A DictionaryEncoder to be used with the Encodable protocol, based on the default PropertyListEncoder from the Swift project

DictionaryEncoder.swift

//===----------------------------------------------------------------------===//
//
// This source file is a modification of the PropertyListEncoder from the Swift project.
// The modification consists of skipping the final serialization into binary format (in the encode function) and instead returning the dictionary representation of the object to be encoded.
//
//===----------------------------------------------------------------------===//

import Foundation

//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//

//===----------------------------------------------------------------------===//
// Plist Encoder
//===----------------------------------------------------------------------===//

/// `DictionaryEncoder` facilitates the encoding of `Encodable` values into property lists.
open class DictionaryEncoder {

    // MARK: - Options

    /// The output format to write the property list data in. Defaults to `.binary`.
    open var outputFormat: PropertyListSerialization.PropertyListFormat = .binary

    /// Contextual user-provided information for use during encoding.
    open var userInfo: [CodingUserInfoKey : Any] = [:]

    /// Options set on the top-level encoder to pass down the encoding hierarchy.
    fileprivate struct _Options {
        let outputFormat: PropertyListSerialization.PropertyListFormat
        let userInfo: [CodingUserInfoKey : Any]
    }

    /// The options set on the top-level encoder.
    fileprivate var options: _Options {
        return _Options(outputFormat: outputFormat, userInfo: userInfo)
    }

    // MARK: - Constructing a Property List Encoder

    /// Initializes `self` with default strategies.
    public init() {}

    // MARK: - Encoding Values

    /// Encodes the given top-level value and returns its property list representation.
    ///
    /// - parameter value: The value to encode.
    /// - returns: A new `Data` value containing the encoded property list data.
    /// - throws: `EncodingError.invalidValue` if a non-conforming floating-point value is encountered during encoding, and the encoding strategy is `.throw`.
    /// - throws: An error if any value throws an error during encoding.
    open func encode<Value : Encodable, ResultType>(_ value: Value) throws -> ResultType {
        let topLevel = try encodeToTopLevelContainer(value)
        if topLevel is NSNumber {
            throw EncodingError.invalidValue(value,
                                             EncodingError.Context(codingPath: [],
                                                                   debugDescription: "Top-level \(Value.self) encoded as number property list fragment."))
        } else if topLevel is NSString {
            throw EncodingError.invalidValue(value,
                                             EncodingError.Context(codingPath: [],
                                                                   debugDescription: "Top-level \(Value.self) encoded as string property list fragment."))
        } else if topLevel is NSDate {
            throw EncodingError.invalidValue(value,
                                             EncodingError.Context(codingPath: [],
                                                                   debugDescription: "Top-level \(Value.self) encoded as date property list fragment."))
        }

        guard let result = topLevel as? ResultType else {
            throw EncodingError.invalidValue(value,
                                             EncodingError.Context(codingPath: [],
                                                                   debugDescription: "Result could not be represented as \(ResultType.self)"))
        }

        return result
    }

    /// Encodes the given top-level value and returns its plist-type representation.
    ///
    /// - parameter value: The value to encode.
    /// - returns: A new top-level array or dictionary representing the value.
    /// - throws: `EncodingError.invalidValue` if a non-conforming floating-point value is encountered during encoding, and the encoding strategy is `.throw`.
    /// - throws: An error if any value throws an error during encoding.
    internal func encodeToTopLevelContainer<Value : Encodable>(_ value: Value) throws -> Any {
        let encoder = _DictEncoder(options: self.options)
        guard let topLevel = try encoder.box_(value) else {
            throw EncodingError.invalidValue(value,
                                             EncodingError.Context(codingPath: [],
                                                                   debugDescription: "Top-level \(Value.self) did not encode any values."))
        }

        return topLevel
    }
}

// MARK: - _DictEncoder

fileprivate class _DictEncoder : Encoder {
    // MARK: Properties

    /// The encoder's storage.
    fileprivate var storage: _PlistEncodingStorage

    /// Options set on the top-level encoder.
    fileprivate let options: DictionaryEncoder._Options

    /// The path to the current point in encoding.
    fileprivate(set) public var codingPath: [CodingKey]

    /// Contextual user-provided information for use during encoding.
    public var userInfo: [CodingUserInfoKey : Any] {
        return self.options.userInfo
    }

    // MARK: - Initialization

    /// Initializes `self` with the given top-level encoder options.
    fileprivate init(options: DictionaryEncoder._Options, codingPath: [CodingKey] = []) {
        self.options = options
        self.storage = _PlistEncodingStorage()
        self.codingPath = codingPath
    }

    /// Returns whether a new element can be encoded at this coding path.
    ///
    /// `true` if an element has not yet been encoded at this coding path; `false` otherwise.
    fileprivate var canEncodeNewValue: Bool {
        // Every time a new value gets encoded, the key it's encoded for is pushed onto the coding path (even if it's a nil key from an unkeyed container).
        // At the same time, every time a container is requested, a new value gets pushed onto the storage stack.
        // If there are more values on the storage stack than on the coding path, it means the value is requesting more than one container, which violates the precondition.
        //
        // This means that anytime something that can request a new container goes onto the stack, we MUST push a key onto the coding path.
        // Things which will not request containers do not need to have the coding path extended for them (but it doesn't matter if it is, because they will not reach here).
        return self.storage.count == self.codingPath.count
    }

    // MARK: - Encoder Methods
    public func container<Key>(keyedBy: Key.Type) -> KeyedEncodingContainer<Key> {
        // If an existing keyed container was already requested, return that one.
        let topContainer: NSMutableDictionary
        if self.canEncodeNewValue {
            // We haven't yet pushed a container at this level; do so here.
            topContainer = self.storage.pushKeyedContainer()
        } else {
            guard let container = self.storage.containers.last as? NSMutableDictionary else {
                preconditionFailure("Attempt to push new keyed encoding container when already previously encoded at this path.")
            }

            topContainer = container
        }

        let container = _PlistKeyedEncodingContainer<Key>(referencing: self, codingPath: self.codingPath, wrapping: topContainer)
        return KeyedEncodingContainer(container)
    }

    public func unkeyedContainer() -> UnkeyedEncodingContainer {
        // If an existing unkeyed container was already requested, return that one.
        let topContainer: NSMutableArray
        if self.canEncodeNewValue {
            // We haven't yet pushed a container at this level; do so here.
            topContainer = self.storage.pushUnkeyedContainer()
        } else {
            guard let container = self.storage.containers.last as? NSMutableArray else {
                preconditionFailure("Attempt to push new unkeyed encoding container when already previously encoded at this path.")
            }

            topContainer = container
        }

        return _PlistUnkeyedEncodingContainer(referencing: self, codingPath: self.codingPath, wrapping: topContainer)
    }

    public func singleValueContainer() -> SingleValueEncodingContainer {
        return self
    }
}

// MARK: - Encoding Storage and Containers

fileprivate struct _PlistEncodingStorage {
    // MARK: Properties

    /// The container stack.
    /// Elements may be any one of the plist types (NSNumber, NSString, NSDate, NSArray, NSDictionary).
    private(set) fileprivate var containers: [NSObject] = []

    // MARK: - Initialization

    /// Initializes `self` with no containers.
    fileprivate init() {}

    // MARK: - Modifying the Stack

    fileprivate var count: Int {
        return self.containers.count
    }

    fileprivate mutating func pushKeyedContainer() -> NSMutableDictionary {
        let dictionary = NSMutableDictionary()
        self.containers.append(dictionary)
        return dictionary
    }

    fileprivate mutating func pushUnkeyedContainer() -> NSMutableArray {
        let array = NSMutableArray()
        self.containers.append(array)
        return array
    }

    fileprivate mutating func push(container: NSObject) {
        self.containers.append(container)
    }

    fileprivate mutating func popContainer() -> NSObject {
        precondition(!self.containers.isEmpty, "Empty container stack.")
        return self.containers.popLast()!
    }
}

// MARK: - Encoding Containers

fileprivate struct _PlistKeyedEncodingContainer<K : CodingKey> : KeyedEncodingContainerProtocol {
    typealias Key = K

    // MARK: Properties

    /// A reference to the encoder we're writing to.
    private let encoder: _DictEncoder

    /// A reference to the container we're writing to.
    private let container: NSMutableDictionary

    /// The path of coding keys taken to get to this point in encoding.
    private(set) public var codingPath: [CodingKey]

    // MARK: - Initialization

    /// Initializes `self` with the given references.
    fileprivate init(referencing encoder: _DictEncoder, codingPath: [CodingKey], wrapping container: NSMutableDictionary) {
        self.encoder = encoder
        self.codingPath = codingPath
        self.container = container
    }

    // MARK: - KeyedEncodingContainerProtocol Methods

    public mutating func encodeNil(forKey key: Key)               throws { self.container[key.stringValue] = _plistNullNSString }
    public mutating func encode(_ value: Bool, forKey key: Key)   throws { self.container[key.stringValue] = self.encoder.box(value) }
    public mutating func encode(_ value: Int, forKey key: Key)    throws { self.container[key.stringValue] = self.encoder.box(value) }
    public mutating func encode(_ value: Int8, forKey key: Key)   throws { self.container[key.stringValue] = self.encoder.box(value) }
    public mutating func encode(_ value: Int16, forKey key: Key)  throws { self.container[key.stringValue] = self.encoder.box(value) }
    public mutating func encode(_ value: Int32, forKey key: Key)  throws { self.container[key.stringValue] = self.encoder.box(value) }
    public mutating func encode(_ value: Int64, forKey key: Key)  throws { self.container[key.stringValue] = self.encoder.box(value) }
    public mutating func encode(_ value: UInt, forKey key: Key)   throws { self.container[key.stringValue] = self.encoder.box(value) }
    public mutating func encode(_ value: UInt8, forKey key: Key)  throws { self.container[key.stringValue] = self.encoder.box(value) }
    public mutating func encode(_ value: UInt16, forKey key: Key) throws { self.container[key.stringValue] = self.encoder.box(value) }
    public mutating func encode(_ value: UInt32, forKey key: Key) throws { self.container[key.stringValue] = self.encoder.box(value) }
    public mutating func encode(_ value: UInt64, forKey key: Key) throws { self.container[key.stringValue] = self.encoder.box(value) }
    public mutating func encode(_ value: String, forKey key: Key) throws { self.container[key.stringValue] = self.encoder.box(value) }
    public mutating func encode(_ value: Float, forKey key: Key)  throws { self.container[key.stringValue] = self.encoder.box(value) }
    public mutating func encode(_ value: Double, forKey key: Key) throws { self.container[key.stringValue] = self.encoder.box(value) }

    public mutating func encode<T : Encodable>(_ value: T, forKey key: Key) throws {
        self.encoder.codingPath.append(key)
        defer { self.encoder.codingPath.removeLast() }
        self.container[key.stringValue] = try self.encoder.box(value)
    }

    public mutating func nestedContainer<NestedKey>(keyedBy keyType: NestedKey.Type, forKey key: Key) -> KeyedEncodingContainer<NestedKey> {
        let dictionary = NSMutableDictionary()
        self.container[key.stringValue] = dictionary

        self.codingPath.append(key)
        defer { self.codingPath.removeLast() }

        let container = _PlistKeyedEncodingContainer<NestedKey>(referencing: self.encoder, codingPath: self.codingPath, wrapping: dictionary)
        return KeyedEncodingContainer(container)
    }

    public mutating func nestedUnkeyedContainer(forKey key: Key) -> UnkeyedEncodingContainer {
        let array = NSMutableArray()
        self.container[key.stringValue] = array

        self.codingPath.append(key)
        defer { self.codingPath.removeLast() }
        return _PlistUnkeyedEncodingContainer(referencing: self.encoder, codingPath: self.codingPath, wrapping: array)
    }

    public mutating func superEncoder() -> Encoder {
        return _PlistReferencingEncoder(referencing: self.encoder, at: _PlistKey.super, wrapping: self.container)
    }

    public mutating func superEncoder(forKey key: Key) -> Encoder {
        return _PlistReferencingEncoder(referencing: self.encoder, at: key, wrapping: self.container)
    }
}

fileprivate struct _PlistUnkeyedEncodingContainer : UnkeyedEncodingContainer {
    // MARK: Properties

    /// A reference to the encoder we're writing to.
    private let encoder: _DictEncoder

    /// A reference to the container we're writing to.
    private let container: NSMutableArray

    /// The path of coding keys taken to get to this point in encoding.
    private(set) public var codingPath: [CodingKey]

    /// The number of elements encoded into the container.
    public var count: Int {
        return self.container.count
    }

    // MARK: - Initialization

    /// Initializes `self` with the given references.
    fileprivate init(referencing encoder: _DictEncoder, codingPath: [CodingKey], wrapping container: NSMutableArray) {
        self.encoder = encoder
        self.codingPath = codingPath
        self.container = container
    }

    // MARK: - UnkeyedEncodingContainer Methods

    public mutating func encodeNil()             throws { self.container.add(_plistNullNSString) }
    public mutating func encode(_ value: Bool)   throws { self.container.add(self.encoder.box(value)) }
    public mutating func encode(_ value: Int)    throws { self.container.add(self.encoder.box(value)) }
    public mutating func encode(_ value: Int8)   throws { self.container.add(self.encoder.box(value)) }
    public mutating func encode(_ value: Int16)  throws { self.container.add(self.encoder.box(value)) }
    public mutating func encode(_ value: Int32)  throws { self.container.add(self.encoder.box(value)) }
    public mutating func encode(_ value: Int64)  throws { self.container.add(self.encoder.box(value)) }
    public mutating func encode(_ value: UInt)   throws { self.container.add(self.encoder.box(value)) }
    public mutating func encode(_ value: UInt8)  throws { self.container.add(self.encoder.box(value)) }
    public mutating func encode(_ value: UInt16) throws { self.container.add(self.encoder.box(value)) }
    public mutating func encode(_ value: UInt32) throws { self.container.add(self.encoder.box(value)) }
    public mutating func encode(_ value: UInt64) throws { self.container.add(self.encoder.box(value)) }
    public mutating func encode(_ value: Float)  throws { self.container.add(self.encoder.box(value)) }
    public mutating func encode(_ value: Double) throws { self.container.add(self.encoder.box(value)) }
    public mutating func encode(_ value: String) throws { self.container.add(self.encoder.box(value)) }

    public mutating func encode<T : Encodable>(_ value: T) throws {
        self.encoder.codingPath.append(_PlistKey(index: self.count))
        defer { self.encoder.codingPath.removeLast() }
        self.container.add(try self.encoder.box(value))
    }

    public mutating func nestedContainer<NestedKey>(keyedBy keyType: NestedKey.Type) -> KeyedEncodingContainer<NestedKey> {
        self.codingPath.append(_PlistKey(index: self.count))
        defer { self.codingPath.removeLast() }

        let dictionary = NSMutableDictionary()
        self.container.add(dictionary)

        let container = _PlistKeyedEncodingContainer<NestedKey>(referencing: self.encoder, codingPath: self.codingPath, wrapping: dictionary)
        return KeyedEncodingContainer(container)
    }

    public mutating func nestedUnkeyedContainer() -> UnkeyedEncodingContainer {
        self.codingPath.append(_PlistKey(index: self.count))
        defer { self.codingPath.removeLast() }

        let array = NSMutableArray()
        self.container.add(array)
        return _PlistUnkeyedEncodingContainer(referencing: self.encoder, codingPath: self.codingPath, wrapping: array)
    }

    public mutating func superEncoder() -> Encoder {
        return _PlistReferencingEncoder(referencing: self.encoder, at: self.container.count, wrapping: self.container)
    }
}

extension _DictEncoder : SingleValueEncodingContainer {
    // MARK: - SingleValueEncodingContainer Methods

    private func assertCanEncodeNewValue() {
        precondition(self.canEncodeNewValue, "Attempt to encode value through single value container when previously value already encoded.")
    }

    public func encodeNil() throws {
        assertCanEncodeNewValue()
        self.storage.push(container: _plistNullNSString)
    }

    public func encode(_ value: Bool) throws {
        assertCanEncodeNewValue()
        self.storage.push(container: self.box(value))
    }

    public func encode(_ value: Int) throws {
        assertCanEncodeNewValue()
        self.storage.push(container: self.box(value))
    }

    public func encode(_ value: Int8) throws {
        assertCanEncodeNewValue()
        self.storage.push(container: self.box(value))
    }

    public func encode(_ value: Int16) throws {
        assertCanEncodeNewValue()
        self.storage.push(container: self.box(value))
    }

    public func encode(_ value: Int32) throws {
        assertCanEncodeNewValue()
        self.storage.push(container: self.box(value))
    }

    public func encode(_ value: Int64) throws {
        assertCanEncodeNewValue()
        self.storage.push(container: self.box(value))
    }

    public func encode(_ value: UInt) throws {
        assertCanEncodeNewValue()
        self.storage.push(container: self.box(value))
    }

    public func encode(_ value: UInt8) throws {
        assertCanEncodeNewValue()
        self.storage.push(container: self.box(value))
    }

    public func encode(_ value: UInt16) throws {
        assertCanEncodeNewValue()
        self.storage.push(container: self.box(value))
    }

    public func encode(_ value: UInt32) throws {
        assertCanEncodeNewValue()
        self.storage.push(container: self.box(value))
    }

    public func encode(_ value: UInt64) throws {
        assertCanEncodeNewValue()
        self.storage.push(container: self.box(value))
    }

    public func encode(_ value: String) throws {
        assertCanEncodeNewValue()
        self.storage.push(container: self.box(value))
    }

    public func encode(_ value: Float) throws {
        assertCanEncodeNewValue()
        self.storage.push(container: self.box(value))
    }

    public func encode(_ value: Double) throws {
        assertCanEncodeNewValue()
        self.storage.push(container: self.box(value))
    }

    public func encode<T : Encodable>(_ value: T) throws {
        assertCanEncodeNewValue()
        try self.storage.push(container: self.box(value))
    }
}

// MARK: - Concrete Value Representations

extension _DictEncoder {

    /// Returns the given value boxed in a container appropriate for pushing onto the container stack.
    fileprivate func box(_ value: Bool)   -> NSObject { return NSNumber(value: value) }
    fileprivate func box(_ value: Int)    -> NSObject { return NSNumber(value: value) }
    fileprivate func box(_ value: Int8)   -> NSObject { return NSNumber(value: value) }
    fileprivate func box(_ value: Int16)  -> NSObject { return NSNumber(value: value) }
    fileprivate func box(_ value: Int32)  -> NSObject { return NSNumber(value: value) }
    fileprivate func box(_ value: Int64)  -> NSObject { return NSNumber(value: value) }
    fileprivate func box(_ value: UInt)   -> NSObject { return NSNumber(value: value) }
    fileprivate func box(_ value: UInt8)  -> NSObject { return NSNumber(value: value) }
    fileprivate func box(_ value: UInt16) -> NSObject { return NSNumber(value: value) }
    fileprivate func box(_ value: UInt32) -> NSObject { return NSNumber(value: value) }
    fileprivate func box(_ value: UInt64) -> NSObject { return NSNumber(value: value) }
    fileprivate func box(_ value: Float)  -> NSObject { return NSNumber(value: value) }
    fileprivate func box(_ value: Double) -> NSObject { return NSNumber(value: value) }
    fileprivate func box(_ value: String) -> NSObject { return NSString(string: value) }

    fileprivate func box<T : Encodable>(_ value: T) throws -> NSObject {
        return try self.box_(value) ?? NSDictionary()
    }

    fileprivate func box_<T : Encodable>(_ value: T) throws -> NSObject? {
        if T.self == Date.self || T.self == NSDate.self {
            // PropertyListSerialization handles NSDate directly.
            return (value as! NSDate)
        } else if T.self == Data.self || T.self == NSData.self {
            // PropertyListSerialization handles NSData directly.
            return (value as! NSData)
        }

        // The value should request a container from the _DictEncoder.
        let depth = self.storage.count
        do {
            try value.encode(to: self)
        } catch let error {
            // If the value pushed a container before throwing, pop it back off to restore state.
            if self.storage.count > depth {
                let _ = self.storage.popContainer()
            }

            throw error
        }

        // The top container should be a new container.
        guard self.storage.count > depth else {
            return nil
        }

        return self.storage.popContainer()
    }
}

// MARK: - _PlistReferencingEncoder

/// _PlistReferencingEncoder is a special subclass of _DictEncoder which has its own storage, but references the contents of a different encoder.
/// It's used in superEncoder(), which returns a new encoder for encoding a superclass -- the lifetime of the encoder should not escape the scope it's created in, but it doesn't necessarily know when it's done being used (to write to the original container).
fileprivate class _PlistReferencingEncoder : _DictEncoder {
    // MARK: Reference types.

    /// The type of container we're referencing.
    private enum Reference {
        /// Referencing a specific index in an array container.
        case array(NSMutableArray, Int)

        /// Referencing a specific key in a dictionary container.
        case dictionary(NSMutableDictionary, String)
    }

    // MARK: - Properties

    /// The encoder we're referencing.
    private let encoder: _DictEncoder

    /// The container reference itself.
    private let reference: Reference

    // MARK: - Initialization

    /// Initializes `self` by referencing the given array container in the given encoder.
    fileprivate init(referencing encoder: _DictEncoder, at index: Int, wrapping array: NSMutableArray) {
        self.encoder = encoder
        self.reference = .array(array, index)
        super.init(options: encoder.options, codingPath: encoder.codingPath)

        self.codingPath.append(_PlistKey(index: index))
    }

    /// Initializes `self` by referencing the given dictionary container in the given encoder.
    fileprivate init(referencing encoder: _DictEncoder, at key: CodingKey, wrapping dictionary: NSMutableDictionary) {
        self.encoder = encoder
        self.reference = .dictionary(dictionary, key.stringValue)
        super.init(options: encoder.options, codingPath: encoder.codingPath)

        self.codingPath.append(key)
    }

    // MARK: - Coding Path Operations

    fileprivate override var canEncodeNewValue: Bool {
        // With a regular encoder, the storage and coding path grow together.
        // A referencing encoder, however, inherits its parents coding path, as well as the key it was created for.
        // We have to take this into account.
        return self.storage.count == self.codingPath.count - self.encoder.codingPath.count - 1
    }

    // MARK: - Deinitialization

    // Finalizes `self` by writing the contents of our storage to the referenced encoder's storage.
    deinit {
        let value: Any
        switch self.storage.count {
        case 0: value = NSDictionary()
        case 1: value = self.storage.popContainer()
        default: fatalError("Referencing encoder deallocated with multiple containers on stack.")
        }

        switch self.reference {
        case .array(let array, let index):
            array.insert(value, at: index)

        case .dictionary(let dictionary, let key):
            dictionary[NSString(string: key)] = value
        }
    }
}

//===----------------------------------------------------------------------===//
// Shared Plist Null Representation
//===----------------------------------------------------------------------===//

// Since plists do not support null values by default, we will encode them as "$null".
fileprivate let _plistNull = "$null"
fileprivate let _plistNullNSString = NSString(string: _plistNull)

//===----------------------------------------------------------------------===//
// Shared Key Types
//===----------------------------------------------------------------------===//

fileprivate struct _PlistKey : CodingKey {
    public var stringValue: String
    public var intValue: Int?

    public init?(stringValue: String) {
        self.stringValue = stringValue
        self.intValue = nil
    }

    public init?(intValue: Int) {
        self.stringValue = "\(intValue)"
        self.intValue = intValue
    }

    fileprivate init(index: Int) {
        self.stringValue = "Index \(index)"
        self.intValue = index
    }

    fileprivate static let `super` = _PlistKey(stringValue: "super")!
}