key {
+			ItemKey::TrackTitle => self.name.is_some(),
+			ItemKey::TrackArtist => self.author.is_some(),
+			ItemKey::CopyrightMessage => self.copyright.is_some(),
+			ItemKey::Comment => self.annotations.is_some() || self.comments.is_some(),
+			_ => false,
+		}
+	}
 
 	fn is_empty(&self) -> bool {
 		matches!(

src/iff/wav/tag/mod.rs

@@ -131,6 +131,13 @@ impl Accessor for RIFFInfoList {
 
 impl TagExt for RIFFInfoList {
 	type Err = LoftyError;
+	type RefKey<'a> = &'a str;
+
+	fn contains<'a>(&'a self, key: Self::RefKey<'a>) -> bool {
+		self.items
+			.iter()
+			.any(|(item_key, _)| item_key.eq_ignore_ascii_case(key))
+	}
 
 	fn is_empty(&self) -> bool {
 		self.items.is_empty()

src/mp4/ilst/mod.rs

@@ -325,6 +325,11 @@ impl Accessor for Ilst {
 
 impl TagExt for Ilst {
 	type Err = LoftyError;
+	type RefKey<'a> = &'a AtomIdent;
+
+	fn contains<'a>(&'a self, key: Self::RefKey<'a>) -> bool {
+		self.atoms.iter().any(|atom| &atom.ident == key)
+	}
 
 	fn is_empty(&self) -> bool {
 		self.atoms.is_empty()

src/ogg/tag.rs

@@ -368,6 +368,13 @@ impl Accessor for VorbisComments {
 
 impl TagExt for VorbisComments {
 	type Err = LoftyError;
+	type RefKey<'a> = &'a str;
+
+	fn contains<'a>(&'a self, key: Self::RefKey<'a>) -> bool {
+		self.items
+			.iter()
+			.any(|(item_key, _)| item_key.eq_ignore_ascii_case(key))
+	}
 
 	fn is_empty(&self) -> bool {
 		self.items.is_empty() && self.pictures.is_empty()

src/tag/mod.rs

@@ -507,6 +507,11 @@ impl Tag {
 
 impl TagExt for Tag {
 	type Err = LoftyError;
+	type RefKey<'a> = &'a ItemKey;
+
+	fn contains<'a>(&'a self, key: Self::RefKey<'a>) -> bool {
+		self.items.iter().any(|item| item.key() == key)
+	}
 
 	fn is_empty(&self) -> bool {
 		self.items.is_empty() && self.pictures.is_empty()

src/traits.rs

@@ -131,10 +131,26 @@ use std::path::Path;
 pub trait TagExt: Accessor + Into<Tag> + Sized {
 	/// The associated error which can be returned from IO operations
 	type Err;
+	/// The type of key used in the tag for non-mutating functions
+	type RefKey<'a>
+	where
+		Self: 'a;
 
-	// TODO:
-	// type Key;
-	// fn contains(key: Key) -> bool;
+	/// Whether the tag contains an item with the key
+	///
+	/// # Example
+	///
+	/// ```rust
+	/// use lofty::{Accessor, ItemKey, Tag, TagExt};
+	/// # let tag_type = lofty::TagType::ID3v2;
+	///
+	/// let mut tag = Tag::new(tag_type);
+	/// assert!(tag.is_empty());
+	///
+	/// tag.set_artist(String::from("Foo artist"));
+	/// assert!(tag.contains(&ItemKey::TrackArtist));
+	/// ```
+	fn contains<'a>(&'a self, key: Self::RefKey<'a>) -> bool;
 
 	/// Whether the tag has any items
 	///