Swift 4.2 coalescing while downcasting multiple variables in a single if-let

Question

I recently started learning Swift, now I'm trying to safely unwrap multiple variables that come from a JSON response which might contain or not that key.

Example of the JSON response:

{
    "products: [{
        "foo": "foo"
        "bar": "bar"
    }, {
        "foo": "foo"
    }]
}

Here I'm trying the following:

let dataTask = URLSession.shared.dataTask(with: myURL) { (data, response, error) in
    guard let safeData = data else { return }

    do {
        let json = try JSONSerialization.jsonObject(with: safeData, options: .mutableLeaves)
        if let jsonDict = json as? [String : Any] {
            let productArray = jsonDict["products"] as? [[String : Any]]
            for product in productArray! {
                if let foo = product["foo"] as? String, let bar = product["bar"] as? String {
                    let prod = Product(foo: foo, bar: bar)
                    products.append(prod)
                }
            }
        }
    } catch {
        print ("Error: \(error)")
    }
}

What I want to do is to give bar a default value (coalescing) if the value is nil, such as "Not Available" in order to display it in a label.

Is it possible? How could I do that?

Shehata Gamal · Accepted Answer · 2018-12-27T03:41:21.477

2

You can try

 let prod = Product(foo:product["foo"] as? String ?? "Not Available" , bar: product["bar"] as? String ?? "Not Available" )

struct Root: Decodable {
    let products: [Product]
}

struct Product: Decodable {
    let foo: String
    let bar: String?

    enum CodingKeys: String, CodingKey {
        case foo , bar  
    }

    init(from decoder: Decoder) throws {

         let container = try decoder.container(keyedBy: CodingKeys.self)


        do { 
            let foo = try container.decode(String.self, forKey: .foo)  
            let bar = try container.decodeIfPresent(String.self, forKey: .bar) ?? "Not Available"
            self.init(foo:foo, bar:bar)

        } catch let error {
            print(error)
            throw error
        }
     }
}

edited Dec 27 '18 at 03:41

answered Dec 27 '18 at 03:26

Shehata Gamal

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Is this the only possible way? My actual `Product` contains around 10 properties, so I wanted to make it clearer having them in their own variables. – Frakcool Dec 27 '18 at 03:28
I guess a more clean way is through `Codable` – Shehata Gamal Dec 27 '18 at 03:29
You should be using Decodable, not JSONSerialization. – matt Dec 27 '18 at 03:30
Oh, thanks! I'll look into it @matt. Can you add the `Codable` part into your answer Sh_Khan? I'll accept once I'm allowed to after you add it. – Frakcool Dec 27 '18 at 03:32

Kamran · Answer 2 · 2018-12-28T18:40:49.100

As the Keys in json are Decodable friendly, you can use this minimal code,

struct ProductResponse: Decodable {
    let products: [Product]
}

struct Product: Decodable {
    let foo: String
    let bar: String?
}

let dataTask = URLSession.shared.dataTask(with: myURL) { (data, response, error) in
    guard let data = data else { return }

    do {
        let productResponse = JSONDecoder().decode(ProductResponse.self, from: data)
        print(productResponse.products.forEach({ print($0.foo)}))
    } catch {
        print ("Error: \(error)")
    }
}

Assigning a default value to bar at parsing level doesn't seem natural. Product is a simple type with just two properties but for a type with tens of properties you will hate implementing the init(from decoder: Decoder) and CodingKeys enumerations just because of one or two properties that need a default value.

I would suggest a better approach by introducing an extension to Optional as below,

extension Optional where Wrapped == String {

    /// Unwrapped string
    /// `u7d` comes from starting `u` in `Unwrapped`, 
    ///  7 letters in between and last letter `d`.
    public func u7d(_ defaultString: String = "N/A") -> String {
        guard let value = self, value.isEmpty == false else { return defaultString }
        return value
    }
}

So now when you want to use a default value if this property is nil, you can just unwrap by passing that default value as below,

productResponse.products.forEach({ product in
    print(product.bar.u7d("Not Available"))
})

This has some key benefits as below,

Your if statement result will stay as expected when you will compare this optional property with nil.
You can pass different default values at different places without any if statement.
As UITextField, UITextView and UILabel accept Optional text and in many cases you will need to show a placeholder when the string attribute is nil from the api response. So in those cases you don't have to re-engineer the string attribute to know if it has default value or api returned value.

Swift 4.2 coalescing while downcasting multiple variables in a single if-let

2 Answers2