AC-iOS Codable and UserDefaults: Part II JSON Codables (Swift 4.x)

Readings

1. Ultimate Guide to Parsing With Swift
2. An Introduction to NSUserDefaults

Required Resources

1. Postman via Chrome Store

---

0. Objectives

1. Become familiar with using UserDefaults to store data
2. Understand that UserDefaults is a light-weight, persistant storage option for small amounts of data that relate to how your app should be configured, based on the user's selection/choices.
3. Become with the Codable protocol that allows for easy conversion between Swift objects and storeable Data

---

1. JSONDecoder and Codable

Codeable makes a world of difference when it comes to working with JSON. Rather than using our old friend JSONSerialization, we use JSONDecoder when we can guarantee that the object we're creating conforms to Codable. Let's start off with a simple eample using another old friend, Cat! (conveniently provided as its own file in the starter project):

struct Cat: Codable {
	let name: String
	let breed: String
	let snack: String
}

We'll be using the provided CatRequester to make our API requests. At the top of the class you'll notice a list of URL to use for various examples:

	let example1URL = URL(string: "https://api.myjson.com/bins/1h4707")!
	let example2URL = URL(string: "https://api.myjson.com/bins/fq67r")!
	let example3URL = URL(string: "https://api.myjson.com/bins/oatbr")!
	let example4URL = URL(string: "https://api.myjson.com/bins/vg0l3")!

Basic JSON Request

We're first going fill out the function labeled makeBasicCatRequest and make a request using example1URL. If we plug in example1URL into Postman, we get

{
    "name": "Miley",
    "breed": "American Shorthair",
    "snack": "Chicken"
}

The key to using a model that conforms to Codable along with JSONDecoder is to ensure that the names of instance properties of the model match the keys in the JSON response. Our Cat model has three properties, name, breed, snack which correspond to the keys being returned in the JSON response, name, breed and snack. You don't have to know the full details yet on how this works, but just understand that this is how you make this kind of decoding work.

Now, we'll do a basic URLSession data task to make a model:

func makeBasicCatRequest() {
	urlSession.dataTask(with: example1URL) { (data: Data?, _, _) in
		if data != nil {
			do {
				let cat = try JSONDecoder().decode(Cat.self, from: data!)
				print("\n\nNice to meet you, I'm ", cat.name)
			}
			catch {
				print("Error converting Data into Cat!", error)
			}
		}
	}.resume()
}

Incorrectly Named JSON Keys

Ok, now replace example1URL with example2URL and observe the difference. Putting in example2URL into Postman gives us:

{
    "fullname": "Miley",
    "breed": "American Shorthair",
    "snack": "Chicken"
}

With the JSON key (fullname) no longer matching the model's property name (Cat.name), attempting to decode results in an error.

Nested JSON Structure

Let's now take a look at a nested dictionary structure. Replace example2URL with example3URL and verify that in Postman you're getting this JSON response:

{
  "cat": {
    "name": "Miley",
    "breed": "American Shorthair",
    "snack": "Chicken"
  }
}

In this example, our Cat dictionary is wrapped up using a "cat" key/value pair. To access the deeper level of the dictionary, (to get name, breed, and snack) we can simply create a wrapper struct like so:

// Nested Cat object wrapped in a "cat" key
struct CatContainer: Codable {
	let cat: Cat
}

Now, we just need to update the code so that JSONDecoder expects to decode a CatContainer rather than a Cat directly.

	// just change the expected type from `Cat` to `CatContainer`!
	let catContainer = try JSONDecoder().decode(CatContainer.self, from: data!)
	print("\n\nNice to meet you, I'm ", catContainer.cat.name)

JSON Array Structure

Let's look at one more example where the root object is an array of Cats. We're going to use example4URL which should have a JSON response like:

{
  "cats": [
    {
      "name": "Miley",
      "breed": "American Shorthair",
      "snack": "Chicken"
    },
    {
      "name": "Bale",
      "breed": "Russian Blue",
      "snack": "Kibble"
    }
  ]
}

You might already be able to guess what needs to be done for this to work... make a wrapper struct!

struct CatArrayContainer: Codable {
	let cats: [Cat]
}

And now we just update our JSONDecoder code to make use of the new wrapper:

let catArrayContainer = try JSONDecoder().decode(CatArrayContainer.self, from: data!)
for cat in catArrayContainer.cats {
	print("\n\nNice to meet you, I'm ", cat.name)
}

---

2. Exercises

For this set of exercises, follow these general guidelines:

1. Create a new empty swift file named Pods.swift to add all of the structs you will be writing
2. Create a new file, PodRequestor.swift to create a request for each example problem. You should name these functions like func example1Request, func example2Request, etc...
3. Call each new request in either the AppDelegate's didFinishLaunching or in ViewController's viewDidLoad
4. There are no tests for these exercises, so instead print out to console the result of each successful request. For example after creating a Podcast object, have print("Podcast created: ", podcast.podcast)
5. Exercise answers are provided to you under Exercises/Exercises.md. Be sure to attempt a problem before checking the answer

Note: For each of these exercises, make sure that you're checking out the JSON response for each URL using Postman. The response should match the snippets provided with the exercise questions, but you are to verify this.

Pod(s) Save America

---

Example 1: https://api.myjson.com/bins/tq46v

• Create a new model, Podcast that conforms to Codable
• Make a request the the URL listed and create an instance of Podcast

{
    "podcast": "Pod Save America",
    "producer": "Crooked Media",
    "url": "https://itunes.apple.com/us/podcast/pod-save-america/id1192761536?mt=2"
}

---

Example 2: https://api.myjson.com/bins/182vl3

• Create a new wrapper, PodInfo to house a Podcast object

{
    "pod": {
        "podcast": "Pod Save America",
        "producer": "Crooked Media",
        "url": "https://itunes.apple.com/us/podcast/pod-save-america/id1192761536?mt=2"
    }
}

---

Example 3: https://api.myjson.com/bins/n8pev

• Create a new struct, Episode

{
  "title": "Making Redistricting Sexy Again...",
  "time": "1hr 19min",
  "released": "June 6 2017",
  "number": 1
}

---

Example 4: https://api.myjson.com/bins/mn9t3

• Expand Pod to include [Episode]

{
    "pod": {
        "podcast": "Pod Save America",
        "producer": "Crooked Media",
        "url": "https://itunes.apple.com/us/podcast/pod-save-america/id1192761536?mt=2",
        "episodes": [
            {
                "title": "Making Redistricting Sexy Again...",
                "time": "1hr 19min",
                "released": "June 6 2017",
                "number": 1
            }
        ]
    }
}

---

Example 5: https://api.myjson.com/bins/18qgcn

• Extend PodInfo to expect an array of Podcast

{
  "pods": [
    {
      "podcast": "Pod Save America",
      "producer": "Crooked Media",
      "url": "https://itunes.apple.com/us/podcast/pod-save-america/id1192761536?mt=2",
      "episodes": [
        {
          "title": "Making Redistricting Sexy Again...",
          "time": "1hr 19min",
          "released": "June 6 2017",
          "number": 1
        }
      ]
    },
    {
      "podcast": "The Daily",
      "producer": "New York Times",
      "url": "https://itunes.apple.com/us/podcast/the-daily/id1200361736?mt=2",
      "episodes": [
        {
          "title": "Friday July 7th",
          "time": "22min",
          "released": "June 7 2017",
          "number": 1
        }
      ]
    }
  ]
}

---

Example 6: https://api.myjson.com/bins/7xv5z

• Extend PodInfo to have a new property for meta data. The meta property should be of type PodMeta and also Codable

{
    "meta": {
        "date_requested": "2017-07-07 17:23:50 +0000"
    },
    "pods": [
        {
            "podcast": "Pod Save America",
            "producer": "Crooked Media",
            "url": "https://itunes.apple.com/us/podcast/pod-save-america/id1192761536?mt=2",
            "episodes": [
                {
                    "title": "Making Redistricting Sexy Again...",
                    "time": "1hr 19min",
                    "released": "June 6 2017",
                    "number": 1
                }
            ]
        },
        {
            "podcast": "The Daily",
            "producer": "New York Times",
            "url": "https://itunes.apple.com/us/podcast/the-daily/id1200361736?mt=2",
            "episodes": [
                {
                    "title": "Friday July 7th",
                    "time": "22min",
                    "released": "June 7 2017",
                    "number": 1
                }
            ]
        }
    ]
}

---

Once More With Gusto!

The previous examples should have given you an understand on how you can start from a very simple JSON structure and build it up to into something far more complicated.

In this last example, you will be challenged to do this entire building process on your own. You'll be using a URL from the Random User API that I provide in order to create a few new structs. I will also provide you with a sample output JSON using that URL.

What you'll need to do is the following:

1. Create a wrapper, APIResult that will contain an array of User and a dictionary Info
2. The User struct, will have several properties: gender, email, registered
3. The User struct will also have several Codable properties: name: Name, picture: Picture
4. The Name struct should have three properties: title, first, last
5. The Picture struct should have three properties: large, medium, thumbnail
6. The Info struct will have four properties: seed, results, page, version. Please be aware that results and pages are not String

URL: https://randomuser.me/api/?nat=us&inc=gender,name,email,registered,picture

{
    "results": [
        {
            "gender": "male",
            "name": {
                "title": "mr",
                "first": "cory",
                "last": "wagner"
            },
            "email": "cory.wagner@example.com",
            "registered": "2011-07-21 03:55:39",
            "picture": {
                "large": "https://randomuser.me/api/portraits/men/17.jpg",
                "medium": "https://randomuser.me/api/portraits/med/men/17.jpg",
                "thumbnail": "https://randomuser.me/api/portraits/thumb/men/17.jpg"
            }
        }
    ],
    "info": {
        "seed": "a10c8066754a7d56",
        "results": 1,
        "page": 1,
        "version": "1.1"
    }
}

---

Advanced: Formatting Time

In example 6, you received a new key meta that had a single key/value date. That problem only requires you to express the date passed as a String but now you are tasked with parsing the value as a Date instead. You must convert the formatted date by using DateFormatter so that it reads: "Month Day, Year <Hour:Minute>" in console.

Implementation hints

1. You're going to need to create & use DateFormatter
2. You need to change Meta.date_requested from a String to Date
3. You'll need to change JSONDecoder's dateDecodingStrategy to .dateFormatted(DateFormatter)
4. You're going to need to update your DateFormatter's .dateFormat property twice: once for decoding the string value of the date from the JSON into Date and then once again to output the Date as a String

Resources For Advanced

1. Ultimate Guide to JSON Parsing with Swift 4
2. NSDateFormatter.com