MultipartKit

Overview

MultipartKit is a Swift package for parsing and serializing multipart/form-data requests. It provides hooks for encoding and decoding requests in Swift and Codable support for handling multipart/form-data data through a FormDataEncoder and FormDataDecoder. The parser delivers its output as it is parsed through callbacks suitable for streaming.

Multipart Form Data

Let’s define a Codable type and a choose a boundary used to separate the multipart parts.

struct User: Codable {
    let name: String
    let email: String
}
let user = User(name: "Ed", email: "ed@example.com")
let boundary = "abc123"

We can encode this instance of a our type using a FormDataEncoder.

let encoded = try FormDataEncoder().encode(foo, boundary: boundary)

The output looks then looks like this.

--abc123
Content-Disposition: form-data; name="name"

Ed
--abc123
Content-Disposition: form-data; name="email"

ed@example.com
--abc123--

In order to decode this message we feed this output and the same boundary to a FormDataDecoder and we get back an identical instance to the one we started with.

let decoded = try FormDataDecoder().decode(User.self, from: encoded, boundary: boundary)

A note on `null`

As there is no standard defined for how to represent null in Multipart (unlike, for instance, JSON), FormDataEncoder and FormDataDecoder do not support encoding or decoding null respectively.

Nesting and Collections

Nested structures can be represented by naming the parts such that they describe a path using square brackets to denote contained properties or elements in a collection. The following example shows what that looks like in practice.

struct Nested: Encodable {
    let tag: String
    let flag: Bool
    let nested: [Nested]
}
let boundary = "abc123"
let nested = Nested(tag: "a", flag: true, nested: [Nested(tag: "b", flag: false, nested: [])])
let encoded = try FormDataEncoder().encode(nested, boundary: boundary)

This results in the content below.

--abc123
Content-Disposition: form-data; name="tag"

a
--abc123
Content-Disposition: form-data; name="flag"

true
--abc123
Content-Disposition: form-data; name="nested[0][tag]"

b
--abc123
Content-Disposition: form-data; name="nested[0][flag]"

false
--abc123--

Note that the array elements always include the index (as opposed to just []) in order to support complex nesting.