Elegantly populate dictionary from a struct checking nil values

Question

Given a struct A, I want to populate a NSDictionary with values from that struct, provided they are not nil.
To do that I insert all the valu

Answer 1

As @vacawama says, you should generally steer clear of Dictionary types with optional values, as the Dictionary API is built around using nil to indicate the lack of a value for a given key. Therefore using an optional type for a Dictionary's Value results in confusing double-wrapped optionals, which can often exhibit unintuitive behaviour.

Another (more overkill) solution would be to directly utilise dictionary literals by defining your own 'value unwrapping' ExpressibleByDictionaryLiteral type.

extension Dictionary {

    /// A dictionary wrapper that unwraps optional values in the dictionary literal
    /// that it's created with.
    struct ValueUnwrappingLiteral : ExpressibleByDictionaryLiteral {

        typealias WrappedValue = Dictionary.Value

        fileprivate let base: [Key : WrappedValue]

        init(dictionaryLiteral elements: (Key, WrappedValue?)...) {

            var base = [Key : WrappedValue]()

            // iterate through the literal's elements, unwrapping the values,
            // and updating the base dictionary with them.
            for case let (key, value?) in elements {

                if base.updateValue(value, forKey: key) != nil {
                    // duplicate keys are not permitted.
                    fatalError("Dictionary literal may not contain duplicate keys")
                }
            }

            self.base = base
        }
    }

    init(unwrappingValues literal: ValueUnwrappingLiteral) {
        self = literal.base // simply get the base of the unwrapping literal.
    }
}

You can then simply pass instances of this type to our init(unwrappingValues:) initialiser:

struct A {
    var first: String?
    var second: String?
    var third: String?
}

let a = A(first: "foo", second: nil, third: "baz")

let dictionary = Dictionary(unwrappingValues: [
    "first"  : a.first,
    "second" : a.second,
    "third"  : a.third
])

print(dictionary) // ["third": "baz", "first": "foo"]