Sample CodeReviewed 2026-07-21View on Apple Developer

Using JSON with custom types

At a glance

Item	Summary
Purpose	Demonstrate array decoding, key renaming, nested source shapes, and dynamic JSON keys with `Codable`.
Deliverable	A multi-page Xcode playground, not an app with a runtime UI or shared object graph.
Architecture	Each page is an independent executable lesson with local JSON, local types, encoder/decoder calls, and printed output.

Project structure

Using-JSON-with-Custom-Types.playground/Pages/
├── Read Data from Arrays.xcplaygroundpage/Contents.swift
├── Change Key Names.xcplaygroundpage/Contents.swift
├── Access Nested Data.xcplaygroundpage/Contents.swift
├── Merge Data from Different Depths.xcplaygroundpage/Contents.swift
└── Table of Contents.xcplaygroundpage/Contents.swift

Declarations with the same names on different pages are isolated by playground-page execution; they do not form one module-wide model layer.

Overall architecture

flowchart LR
    JSON["external JSON shape"] --> Decoder["JSONDecoder"]
    Decoder --> DTO["GroceryStoreService Decodable DTO"]
    DTO --> Adapter["GroceryStore.init(from:)"]
    Adapter --> Domain["flattened GroceryStore"]
    Domain --> App["consumer / printed output"]

Reference code

Using-JSON-with-Custom-Types.playground/Pages/Access Nested Data.xcplaygroundpage/Contents.swift:143 — the domain initializer deliberately removes transport-only aisle/shelf nesting.

extension GroceryStore {
    // ...
}

Ownership and state

classDiagram
    JSONDecoder ..> GroceryStoreService : creates values
    GroceryStoreService *-- Aisle
    Aisle *-- Shelf
    Shelf *-- Product
    GroceryStore *-- Product : flattened array
    JSONEncoder ..> GroceryStore : reads values
    GroceryStore ..> ProductKey : dynamic keys

Ownership evidence

Using-JSON-with-Custom-Types.playground/Pages/Merge Data from Different Depths.xcplaygroundpage/Contents.swift:87 — encoding creates nested containers from domain values without persistent shared state.

    func encode(to encoder: Encoder) throws {
        // ...
    }

All state is value-semantic and page-local. Decoders create DTO/domain values; adapter initializers copy the needed data. Encoders borrow values to write containers. There is no owner with a lifecycle beyond playground execution.

Class and protocol design

Type technique	Responsibility
`GroceryProduct: Codable`	Use synthesized array element coding when names and shape align.
Private `CodingKeys` enum	Map wire keys such as `product_name` to idiomatic Swift properties.
`GroceryStoreService: Decodable`	Mirror a nested transport shape exactly.
`GroceryStore.init(from:)`	Convert transport values into a flatter domain shape.
`ProductKey: CodingKey` struct	Represent unbounded product names as dynamic JSON keys.
Manual `encode` / `init(from:)`	Merge key names and nested values into one product model.

Protocol-oriented design here means conformance to Codable, Encodable, Decodable, and CodingKey; there is no repository or service interface.

Access control

Boundary	Effect and rationale
`private CodingKeys`	Keeps wire-field names inside the type whose representation they define.
Page types and values are implicit `internal`	They are lesson-local implementation, not a public package API.
`public init(from:)` on the internal `GroceryStore` extension	Satisfies/illustrates decoding visibility, but the internal enclosing type caps external access.
Nested DTO types	Names transport-only aisle/shelf concepts without exposing them to the flattened domain model.

Using-JSON-with-Custom-Types.playground/Pages/Change Key Names.xcplaygroundpage/Contents.swift:47 shows the private key map; the explicitly public decoder initializer is at Using-JSON-with-Custom-Types.playground/Pages/Merge Data from Different Depths.xcplaygroundpage/Contents.swift:124.

Logic ownership and placement

Logic	Owner
Wire-name mapping	Nested `CodingKeys`
Transport-shape mirroring	`GroceryStoreService` and nested DTOs
Transport-to-domain flattening	`GroceryStore.init(from:)`
Dynamic-key generation	`ProductKey`
Manual nested encode/decode	`GroceryStore` protocol conformances
Lesson execution and output	Page-level statements

Design patterns

Pattern	Evidence	Purpose
Synthesized Codable	`Using-JSON-with-Custom-Types.playground/Pages/Read Data from Arrays.xcplaygroundpage/Contents.swift:38`	Handles aligned homogeneous array data with minimal code.
Coding-key mapping	`Using-JSON-with-Custom-Types.playground/Pages/Change Key Names.xcplaygroundpage/Contents.swift:42`	Separates idiomatic Swift names from wire names.
DTO / translation boundary	`Using-JSON-with-Custom-Types.playground/Pages/Access Nested Data.xcplaygroundpage/Contents.swift:115`	Prevents transport nesting from shaping the app model.
Custom dynamic key	`Using-JSON-with-Custom-Types.playground/Pages/Merge Data from Different Depths.xcplaygroundpage/Contents.swift:74`	Supports data-dependent object keys.

Naming conventions

GroceryStoreService signals a source-shaped intermediary; GroceryStore is the app-facing domain value.
Nested types (Aisle, Shelf, ProductKey) keep names scoped to the representation that needs them.
Initializers use init(from:) for translation and Codable decoding, with the parameter type disambiguating their roles.

Architecture takeaways

Model transport JSON exactly in a DTO when its shape is not suitable for app code.
Convert at one initializer boundary and keep the domain representation simple.
Keep wire keys private to their coding type.
Treat each playground page as an independent architecture example, not one combined application.

Source map

Source	Role
`Using-JSON-with-Custom-Types.playground/Pages/Read Data from Arrays.xcplaygroundpage/Contents.swift:38`	Synthesized array coding
`Using-JSON-with-Custom-Types.playground/Pages/Change Key Names.xcplaygroundpage/Contents.swift:42`	Static key mapping
`Using-JSON-with-Custom-Types.playground/Pages/Access Nested Data.xcplaygroundpage/Contents.swift:115`	Source-shaped DTO
`Using-JSON-with-Custom-Types.playground/Pages/Access Nested Data.xcplaygroundpage/Contents.swift:143`	DTO-to-domain adapter
`Using-JSON-with-Custom-Types.playground/Pages/Merge Data from Different Depths.xcplaygroundpage/Contents.swift:73`	Dynamic coding keys

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