Custom decoder configuration in iOS Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom decoder in Swift's Codable system?

A custom decoder lets you control how data is read and converted into your Swift types, especially when the default decoding doesn't fit your data format.

beginner

Why would you use custom decoder configuration?

To handle special cases like date formats, key naming differences, or nested data structures that the default decoder can't handle automatically.

intermediate

How do you customize the date decoding strategy in JSONDecoder?

You set the dateDecodingStrategy property, for example: decoder.dateDecodingStrategy = .iso8601 to decode ISO 8601 date strings.

intermediate

What is the role of keyDecodingStrategy in a decoder?

It controls how keys from JSON map to your Swift property names, like converting snake_case keys to camelCase properties automatically.

advanced

How can you decode nested JSON objects with a custom decoder?

By implementing init(from decoder: Decoder) in your type and manually extracting nested containers using container.nestedContainer(keyedBy:forKey:).

Which property of JSONDecoder lets you change how date strings are decoded?

A dateEncodingStrategy

B keyDecodingStrategy

C dataDecodingStrategy

D dateDecodingStrategy

The dateDecodingStrategy property controls how date strings are interpreted during decoding.

What does the keyDecodingStrategy .convertFromSnakeCase do?

A Ignores key names during decoding

B Converts camelCase keys to snake_case

C Converts snake_case keys to camelCase

D Converts keys to uppercase

It automatically converts JSON keys written in snake_case to Swift property names in camelCase.

To decode nested JSON objects manually, which method do you use inside init(from decoder: Decoder)?

A nestedContainer(keyedBy:forKey:)

B decodeNestedObject()

C decodeIfPresent()

D containerForNested()

nestedContainer(keyedBy:forKey:) lets you access nested JSON containers for manual decoding.

If your JSON date format is custom, how do you handle decoding?

A Use a custom DateFormatter and set it in dateDecodingStrategy

B Use default decoding only

C Ignore the date field

D Convert date to string manually after decoding

You create a DateFormatter matching your format and assign it to dateDecodingStrategy.custom.

What is the purpose of implementing init(from decoder: Decoder) yourself?

A To encode data manually

B To customize how your type decodes data beyond automatic decoding

C To create a new instance without decoding

D To skip decoding

Implementing init(from decoder: Decoder) lets you control decoding logic for complex or custom data.

Explain how you would configure a JSONDecoder to decode dates in a custom format and convert snake_case keys to camelCase properties.

Think about setting properties on JSONDecoder before decoding.

Describe the steps to decode a nested JSON object manually using a custom decoder in Swift.

Focus on how to access nested containers inside the decoder.