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.

Answer 1

I think it makes sense to use decodeifPresent rather than an optional property if you want to use a default value for a property that could be missing from the JSON.

For example, let's examine 3 situations:

1. All the keys are present in the JSON:

Let's suppose you must decode this JSON:

{
    "project_names": ["project1", "project2", "project3"],
    "is_pro": true
}

You can use this struct:

struct Program: Codable {
    let projectNames: [String]
    let isPro: Bool
}

and you will get a Program object with a isPro value equal to true. (I suppose your decoder keyDecodingStrategy is .convertFromSnakeCase in the rest of this example)

---

2. Some keys are missing in the JSON and you're ok to have an optional in Swift:

{
    "project_names": ["project1", "project2", "project3"]
}

You can now use this struct:

struct Program: Codable {
    let projectNames: [String]
    var isPro: Bool?
}

and you will get a Program object with a isPro value equal to nil.

If the JSON looked like this:

{
    "project_names": ["project1", "project2", "project3"],
    "is_pro": true
}

then isPro would be a Bool? with value true. Maybe that's what you want, but probably you would like to have a Bool with a default value of false. That's where decodeIfPresent could be useful.

---

3. Some keys are missing in the JSON and you want a non-optional property with a default value in Swift:

If your struct looks like this:

struct Program: Codable {
    let projectNames: [String]
    var isPro: Bool = false
}

then you will get a parsing error if the "is_pro" attribute is not present in your JSON. Because Codable expects to find a value to parse a Bool property.

In that situation, a good idea would be to have an initializer with decodeIfPresent, like so:

struct Program: Codable {
    let projectNames: [String]
    let isPro: Bool

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        self.projectNames = try container.decode([String].self, forKey: .projectNames)
        self.isPro = try container.decodeIfPresent(Bool.self, forKey: .isPro) ?? false
    }
}

This allows you to have the best of both worlds:

• your struct has a Bool, not a Bool? property
• you are still able to parse a JSON that does NOT contain the "is_pro" field
• you can get a default value of false if the field is not present in the JSON.