Briefly, while using Xcode 9 Beta, I have run into the following warning:

&gt;The use of Swift 3 @objc inference in Swift 4 mode is deprecated. Please address deprecated @objc inference warnings, test your code with “Use of deprecated Swift 3 @objc inference” logging enabled, and disable Swift 3 @objc inference.**

After some research, I still have no idea how to fix the issue. 
I would greatly appreciate any tips on how to fix this issue as well as an explanation of what is going on. 

My goal is to grasp a better understanding of what is happening with my code. 



The use of Swift 3 @objc inference in Swift 4 mode is deprecated?

In Swift 2, I was able to use `dispatch_after` to delay an action using grand central dispatch:

    var dispatchTime: dispatch_time_t = dispatch_time(DISPATCH_TIME_NOW, Int64(0.1 * Double(NSEC_PER_SEC))) 
    dispatch_after(dispatchTime, dispatch_get_main_queue(), { 
        // your function here 
    })

But this no longer seems to compile since Swift 3. What is the preferred way to write this in modern Swift?

How do I write dispatch_after GCD in Swift 3, 4, and 5?

I have the following simple code written in Swift 3:

    let str = &quot;Hello, playground&quot;
    let index = str.index(of: &quot;,&quot;)!
    let newStr = str.substring(to: index)

From Xcode 9 beta 5, I get the following warning:

&gt; &#39;`substring(to:)`&#39; is deprecated: Please use `String` slicing subscript with a &#39;partial range from&#39; operator.

How can this **slicing subscript with partial range from** be used in Swift 4?

How can I use String substring in Swift 4? &#39;substring(to:)&#39; is deprecated: Please use String slicing subscript with a &#39;partial range from&#39; operator

After updating to the latest version of Xcode at the moment (version 10.0) the project is unable to build because it found some errors regarding some &quot;Command CompileSwift failed with a nonzero exit code&quot; error.

**How do I solve this errors?**
They appear in most of the Pods (I use CocoaPods) I use inside my project.

I have tried updating the pod version and the pods to the latest version available, but the problem is still there.

I have searched a lot through the web and there is very little information regarding this issue.

Command CompileSwift failed with a nonzero exit code in Xcode 10

[characters](https://developer.apple.com/documentation/swift/string/1540072-characters) - an instance property of String, is deprecated from with [Xcode 9.1](https://stackoverflow.com/questions/44390162/download-install-xcode-9-1-9-2-9-3-beta-or-xcode-9-version-without-premium-d/44390183#44390183)

It was very useful to get a substring from String by using the `characters` property but now it has been deprecated and Xcode suggests to use `substring`. I&#39;ve tried to check around SO questions and apple developer tutorials/guidelines for the same. But could not see any solution/alternate as suggested.

Here is warning message:

&gt; **&#39;characters&#39; is deprecated: Please use String or Substring**

 
[![enter image description here][1]][1]

I&#39;ve so many string operations are performed/handled using property `characters`.

Anyone have any idea/info about this update?

  [1]: https://i.stack.imgur.com/vBDCE.png

warning: &#39;characters&#39; is deprecated: Please use String or Substring directly

Let&#39;s say I have `Customer` data type which contains a `metadata` property that can contains any JSON dictionary in the customer object

    struct Customer {
      let id: String
      let email: String
      let metadata: [String: Any]
    }


----------


    {  
      &quot;object&quot;: &quot;customer&quot;,
      &quot;id&quot;: &quot;4yq6txdpfadhbaqnwp3&quot;,
      &quot;email&quot;: &quot;john.doe@example.com&quot;,
      &quot;metadata&quot;: {
        &quot;link_id&quot;: &quot;linked-id&quot;,
        &quot;buy_count&quot;: 4
      }
    }

**The `metadata` property can be any arbitrary JSON map object.**

Before I can cast the property from a deserialized JSON from `NSJSONDeserialization` but with the new Swift 4 `Decodable` protocol, I still can&#39;t think of a way to do that.

Do anyone know how to achieve this in Swift 4 with Decodable protocol?

How to decode a property with type of JSON dictionary in Swift [45] decodable protocol

Swift 4 added the new `Codable` protocol. When I use `JSONDecoder` it seems to require all the non-optional properties of my `Codable` class to have keys in the JSON or it throws an error.

Making every property of my class optional seems like an unnecessary hassle since what I really want is to use the value in the json or a default value. (I don&#39;t want the property to be nil.)

Is there a way to do this?

    class MyCodable: Codable {
        var name: String = &quot;Default Appleseed&quot;
    }

    func load(input: String) {
        do {
            if let data = input.data(using: .utf8) {
                let result = try JSONDecoder().decode(MyCodable.self, from: data)
                print(&quot;name: \(result.name)&quot;)
            }
        } catch  {
            print(&quot;error: \(error)&quot;)
            // `Error message: &quot;Key not found when expecting non-optional type
            // String for coding key \&quot;name\&quot;&quot;`
        }
    }

    let goodInput = &quot;{\&quot;name\&quot;: \&quot;Jonny Appleseed\&quot; }&quot;
    let badInput = &quot;{}&quot;
    load(input: goodInput) // works, `name` is Jonny Applessed
    load(input: badInput) // breaks, `name` required since property is non-optional


With JSONDecoder in Swift 4, can missing keys use a default value instead of having to be optional properties?

While using Swift4 and Codable protocols I got the following problem - it looks like there is no way to allow `JSONDecoder` to skip elements in an array.
For example, I have the following JSON:

    [
        {
            &quot;name&quot;: &quot;Banana&quot;,
            &quot;points&quot;: 200,
            &quot;description&quot;: &quot;A banana grown in Ecuador.&quot;
        },
        {
            &quot;name&quot;: &quot;Orange&quot;
        }
    ]

And a &lt;i&gt;Codable&lt;/i&gt; struct:

    struct GroceryProduct: Codable {
        var name: String
        var points: Int
        var description: String?
    }

When decoding this json

    let decoder = JSONDecoder()
    let products = try decoder.decode([GroceryProduct].self, from: json)

Resulting `products` is empty. Which is to be expected, due to the fact that the second object in JSON has no `&quot;points&quot;` key, while `points` is not optional in `GroceryProduct` struct.

Question is how can I allow `JSONDecoder` to &quot;skip&quot; invalid object?

Swift JSONDecode decoding arrays fails if single element decoding fails

I have a struct that implements Swift 4’s `Codable`. Is there a simple built-in way to encode that struct into a dictionary?

    let struct = Foo(a: 1, b: 2)
    let dict = something(struct)
    // now dict is [&quot;a&quot;: 1, &quot;b&quot;: 2]

How can I use Swift’s Codable to encode into a dictionary?

I&#39;m trying to convert my project&#39;s source code from Swift 3 to Swift 4. One warning Xcode is giving me is about my selectors.

For instance, I add a target to a button using a regular selector like this:

    button.addTarget(self, action: #selector(self.myAction), for: .touchUpInside)

This is the warning it shows:

&gt;Argument of &#39;#selector&#39; refers to instance method &#39;myAction()&#39; in &#39;ViewController&#39; that depends on &#39;@objc&#39; attribute inference deprecated in Swift 4
&gt;
&gt; Add &#39;@objc&#39; to expose this instance method to Objective-C

Now, hitting `Fix` on the error message does this to my function:

    // before
    func myAction() { /* ... */ }

    // after
    @objc func myAction() { /* ... */ }

I don&#39;t really want to rename all of my functions to include the `@objc` mark and I&#39;m assuming that&#39;s not necessary.

How do I rewrite the selector to deal with the deprecation?

---

**Related question:**

- https://stackoverflow.com/questions/44379348/the-use-of-swift-3-objc-inference-in-swift-4-mode-is-deprecated

How can I deal with @objc inference deprecation with #selector() in Swift 4?

Swift 4 introduced support for native JSON encoding and decoding via the [`Decodable`][1] protocol. How do I use custom keys for this?

E.g., say I have a struct

    struct Address:Codable {
        var street:String
        var zip:String
        var city:String
        var state:String
    }

I can encode this to JSON.

    let address = Address(street: &quot;Apple Bay Street&quot;, zip: &quot;94608&quot;, city: &quot;Emeryville&quot;, state: &quot;California&quot;)
    
    if let encoded = try? encoder.encode(address) {
        if let json = String(data: encoded, encoding: .utf8) {
            // Print JSON String
            print(json)

            // JSON string is 
               { &quot;state&quot;:&quot;California&quot;, 
                 &quot;street&quot;:&quot;Apple Bay Street&quot;, 
                 &quot;zip&quot;:&quot;94608&quot;, 
                 &quot;city&quot;:&quot;Emeryville&quot; 
               }
        }
    }

I can encode this back to an object.

        let newAddress: Address = try decoder.decode(Address.self, from: encoded)

But If I had a json object that was 

    { 
       &quot;state&quot;:&quot;California&quot;, 
       &quot;street&quot;:&quot;Apple Bay Street&quot;, 
       &quot;zip_code&quot;:&quot;94608&quot;, 
       &quot;city&quot;:&quot;Emeryville&quot; 
    }

How would I tell the decoder on `Address` that `zip_code` maps to `zip`? I believe you use the new `CodingKey` protocol, but I can&#39;t figure out how to use this.


  [1]: https://developer.apple.com/documentation/swift/decodable

How do I use custom keys with Swift 4&#39;s Decodable protocol?

Here is my JSON

    {
        &quot;id&quot;: 1,
        &quot;user&quot;: {
            &quot;user_name&quot;: &quot;Tester&quot;,
            &quot;real_info&quot;: {
             	&quot;full_name&quot;:&quot;Jon Doe&quot;
            }
        },
        &quot;reviews_count&quot;: [
            {
                &quot;count&quot;: 4
            }
        ]
    }
    

Here is the structure I want it saved to (incomplete)

    struct ServerResponse: Decodable {
    	var id: String
    	var username: String
    	var fullName: String
    	var reviewCount: Int
    	
    	enum CodingKeys: String, CodingKey {
           case id, 
	       // How do i get nested values?
    	}
    }

I have looked at [Apple&#39;s Documentation][1] on decoding nested structs, but I still do not understand how to do the different levels of the JSON properly. Any help will be much appreciated.


  [1]: https://developer.apple.com/documentation/foundation/archives_and_serialization/encoding_and_decoding_custom_types

How to decode a nested JSON struct with Swift Decodable protocol?

This code works ok in ios10. i get my label and an image button which is the user photo profile, circular round.. ok. but when running xcode 9 ios11 simulator i get it streched out. the button frame has to be 32x32 , when checking on the sim and getting the view and telling xcode to describe the view i get output as 170x32 or somethint like that.

heres my code.

    let labelbutton = UIButton( type: .system)
        labelbutton.addTarget(self, action:#selector(self.toLogin(_:)), for: .touchUpInside)
        labelbutton.setTitleColor(UIColor.white, for: .normal)
        labelbutton.contentHorizontalAlignment = .right
        labelbutton.titleLabel?.font = UIFont.systemFont(ofSize: 18.00)
        
       
        
        let button = UIButton(type: .custom)
         button.addTarget(self, action:#selector(self.toLogin(_:)), for: .touchUpInside)
         button.frame = CGRect(x: 0, y: 0, width: 32, height: 32)
         button.setTitleColor(UIColor.white, for: .normal)
         button.setTitleColor(UIColor.white, for: .highlighted)
        
      
        var buttomItem : UIBarButtonItem = UIBarButtonItem()
        buttomItem.customView = button
        buttomItem.target = self
        buttomItem.action = &quot;ToLogin&quot;
        
        var labelItem : UIBarButtonItem = UIBarButtonItem()
        labelItem.customView = labelbutton
        labelItem.target = self
        labelItem.action = &quot;ToLogin&quot;
        
        
        if let user = PFUser.current() {
            print(&quot;LOGIN : checkiando si existe usuario &quot;)
                labelbutton.setTitle(USERNAME, for: UIControlState.normal)
                labelbutton.sizeToFit()
            
            if(user[&quot;profile_photo_url&quot;] != nil) {
                print(&quot; ENCONTRO PROFILE PHOTO URL NOT NIL Y ES \(user[&quot;profile_photo_url&quot;])&quot;)
                let photoURL = user[&quot;profile_photo_url&quot;] as! String
                let a = LoginService.sharedInstance
                a.downloadImage(url: photoURL, complete: { (complete) in
                    
                    if (complete) {
                  
                        button.setImage(LoginService.sharedInstance.profile_photo! , for: UIControlState.normal)
                        
                        button.layer.cornerRadius = 0.5 * button.bounds.size.width
                       // button.imageView!.contentMode = .scaleAspectFit
                       // button.imageView!.frame = CGRect(x: 0, y: 0, width: 40, height: 40)
                        //button.imageView!.contentMode = .scaleAspectFit
                        //button.imageView!.clipsToBounds = true
                        //button.imageView!.layer.cornerRadius = 60
                        button.clipsToBounds = true
                        self.NavigationItem.rightBarButtonItems = [buttomItem,labelItem]
                    }
                    
                    
                })
            } else {
                    self.NavigationItem.rightBarButtonItem = labelItem
                
            }
                print(&quot; EL FRAME DEL BUTTON ES \(button.frame)&quot;)
            
        } else {
   
            labelbutton.setTitle(&quot;Login&quot;, for: UIControlState.normal)
            labelbutton.sizeToFit()
            self.NavigationItem.rightBarButtonItem = labelItem
    
        }
   
[![enter image description here][1]][1]


  [1]: https://i.stack.imgur.com/zuB9A.png

navigation bar rightbaritem image-button bug iOS 11

 Why am I getting a &quot;Type &#39;Bookmark&#39; does not conform to protocol &#39;Decodable&#39;&quot; error message?

    class Bookmark: Codable {
       weak var publication: Publication?
       var indexPath: [Int]
       var locationInText = 0
       
       enum CodingKeys: String, CodingKey {
          case indexPath
          case locationInText
       }
       
       init(publication: Publication?, indexPath: [Int]) {
          self.publication = publication
          self.indexPath = indexPath
       }
    }

I do not wish to save the publication var since the Publication owns the Bookmark but the bookmark needs to know which Publication it belongs to. The decode init of Publication will set the bookmark reference to itself.

Codable class does not conform to protocol Decodable

Should the use of class inheritance break the Decodability of class.  For example, the following code 



    class Server : Codable {
        var id : Int?
    }

    class Development : Server {
        var name : String?
        var userId : Int?
    }

    var json = &quot;{\&quot;id\&quot; : 1,\&quot;name\&quot; : \&quot;Large Building Development\&quot;}&quot;
    let jsonDecoder = JSONDecoder()
    let item = try jsonDecoder.decode(Development.self, from:json.data(using: .utf8)!) as Development

    print(item.id ?? &quot;id is nil&quot;)
    print(item.name ?? &quot;name is nil&quot;) here


output is: 


    1
    name is nil



Now if I reverse this, name decodes but id does not.

    class Server {
        var id : Int?
    }

    class Development : Server, Codable {
        var name : String?
        var userId : Int?
    }

    var json = &quot;{\&quot;id\&quot; : 1,\&quot;name\&quot; : \&quot;Large Building Development\&quot;}&quot;
    let jsonDecoder = JSONDecoder()
    let item = try jsonDecoder.decode(Development.self, from:json.data(using: .utf8)!) as Development

    print(item.id ?? &quot;id is nil&quot;)
    print(item.name ?? &quot;name is nil&quot;)

output is:

    id is nil
    Large Building Development


And you can&#39;t express Codable in both classes. 

Using Decodable in Swift 4 with Inheritance

However I have recompiled the framework using the same Xcode and it still gives me this error.

- Base SDK iOS 11.1 for both  
- Swift Language Version Swift 4.0 for both
- Not using Pods/Carthage

I hope someone might know

Module compiled with Swift 4.0 cannot be imported in Swift 4.0.1

I&#39;m using Swift 4&#39;s `JSONEncoder`. I have a `Codable` struct with an optional property, and I&#39;d like this property to show up as `null` value in the produced JSON data when the value is `nil`. However, `JSONEncoder` discards the property and does not add it to the JSON output. Is there a way to configure `JSONEncoder` so that it preserves the key and sets it to `null` in this case?

## Example
The code snippet below produces `{&quot;number&quot;:1}`, but I&#39;d rather like it to give me `{&quot;string&quot;:null,&quot;number&quot;:1}`:

    struct Foo: Codable {
      var string: String? = nil
      var number: Int = 1
    }

    let encoder = JSONEncoder()
    let data = try! encoder.encode(Foo())
    print(String(data: data, encoding: .utf8)!)


Encode nil value as null with JSONEncoder

Xcode 9 (iOS 11) showing me an error/warning while registering for Push (remote) notification.

Here is error message

[![enter image description here][1]][1]

And here is code, I&#39;ve tried:

    let center  = UNUserNotificationCenter.current()
    center.delegate = self
    center.requestAuthorization(options: [.sound, .alert, .badge]) { (granted, error) in
            if error == nil{
                  UIApplication.shared.registerForRemoteNotifications()
            }
     }

**Error/Warning Line:**

&gt; UIApplication.shared.registerForRemoteNotifications()

How to resolve this?


  [1]: https://i.stack.imgur.com/GY0iw.png

UIApplication.registerForRemoteNotifications() must be called from main thread only

After updating to Xcode 9, I tried to build one of my projects.

I use the **FacebookLogin** pod.
I have a compiler error in FacebookLogin/LoginButton.swift

    @testable import FacebookCore
    ❌ Module compiled with Swift 3.1 cannot be imported in Swift 4.0

In my target&#39;s build settings, the Swift language version is set to **Swift 3.2**.

[![Screenshot added][1]][1]

I guess I need to wait for Facebook to update their pod ? Or any other suggestion ?

Thanks !


  [1]: https://i.stack.imgur.com/6jYoK.png

Xcode 9 : Module compiled with Swift 3.1 cannot be imported in Swift 4.0

Using the latest Xcode 9 beta, I&#39;m seemingly completely unable to access properties on Swift classes. Even odder, I can access the class itself to instantiate it or whatever, but completely unable to access properties on it.

So if I have this Swift class:

    import UIKit
    
    class TestViewController: UIViewController {
        var foobar = true
    }

And I try to do this:

    TestViewController *testViewController = [[TestViewController alloc] init]; // success
    testViewController.foobar; // error

What exactly am I doing wrong? New project with Xcode 9.

Unable to access Swift 4 class from Objective-C: &quot;Property not found on object of type&quot;

In Swift 4, I&#39;m getting this error when I try to take a ``Substring`` of a ``String`` using subscript syntax.

&gt; &#39;subscript&#39; is unavailable: cannot subscript String with a CountableClosedRange&lt;Int&gt;, see the documentation comment for discussion

For example:

    let myString: String = &quot;foobar&quot;
    let mySubstring: Substring = myString[1..&lt;3]

Two questions:

1. How can I resolve this error?
2. Where is &quot;the documentation comment for discussion&quot; that was referred to in the error?

&#39;subscript&#39; is unavailable: cannot subscript String with a CountableClosedRange&lt;Int&gt;, see the documentation comment for discussion

I have an extension:

    public extension UIWindow {
        override public func topMostController()-&gt;UIViewController? { ... }
    }

but for my `topMostController` I get the next error:

    Declarations in extensions cannot override yet error

It works well for Swift 3.1, but for Swift 4 I get this error. How can it be fixed? What did they change in Swift 4?

Declarations in extensions cannot override yet error in Swift 4

I have been playing around with `Codable` and reading and writing JSON from and to a file. Now I would like to write a custom `Coder` that can read and write iOS `.strings` files. Can anyone help me with this? I found the protocols `Encoder` and `Decoder`, but I have no idea what I should implement here:

    class StringsEncoder {}
    
    extension StringsEncoder: Encoder {
        var codingPath: [CodingKey?] {
            return []
        }
    
        var userInfo: [CodingUserInfoKey : Any] {
            return [:]
        }
    
        func container&lt;Key&gt;(keyedBy type: Key.Type) -&gt; KeyedEncodingContainer&lt;Key&gt; where Key : CodingKey {
    
        }
    
        func unkeyedContainer() -&gt; UnkeyedEncodingContainer {
    
        }
    
        func singleValueContainer() -&gt; SingleValueEncodingContainer {
    
        }
    }
    
    extension StringsEncoder: Decoder {
        func container&lt;Key&gt;(keyedBy type: Key.Type) throws -&gt; KeyedDecodingContainer&lt;Key&gt; where Key : CodingKey {
    
        }
    
        func unkeyedContainer() throws -&gt; UnkeyedDecodingContainer {
    
        }
    
        func singleValueContainer() throws -&gt; SingleValueDecodingContainer {
    
        }
    }

Custom Swift Encoder/Decoder for the Strings Resource Format

I have defined an `enum` as follows:

    enum Type: String, Codable {
        case text = &quot;text&quot;
        case image = &quot;image&quot;
        case document = &quot;document&quot;
        case profile = &quot;profile&quot;
        case sign = &quot;sign&quot;
        case inputDate = &quot;input_date&quot;
        case inputText = &quot;input_text&quot;
        case inputNumber = &quot;input_number&quot;
        case inputOption = &quot;input_option&quot;
    
        case unknown
    }



that maps a JSON string property.
The automatic serialization and deserialization works fine, but I found that if a different string is encountered, the deserialization fails. 

Is it possible to define an `unknown` case that maps any other available case?

This can be very useful, since this data comes from a RESTFul API that, maybe, can change in the future.

Codable enum with default case in Swift 4

I have an API that will sometimes return a specific key value (in this case `id`) in the JSON as an Int and other times it will return that same key value as a String. How do I use codable to parse that JSON?

    struct GeneralProduct: Codable {
        var price: Double!
        var id: String?
        var name: String!
  
        private enum CodingKeys: String, CodingKey {
            case price = &quot;p&quot;
            case id = &quot;i&quot;
            case name = &quot;n&quot;
        }

        init(price: Double? = nil, id: String? = nil, name: String? = nil) {
            self.price = price
            self.id = id
            self.name = name
        }
    }

I keep getting this error message: `Expected to decode String but found a number instead`. The reason that it returns a number is because the id field is empty and when the id field is empty it defaults to returning 0 as an ID which codable identifies as a number. I can basically ignore the ID key but codable does not give me the option to ignore it to my knowledge. What would be the best way to handle this?

Here is the JSON. It is super simple

Working

    {
      &quot;p&quot;:2.12,
      &quot;i&quot;:&quot;3k3mkfnk3&quot;,
      &quot;n&quot;:&quot;Blue Shirt&quot;
    }

Error - because there is no id in the system, it returns 0 as a default which codable obviously sees as a number opposed to string. 

    {
      &quot;p&quot;:2.19,
      &quot;i&quot;:0,
      &quot;n&quot;:&quot;Black Shirt&quot;
    }


Using codable with value that is sometimes an Int and other times a String

Codable seems a very exciting feature. But I wonder how we can use it in Core Data? In particular, is it possible to directly encode/decode a JSON from/to a NSManagedObject?

I tried a very simple example:

[![enter image description here][1]][1]

and defined `Foo` myself:

    import CoreData

    @objc(Foo)
    public class Foo: NSManagedObject, Codable {}

But when using it like this:

    let json = &quot;&quot;&quot;
    {
        &quot;name&quot;: &quot;foo&quot;,
        &quot;bars&quot;: [{
            &quot;name&quot;: &quot;bar1&quot;,
        }], [{
            &quot;name&quot;: &quot;bar2&quot;
        }]
    }
    &quot;&quot;&quot;.data(using: .utf8)!
    let decoder = JSONDecoder()
    let foo = try! decoder.decode(Foo.self, from: json)
    print(foo)

The compiler failed with this errror:

    super.init isn&#39;t called on all paths before returning from initializer

and the target file was the file that defined `Foo`

I guess I probably did it wrong, since I didn&#39;t even pass a `NSManagedObjectContext`, but I have no idea where to stick it.

Does Core Data support `Codable`?

[1]: https://i.stack.imgur.com/rQbUS.png

How to use swift 4 Codable in Core Data?

How does the Swift 4 Decodable protocol cope with a dictionary containing a key whose name is not known until runtime? For example:

      [
        {
          &quot;categoryName&quot;: &quot;Trending&quot;,
          &quot;Trending&quot;: [
            {
              &quot;category&quot;: &quot;Trending&quot;,
              &quot;trailerPrice&quot;: &quot;&quot;,
              &quot;isFavourit&quot;: null,
              &quot;isWatchlist&quot;: null
            }
          ]
        },
        {
          &quot;categoryName&quot;: &quot;Comedy&quot;,
          &quot;Comedy&quot;: [
            {
              &quot;category&quot;: &quot;Comedy&quot;,
              &quot;trailerPrice&quot;: &quot;&quot;,
              &quot;isFavourit&quot;: null,
              &quot;isWatchlist&quot;: null
            }
          ]
        }
      ]

Here we have an array of dictionaries; the first has keys `categoryName` and `Trending`, while the second has keys `categoryName` and `Comedy`. The value of the `categoryName` key tells me the name of the second key. How do I express that using Decodable?

Swift 4 Decodable with keys not known until decoding time

I&#39;m trying to find the best way to Encode/Decode an array of structs conforming to a swift protocol using the new JSONDecoder/Encoder in Swift 4.

I made up a little example to illustrate the problem:

First we have a protocol Tag and some Types that conform to this protocol.

    protocol Tag: Codable {
        var type: String { get }
        var value: String { get }
    }
    
    struct AuthorTag: Tag {
        let type = &quot;author&quot;
        let value: String
    }
    
    struct GenreTag: Tag {
        let type = &quot;genre&quot;
        let value: String
    }


Then we have a Type Article which has an Array of Tags.


    struct Article: Codable {
        let tags: [Tag]
        let title: String
    }

Finally we encode or decode the Article 


    let article = Article(tags: [AuthorTag(value: &quot;Author Tag Value&quot;), GenreTag(value:&quot;Genre Tag Value&quot;)], title: &quot;Article Title&quot;)
    
    
    let jsonEncoder = JSONEncoder()
    let jsonData = try jsonEncoder.encode(article)
    let jsonString = String(data: jsonData, encoding: .utf8)
    
And this is the JSON structure that I like to have.

    {
     &quot;title&quot;: &quot;Article Title&quot;,
     &quot;tags&quot;: [
         {
           &quot;type&quot;: &quot;author&quot;,
           &quot;value&quot;: &quot;Author Tag Value&quot;
         },
         {
           &quot;type&quot;: &quot;genre&quot;,
           &quot;value&quot;: &quot;Genre Tag Value&quot;
         }
     ]
    }

The problem is that at some point I have to switch on the type property to decode the Array but to Decode the Array I have to know its type.

**EDIT:**

It&#39;s clear to me why Decodable can not work out of the box but at least Encodable should work. The following modified Article struct compiles but crashes with the following error message.

    fatal error: Array&lt;Tag&gt; does not conform to Encodable because Tag does not conform to Encodable.: file /Library/Caches/com.apple.xbs/Sources/swiftlang/swiftlang-900.0.43/src/swift/stdlib/public/core/Codable.swift, line 3280

    struct Article: Encodable {
        let tags: [Tag]
        let title: String
        
        enum CodingKeys: String, CodingKey {
            case tags
            case title
        }
        
        func encode(to encoder: Encoder) throws {
            var container = encoder.container(keyedBy: CodingKeys.self)
            try container.encode(tags, forKey: .tags)
            try container.encode(title, forKey: .title)
        }
    }
    
    let article = Article(tags: [AuthorTag(value: &quot;Author Tag&quot;), GenreTag(value:&quot;A Genre Tag&quot;)], title: &quot;A Title&quot;)
    
    let jsonEncoder = JSONEncoder()
    let jsonData = try jsonEncoder.encode(article)
    let jsonString = String(data: jsonData, encoding: .utf8)

And this is the relevant part from Codeable.swift

    guard Element.self is Encodable.Type else {
        preconditionFailure(&quot;\(type(of: self)) does not conform to Encodable because \(Element.self) does not conform to Encodable.&quot;)
    }

Source: https://github.com/apple/swift/blob/master/stdlib/public/core/Codable.swift

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