Swift 5/6 Multipart form-data Support

MultiPartFormData.swift

//
//  MultiPartFormData.swift
//

import Foundation

/* Example Call site: */
func upload(imageData: Data) async throws {
    guard let url = URL(string: "https://example.com/upload") else { return }
    var multipartFormData = MultipartFormData()
    multipartFormData.addField(named: "Content-Type", value: "image/png")
    multipartFormData.addField(named: "file", filename: "profile.png", data: imageData)
    let request = URLRequest(url: url, formData: multipartFormData)
    _ = try await URLSession.shared.data(for: request)
}

/*----------------------------------*/

public struct MultipartFormData {
    fileprivate let boundary = UUID().uuidString
    private var formData = Data()

    fileprivate var httpBody: Data {
        var data = formData
        data.append("--\(boundary)--")
        return data
    }

    public mutating func addField(named name: String, value: String) {
        formData.addField("--\(boundary)")
        formData.addField("Content-Disposition: form-data; name=\"\(name)\"")
        formData.addField()
        formData.addField(value)
    }

    public mutating func addField(named name: String, filename: String, data: Data) {
        formData.addField("--\(boundary)")
        formData.addField("Content-Disposition: form-data; name=\"\(name)\"; filename=\"\(filename)\"")
        formData.addField()
        formData.addField(data)
    }
}

public extension URLRequest {
    init(url: URL, timeoutInterval: TimeInterval = 60, formData: MultipartFormData) {
        self.init(url: url, timeoutInterval: timeoutInterval)
        httpMethod = "POST"
        setValue("multipart/form-data; boundary=\(formData.boundary)", forHTTPHeaderField: "Content-Type")
        setValue("*/*", forHTTPHeaderField: "Accept")
        httpBody = formData.httpBody
    }
}

fileprivate extension Data {
    mutating func append(_ string: String) {
        append(Data(string.utf8))
    }

    mutating func addField() {
        append(.httpFieldDelimiter)
    }

    mutating func addField(_ string: String) {
        append(string)
        append(.httpFieldDelimiter)
    }

    mutating func addField(_ data: Data) {
        append(data)
        append(.httpFieldDelimiter)
    }
}

fileprivate extension String {
    static let httpFieldDelimiter = "\r\n"
}