Custom Decoder that can be used to create empty Decodable instances

Decodable+Empty.swift

import Foundation

extension Decodable {
    public static func empty() throws -> Self {
        return try Self(from: EmptyDecoder())
    }
}

private class EmptyDecoder: Decoder {
    let codingPath: [CodingKey] = []
    let userInfo: [CodingUserInfoKey: Any] = [:]

    init() { }

    func container<Key: CodingKey>(keyedBy type: Key.Type) throws -> KeyedDecodingContainer<Key> {
        return .init(KeyedContainer<Key>())
    }
    func unkeyedContainer() throws -> UnkeyedDecodingContainer {
        return UnkeyedContainer()
    }
    func singleValueContainer() throws -> SingleValueDecodingContainer {
        return SingleValueContainer()
    }

    struct KeyedContainer<Key: CodingKey>: KeyedDecodingContainerProtocol {
        let allKeys: [Key] = []
        let codingPath: [CodingKey] = []

        init() { }

        func contains(_ key: Key) -> Bool {
            return true
        }
        func decodeNil(forKey key: Key) throws -> Bool {
            return true
        }
        func decode<T: Decodable>(_ type: T.Type, forKey key: Key) throws -> T {
            return try T.empty()
        }

        func nestedContainer<NestedKey: CodingKey>(keyedBy type: NestedKey.Type, forKey key: Key) throws -> KeyedDecodingContainer<NestedKey> {
            return .init(KeyedContainer<NestedKey>())
        }
        func nestedUnkeyedContainer(forKey key: Key) throws -> UnkeyedDecodingContainer {
            return UnkeyedContainer()
        }
        func superDecoder() throws -> Decoder {
            return EmptyDecoder()
        }
        func superDecoder(forKey key: Key) throws -> Decoder {
            return EmptyDecoder()
        }
    }
    struct UnkeyedContainer: UnkeyedDecodingContainer {
        let codingPath: [CodingKey] = []
        var isAtEnd: Bool { return true }
        var count: Int? = 0
        var currentIndex: Int = 0

        init() { }

        mutating func decodeNil() throws -> Bool {
            return true
        }
        mutating func decode<T: Decodable>(_ type: T.Type) throws -> T {
            return try T.empty()
        }
        mutating func nestedContainer<NestedKey: CodingKey>(keyedBy type: NestedKey.Type) throws -> KeyedDecodingContainer<NestedKey> {
            return .init(KeyedContainer<NestedKey>())
        }
        mutating func nestedUnkeyedContainer() throws -> UnkeyedDecodingContainer {
            return UnkeyedContainer()
        }
        mutating func superDecoder() throws -> Decoder {
            return EmptyDecoder()
        }
    }
    struct SingleValueContainer: SingleValueDecodingContainer {
        let codingPath: [CodingKey] = []

        init() { }

        func decodeNil() -> Bool { return true }
        func decode(_ type: Bool.Type) throws -> Bool { return true }
        func decode(_ type: String.Type) throws -> String { return "" }
        func decode(_ type: Double.Type) throws -> Double { return 0 }
        func decode(_ type: Float.Type) throws -> Float { return 0 }
        func decode(_ type: Int.Type) throws -> Int { return 0 }
        func decode(_ type: Int8.Type) throws -> Int8 { return 0 }
        func decode(_ type: Int16.Type) throws -> Int16 { return 0 }
        func decode(_ type: Int32.Type) throws -> Int32 { return 0 }
        func decode(_ type: Int64.Type) throws -> Int64 { return 0 }
        func decode(_ type: UInt.Type) throws -> UInt { return 0 }
        func decode(_ type: UInt8.Type) throws -> UInt8 { return 0 }
        func decode(_ type: UInt16.Type) throws -> UInt16 { return 0 }
        func decode(_ type: UInt32.Type) throws -> UInt32 { return 0 }
        func decode(_ type: UInt64.Type) throws -> UInt64 { return 0 }
        func decode<T: Decodable>(_ type: T.Type) throws -> T { return try T.empty() }
    }
}

Usage.swift

struct Bar: Codable {
    let e: [Int]?
    let f: Bool
}
struct Foo: Codable {
    let a: Int
    let b: String
    let c: Bool?
    let d: Bar
    let e: [Bar]
}

print(try! Foo.empty())

// Foo(a: 0, b: "", c: nil, d: __lldb_expr_52.Bar(e: nil, f: true), e: [])