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.
There's an 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"
}
while it would throw an error when the field is missing
{
"bar": "something"
}
Exhibit 2 will happily parse both. So in normal use cases for JSON parsers you'll want decodeIfPresent for every optional in your model.