What is difference between optional and decodeIfPresent when using Decodable for JSON Parsing?

Question

I am using Codable protocol from Swift 4 first time, I am not able to understand use of decodeIfPresent from Decodable.

/// Decodes a value of the given type for the given key, if present.
///
/// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
///
/// - parameter type: The type of value to decode.
/// - parameter key: The key that the decoded value is associated with.
/// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
/// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
public func decodeIfPresent(_ type: String.Type, forKey key: KeyedDecodingContainer.Key) throws -> String?

Here it suggest that it returns nil, if value not present with associated key. If this is the only reason , then how it differ from optional property, as optional variable also set to nil if value is not present in response.

Answer 1

There's a subtle, but important difference between these two lines of code:

// Exhibit 1
foo = try container.decode(Int?.self, forKey: .foo)
// Exhibit 2
foo = try container.decodeIfPresent(Int.self, forKey: .foo)

Exhibit 1 will parse:

{
  "foo": null,
  "bar": "something"
}

but not:

{
  "bar": "something"
}

while exhibit 2 will happily parse both. So in normal use cases for JSON parsers you'll want decodeIfPresent for every optional in your model.