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XMLCoder

Encoder & Decoder for XML using Swift's Codable protocols.

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This package is a fork of the original ShawnMoore/XMLParsing with more features and improved test coverage. Automatically generated documentation is available on our GitHub Pages.

Join our Discord for any questions and friendly banter.

Example

import XMLCoder
import Foundation

let sourceXML = """
<note>
    <to>Bob</to>
    <from>Jane</from>
    <heading>Reminder</heading>
    <body>Don't forget to use XMLCoder!</body>
</note>
"""

struct Note: Codable {
    let to: String
    let from: String
    let heading: String
    let body: String
}

let note = try! XMLDecoder().decode(Note.self, from: Data(sourceXML.utf8))

let encodedXML = try! XMLEncoder().encode(note, withRootKey: "note")

Advanced features

The following features are available in 0.4.0 release or later (unless stated otherwise):

Stripping namespace prefix

Sometimes you need to handle an XML namespace prefix, like in the XML below:

<h:table xmlns:h="http://www.w3.org/TR/html4/">
  <h:tr>
    <h:td>Apples</h:td>
    <h:td>Bananas</h:td>
  </h:tr>
</h:table>

Stripping the prefix from element names is enabled with shouldProcessNamespaces property:

struct Table: Codable, Equatable {
    struct TR: Codable, Equatable {
        let td: [String]
    }

    let tr: [TR]
}


let decoder = XMLDecoder()

// Setting this property to `true` for the namespace prefix to be stripped
// during decoding so that key names could match.
decoder.shouldProcessNamespaces = true

let decoded = try decoder.decode(Table.self, from: xmlData)

Dynamic node coding

XMLCoder provides two helper protocols that allow you to customize whether nodes are encoded and decoded as attributes or elements: DynamicNodeEncoding and DynamicNodeDecoding.

The declarations of the protocols are very simple:

protocol DynamicNodeEncoding: Encodable {
    static func nodeEncoding(for key: CodingKey) -> XMLEncoder.NodeEncoding
}

protocol DynamicNodeDecoding: Decodable {
    static func nodeDecoding(for key: CodingKey) -> XMLDecoder.NodeDecoding
}

The values returned by corresponding static functions look like this:

enum NodeDecoding {
    // decodes a value from an attribute
    case attribute

    // decodes a value from an element
    case element

    // the default, attempts to decode as an element first,
    // otherwise reads from an attribute
    case elementOrAttribute
}

enum NodeEncoding {
    // encodes a value in an attribute
    case attribute

    // the default, encodes a value in an element
    case element

    // encodes a value in both attribute and element
    case both
}

Add conformance to an appropriate protocol for types you'd like to customize. Accordingly, this example code:

struct Book: Codable, Equatable, DynamicNodeEncoding {
    let id: UInt
    let title: String
    let categories: [Category]

    enum CodingKeys: String, CodingKey {
        case id
        case title
        case categories = "category"
    }

    static func nodeEncoding(for key: CodingKey) -> XMLEncoder.NodeEncoding {
        switch key {
        case Book.CodingKeys.id: return .both
        default: return .element
        }
    }
}

works for this XML:

<book id="123">
    <id>123</id>
    <title>Cat in the Hat</title>
    <category>Kids</category>
    <category>Wildlife</category>
</book>

Please refer to PR #70 by @JoeMatt for more details.

Coding key value intrinsic

Suppose that you need to decode an XML that looks similar to this:

<?xml version="1.0" encoding="UTF-8"?>
<foo id="123">456</foo>

By default you'd be able to decode foo as an element, but then it's not possible to decode the id attribute. XMLCoder handles certain CodingKey values in a special way to allow proper coding for this XML. Just add a coding key with stringValue that equals "" (empty string). What follows is an example type declaration that encodes the XML above, but special handling of coding keys with those values works for both encoding and decoding.

struct Foo: Codable, DynamicNodeEncoding {
    let id: String
    let value: String

    enum CodingKeys: String, CodingKey {
        case id
        case value = ""
    }

    static func nodeEncoding(forKey key: CodingKey)
    -> XMLEncoder.NodeEncoding {
        switch key {
        case CodingKeys.id:
            return .attribute
        default:
            return .element
        }
    }
}

Thanks to @JoeMatt for implementing this in in PR #73.

Preserving whitespaces in element content

By default whitespaces are trimmed in element content during decoding. This includes string values decoded with value intrinsic keys. Starting with version 0.5 you can now set a property trimValueWhitespaces to false (the default value is true) on XMLDecoder instance to preserve all whitespaces in decoded strings.

Remove whitespace elements

When decoding pretty-printed XML while trimValueWhitespaces is set to false, it's possible for whitespace elements to be added as child elements on an instance of XMLCoderElement. These whitespace elements make it impossible to decode data structures that require custom Decodable logic. Starting with version 0.13.0 you can set a property removeWhitespaceElements to true (the default value is false) on XMLDecoder to remove these whitespace elements.

Choice element coding

Starting with version 0.8, you can encode and decode enums with associated values by conforming your CodingKey type additionally to XMLChoiceCodingKey. This allows encoding and decoding XML elements similar in structure to this example:

<container>
    <int>1</int>
    <string>two</string>
    <string>three</string>
    <int>4</int>
    <int>5</int>
</container>

To decode these elements you can use this type:

enum IntOrString: Codable {
    case int(Int)
    case string(String)
    
    enum CodingKeys: String, XMLChoiceCodingKey {
        case int
        case string
    }
    
    enum IntCodingKeys: String, CodingKey { case _0 = "" }
    enum StringCodingKeys: String, CodingKey { case _0 = "" }
}

This is described in more details in PR #119 by @jsbean and @bwetherfield.

Choice elements with (inlined) complex associated values

Lets extend previous example replacing simple types with complex in assosiated values. This example would cover XML like:

<container>
    <nested attr="n1_a1">
        <val>n1_v1</val>
        <labeled>
            <val>n2_val</val>
        </labeled>
    </nested>
    <simple attr="n1_a1">
        <val>n1_v1</val>
    </simple>
</container>

enum InlineChoice: Equatable, Codable {
    case simple(Nested1)
    case nested(Nested1, labeled: Nested2)
    
    enum CodingKeys: String, CodingKey, XMLChoiceCodingKey {
        case simple, nested
    }
    
    enum SimpleCodingKeys: String, CodingKey { case _0 = "" }
    
    enum NestedCodingKeys: String, CodingKey {
        case _0 = ""
        case labeled
    }
    
    struct Nested1: Equatable, Codable, DynamicNodeEncoding {
        var attr = "n1_a1"
        var val = "n1_v1"
        
        public static func nodeEncoding(for key: CodingKey) -> XMLEncoder.NodeEncoding {
            switch key {
            case CodingKeys.attr: return .attribute
            default: return .element
            }
        }
    }

    struct Nested2: Equatable, Codable {
        var val = "n2_val"
    }
}

Integrating with Combine

Starting with XMLCoder version 0.9, when Apple's Combine framework is available, XMLDecoder conforms to the TopLevelDecoder protocol, which allows it to be used with the decode(type:decoder:) operator:

import Combine
import Foundation
import XMLCoder

func fetchBook(from url: URL) -> AnyPublisher<Book, Error> {
    return URLSession.shared.dataTaskPublisher(for: url)
        .map(\.data)
        .decode(type: Book.self, decoder: XMLDecoder())
        .eraseToAnyPublisher()
}

This was implemented in PR #132 by @sharplet.

Additionally, starting with XMLCoder 0.11 XMLEncoder conforms to the TopLevelEncoder protocol:

import Combine
import XMLCoder

func encode(book: Book) -> AnyPublisher<Data, Error> {
    return Just(book)
        .encode(encoder: XMLEncoder())
        .eraseToAnyPublisher()
}

The resulting XML in the example above will start with <book, to customize capitalization of the root element (e.g. <Book) you'll need to set an appropriate keyEncoding strategy on the encoder. To change the element name altogether you'll have to change the name of the type, which is an unfortunate limitation of the TopLevelEncoder API.

Root element attributes

Sometimes you need to set attributes on the root element, which aren't directly related to your model type. Starting with XMLCoder 0.11 the encode function on XMLEncoder accepts a new rootAttributes argument to help with this:

struct Policy: Encodable {
    var name: String
}

let encoder = XMLEncoder()
let data = try encoder.encode(Policy(name: "test"), rootAttributes: [
    "xmlns": "http://www.nrf-arts.org/IXRetail/namespace",
    "xmlns:xsd": "http://www.w3.org/2001/XMLSchema",
    "xmlns:xsi": "http://www.w3.org/2001/XMLSchema-instance",
])

The resulting XML will look like this:

<policy xmlns="http://www.nrf-arts.org/IXRetail/namespace"
        xmlns:xsd="http://www.w3.org/2001/XMLSchema"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <name>test</name>
</policy>

This was implemented in PR #160 by @portellaa.

Property wrappers

If your version of Swift allows property wrappers to be used, you may prefer this API to the more verbose dynamic node coding.

For example, this type

struct Book: Codable {
    @Element var id: Int
}

will encode value Book(id: 42) as <Book><id>42</id></Book>. And vice versa, it will decode the latter into the former.

Similarly,

struct Book: Codable {
    @Attribute var id: Int
}

will encode value Book(id: 42) as <Book id="42"></Book> and vice versa for decoding.

If you don't know upfront if a property will be present as an element or an attribute during decoding, use @ElementAndAttribute:

struct Book: Codable {
    @ElementAndAttribute var id: Int
}

This will encode value Book(id: 42) as <Book id="42"><id>42</id></Book>. It will decode both <Book><id>42</id></Book> and <Book id="42"></Book> as Book(id: 42).

This feature is available starting with XMLCoder 0.13.0 and was implemented by @bwetherfield.

XML Headers

You can add an XML header and/or doctype when encoding an object by supplying it to the encode function. These arguments are both optional, and will only render when explicitly provided.

struct User: Codable {
    @Element var username: String
}

let data = try encoder.encode(
    User(username: "Joanis"),
    withRootKey: "user",
    header: XMLHeader(version: 1.0, encoding: "UTF-8"),
    doctype: .system(
        rootElement: "user",
        dtdLocation: "http://example.com/myUser_v1.dtd"
    )
)

Installation

Requirements

Apple Platforms

Linux

Windows

Swift Package Manager

Swift Package Manager is a tool for managing the distribution of Swift code. It’s integrated with the Swift build system to automate the process of downloading, compiling, and linking dependencies on all platforms.

Once you have your Swift package set up, adding XMLCoder as a dependency is as easy as adding it to the dependencies value of your Package.swift.

dependencies: [
    .package(url: "https://github.com/CoreOffice/XMLCoder.git", from: "0.15.0")
]

If you're using XMLCoder in an app built with Xcode, you can also add it as a direct dependency using Xcode's GUI.

CocoaPods

CocoaPods is a dependency manager for Swift and Objective-C Cocoa projects for Apple's platfoms. You can install it with the following command:

$ gem install cocoapods

Navigate to the project directory and create Podfile with the following command:

$ pod install

Inside of your Podfile, specify the XMLCoder pod:

# Uncomment the next line to define a global platform for your project
# platform :ios, '9.0'

target 'YourApp' do
  # Comment the next line if you're not using Swift or don't want
  # to use dynamic frameworks
  use_frameworks!

  # Pods for YourApp
  pod 'XMLCoder', '~> 0.14.0'
end

Then, run the following command:

$ pod install

Open the the YourApp.xcworkspace file that was created. This should be the file you use everyday to create your app, instead of the YourApp.xcodeproj file.

Carthage

Carthage is a dependency manager for Apple's platfoms that builds your dependencies and provides you with binary frameworks.

Carthage can be installed with Homebrew using the following command:

$ brew update
$ brew install carthage

Inside of your Cartfile, add GitHub path to XMLCoder:

github "CoreOffice/XMLCoder" ~> 0.15.0

Then, run the following command to build the framework:

$ carthage update

Drag the built framework into your Xcode project.

Usage with Vapor

extension XMLEncoder: ContentEncoder {
    public func encode<E: Encodable>(
        _ encodable: E,
        to body: inout ByteBuffer,
        headers: inout HTTPHeaders
    ) throws {
        headers.contentType = .xml
        
        // Note: You can provide an XMLHeader or DocType if necessary
        let data = try self.encode(encodable)
        body.writeData(data)
    }
}

extension XMLDecoder: ContentDecoder {
    public func decode<D: Decodable>(
        _ decodable: D.Type,
        from body: ByteBuffer,
        headers: HTTPHeaders
    ) throws -> D {
        // Force wrap is acceptable, as we're guaranteed these bytes exist through `readableBytes`
        let body = body.readData(length: body.readableBytes)!
        return try self.decode(D.self, from: body)
    }
}

Contributing

This project adheres to the Contributor Covenant Code of Conduct. By participating, you are expected to uphold this code. Please report unacceptable behavior to coreoffice@desiatov.com.

Coding Style

This project uses SwiftFormat and SwiftLint to enforce formatting and coding style. We encourage you to run SwiftFormat within a local clone of the repository in whatever way works best for you either manually or automatically via an Xcode extension, build phase or git pre-commit hook etc.

To guarantee that these tools run before you commit your changes on macOS, you're encouraged to run this once to set up the pre-commit hook:

brew bundle # installs SwiftLint, SwiftFormat and pre-commit
pre-commit install # installs pre-commit hook to run checks before you commit

Refer to the pre-commit documentation page for more details and installation instructions for other platforms.

SwiftFormat and SwiftLint also run on CI for every PR and thus a CI build can fail with incosistent formatting or style. We require CI builds to pass for all PRs before merging.

Test Coverage

Our goal is to keep XMLCoder stable and to serialize any XML correctly according to XML 1.0 standard. All of this can be easily tested automatically and we're slowly improving test coverage of XMLCoder and don't expect it to decrease. PRs that decrease the test coverage have a much lower chance of being merged. If you add any new features, please make sure to add tests, likewise for changes and any refactoring in existing code.