Dynamic Decoding For Deeply-Nested JSON (Playground Demo)

This is an interactive explanation + demo of a small decoding utility that makes it easy to extract values buried deep inside gnarly JSON responses — without writing 20 layers of structs or a forest of CodingKeys.

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Many of the JSON payloads we get from our APIs often nest the data we want several layers deep.
Decoding these with standard Decodable means defining either full intermediate models or multiple CodingKey hierarchies just to reach a single value.

Example JSON

{
  "mission_log": {
    "entries": [
      {
        "crew": {
          "commander": {
            "id": "crew-01",
            "name": "Cmdr. Ayla Chen"
          }
        }
      }
    ]
  }
}

To decode just "Cmdr. Ayla Chen" with traditional Decodable, you’d need to define nested types and CodingKeys for mission_log, entries, crew, and commander.

This utility extends JSONDecoder with a path parameter that lets you jump straight to deeply nested values using Swift key paths – no intermediate types required.

End Result

let name: String = try JSONDecoder().decode(
    String.self,
    from: data,
    path: \.mission_log.entries.0.crew.commander.name
)

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Explore More in the Playground

Examples.playground contains runnable code snippets and more in-depth explanations

• Open this project in Xcode.
• Open Examples.playground in the editor
• Switch to Editor → Show Rendered Markup in Xcode to view formatted explanations alongside the code snippets.
• You can run the code snippets in the playground and view the output in the console or Live View side bar.
• You can modify the examples or add your own to see how different paths behave.