Clean up RFC 2045 “media type” in BodyStructure (apple#717)

Each “part” in a `BodyStructure` has an RFC 2045 “media type” — which is a combination of “top level type” + “sub-stype”.

But the types that were previously used, were inconsistently used / named. This PR cleans up the use of these and makes them consistent.

Additionally it adds a
```swift
extension BodyStructure {
    public var mediaType: Media.MediaType
}
```
such that it’s easy to get to a part’s media type, regardless of its kind.

Sources/NIOIMAPCore/Grammar/Body/Body.swift

@@ -195,46 +195,6 @@ extension BodyStructure {
  }
 }
 
-// MARK: - Types
-
-extension BodyStructure {
- /// The subtype of a multi-part body.
- public struct MediaSubtype: CustomDebugStringConvertible, Hashable {
- /// When used with a `multipart` type, specifies the same data as different formats.
- public static var alternative: Self {
- .init("alternative")
- }
-
- /// When used with a `multipart` type, specifies compound objects consisting of several related body parts.
- public static var related: Self {
- .init("related")
- }
-
- /// When used with a `multipart` type, specifies a generic set of mixed data types.
- public static var mixed: Self {
- .init("mixed")
- }
-
- /// The subtype as a lowercased string
- internal let stringValue: String
-
- /// The subtype as a lowercased string
- public var debugDescription: String { stringValue }
-
- /// Creates a new `MediaSubtype` from the given `String`, which will be lowercased.
- /// - parameter rawValue: The subtype as a `String`. Note that the string will be lowercased.
- public init(_ stringValue: String) {
- self.stringValue = stringValue.lowercased()
- }
- }
-}
-
-extension String {
- public init(_ other: BodyStructure.MediaSubtype) {
- self = other.stringValue
- }
-}
-
 // MARK: - Encoding
 
 extension EncodeBuffer {

Sources/NIOIMAPCore/Grammar/Body/Multipart.swift

@@ -23,7 +23,7 @@ extension BodyStructure {
  public var parts: [BodyStructure]
 
  /// The subtype of the message, e.g. `mixed` (from `multipart/mixed`)
- public var mediaSubtype: MediaSubtype
+ public var mediaSubtype: Media.Subtype
 
  /// Optional additional fields that are not required to form a valid `Multipart`
  public var `extension`: Extension?
@@ -32,7 +32,7 @@ extension BodyStructure {
  /// - parameter parts: The sub-parts that form the `Multipart`
  /// - parameter mediaSubtype: The subtype of the message, e.g. *multipart/mixed*
  /// - parameter extension: Optional additional fields that are not required to form a valid `Multipart` body
- public init(parts: [BodyStructure], mediaSubtype: MediaSubtype, extension: Extension? = nil) {
+ public init(parts: [BodyStructure], mediaSubtype: Media.Subtype, extension: Extension? = nil) {
  self.parts = parts
  self.mediaSubtype = mediaSubtype
  self.extension = `extension`
@@ -68,9 +68,7 @@ extension EncodeBuffer {
  result += self.writeBody(body)
  } +
  self.writeSpace() +
- self.writeString("\"multipart/") +
  self.writeMediaSubtype(part.mediaSubtype) +
- self.writeString("\"") +
  self.writeIfExists(part.extension) { (ext) -> Int in
  self.writeSpace() +
  self.writeBodyExtensionMultipart(ext)
@@ -83,8 +81,4 @@ extension EncodeBuffer {
  self.writeBodyDispositionAndLanguage(dspLanguage)
  }
  }
-
- @discardableResult mutating func writeMediaSubtype(_ type: BodyStructure.MediaSubtype) -> Int {
- self.writeString(type.stringValue)
- }
 }

Sources/NIOIMAPCore/Grammar/Body/Singlepart.swift

@@ -44,7 +44,7 @@ extension BodyStructure.Singlepart {
  /// Represents the type of a single-part message.
  public indirect enum Kind: Hashable {
  /// A simple message containing only one kind of data.
- case basic(Media.Basic)
+ case basic(Media.MediaType)
 
  /// A "full" email message containing an envelope, and a child body.
  case message(Message)
@@ -55,8 +55,8 @@ extension BodyStructure.Singlepart {
 
  /// Represents a typical "full" email message, containing an envelope and a child message.
  public struct Message: Hashable {
- /// Indication if the message contains an encapsulated message.
- public var message: Media.Message
+ /// The RFC 2045 sub-type. This will usually be `rfc822`.
+ public var message: Media.Subtype
 
  /// The envelope of the message, potentially including the message sender, bcc list, etc.
  public var envelope: Envelope
@@ -72,7 +72,7 @@ extension BodyStructure.Singlepart {
  /// - parameter envelope: The envelope of the message
  /// - parameter body: The encapsulated message. Note that this may be a multi-part.
  /// - parameter lineCount: The number of lines in the message
- public init(message: Media.Message, envelope: Envelope, body: BodyStructure, lineCount: Int) {
+ public init(message: Media.Subtype, envelope: Envelope, body: BodyStructure, lineCount: Int) {
  self.message = message
  self.envelope = envelope
  self.body = body
@@ -82,17 +82,17 @@ extension BodyStructure.Singlepart {
 
  /// Represents a text-based message body.
  public struct Text: Hashable {
- /// The type of text message, e.g. `text/html` or `text/plain`
- public var mediaText: String
+ /// The media sub-type of a text part, e.g. `html` or `plain` for `text/html` and `text/plain` respectively.
+ public var mediaSubtype: Media.Subtype
 
  /// The number of lines in the message.
  public var lineCount: Int
 
  /// Creates a new `Text`.
  /// - parameter mediaText: The type of text message, e.g. `text/html` or `text/plain`
  /// - parameter lineCount: The number of lines in the message.
- public init(mediaText: String, lineCount: Int) {
- self.mediaText = mediaText
+ public init(mediaSubtype: Media.Subtype, lineCount: Int) {
+ self.mediaSubtype = mediaSubtype
  self.lineCount = lineCount
  }
  }
@@ -123,7 +123,7 @@ extension EncodeBuffer {
  var size = 0
  switch part.kind {
  case .basic(let basic):
- size += self.writeBodyKindBasic(mediaKind: basic, fields: part.fields)
+ size += self.writeBodyKindBasic(mediaType: basic, fields: part.fields)
  case .message(let message):
  size += self.writeBodyKindMessage(message, fields: part.fields)
  case .text(let text):
@@ -138,14 +138,16 @@ extension EncodeBuffer {
  }
 
  @discardableResult private mutating func writeBodyKindText(_ body: BodyStructure.Singlepart.Text, fields: BodyStructure.Fields) -> Int {
- self.writeMediaText(body.mediaText) +
+ self.writeString(#""TEXT" "#) +
+ self.writeMediaSubtype(body.mediaSubtype) +
  self.writeSpace() +
  self.writeBodyFields(fields) +
  self.writeString(" \(body.lineCount)")
  }
 
  @discardableResult private mutating func writeBodyKindMessage(_ message: BodyStructure.Singlepart.Message, fields: BodyStructure.Fields) -> Int {
- self.writeMediaMessage(message.message) +
+ self.writeString(#""MESSAGE" "#) +
+ self.writeMediaSubtype(message.message) +
  self.writeSpace() +
  self.writeBodyFields(fields) +
  self.writeSpace() +
@@ -155,8 +157,8 @@ extension EncodeBuffer {
  self.writeString(" \(message.lineCount)")
  }
 
- @discardableResult private mutating func writeBodyKindBasic(mediaKind: Media.Basic, fields: BodyStructure.Fields) -> Int {
- self.writeMediaBasic(mediaKind) +
+ @discardableResult private mutating func writeBodyKindBasic(mediaType: Media.MediaType, fields: BodyStructure.Fields) -> Int {
+ self.writeMediaType(mediaType) +
  self.writeSpace() +
  self.writeBodyFields(fields)
  }