I've got the following snippet:
protocol MyProtocol: Identifiable where ID == UUID {
var id: UUID { get }
}
var test: [MyProtocol] = []
Protocol 'MyProtocol' can only be used as a generic constraint because it has Self or associated type requirements
Why doesn't this work? Shouldn't the where ID == UUID remove the ambiguity the error is concerned with? Am I missing something here?
I think this question is similar to this one: Usage of protocols as array types and function parameters in swift
However, I would have assumed that adding where ID == UUID should fix the problem? Why is that not the case?
Thanks!
Edit
So, this problem has occurred while experimenting with SwiftUI and struct data models. I've always used classes for any kind of data model but it seems like SwiftUI wants to get you to use structs as often as possible (I still don't see how that's realistically possible but that's why I'm experimenting with it).
In this particular case, I tried to have a manager that contains structs that all conform to MyProtocol. For example:
protocol MyProtocol: Identifiable where ID == UUID {
var id: UUID { get }
}
struct A: MyProtocol { // First data model
var id: UUID = UUID()
}
struct B: MyProtocol { // Second data model
var id: UUID = UUID()
}
class DataManager: ObservableObject {
var myData: [MyProtocol]
}
...
I don't actually have to declare Identifiable on MyProtocol but I thought it would be nicer and cleaner.
Because this is not a current feature of Swift. Once there is an associated type, there is always an associated type. It doesn't go away just because you constrain it. And once it has an associated type, it is not concrete.
There is no way to "inherit" protocols this way. What you mean is:
protocol MyProtocol {
var id: UUID { get }
}
And then you can attach Identifiable to structs that require it:
struct X: MyProtocol, Identifiable {
var id: UUID
}
(note that no where clause is required.)
There is no Swift feature today that allows you to say "types that conform to X implicitly conform to Y." There is also no Swift feature today that allows for an Array of "things that conform to Identifiable with ID==UUID." (That's called a generalized existential, and it's not currently available.)
Most likely you should go back to your calling code and explore why you require this. If you post the code that iterates over test and specifically requires the Identifiable conformance, then we may be able to help you find a design that doesn't require that.
Related
I have a root object that consists mainly of an array of tuples and then a bunch of state enums. The tuples are described this way:
typealias Program = [(Number, Statement)]
The root object is currently NSCoding, but I'd like to add Codable to make the switch to JSON. In most cases, making the various parts become Codable requires nothing more than adding , Codable to their declarations, but that's not working for the tuples. I think this is just a syntax issue, but neither of these work:
typealias Program, Codable = [(Number, Statement)]
typealias Program = [(Number, Statement)], Codable
Is this possible? The horrible documentation only mentions typealias when describing the protocol itself, and perhaps my Google-fu is failing, but I'm not seeing cogent references here on SO.
UPDATE: root object(s):
typealias Program = [(Number, Statement)]
public enum InterpreterState: Int, Codable {
case Idle
case ReadingStatement // while parsing code
case Running // while running it
case ReadingInput // while waiting on user input
}
public final class Interpreter: NSObject, NSCoding {
/// Array of program lines
var program: Program = Array()
/// State
var state: InterpreterState = .Idle
etc.
Tuples are not Codable. If you think about it, what should the keys for (Number, Statement) be? (I might have made it sound like named tuples are codable, but they aren't either)
Create your own struct:
struct ProgramLine: Codable {
let lineNumber: Number // assuming these types are codable
let statement: Statement
}
And then your type alias will be codable too (without you doing anything)!
typealias Program = [ProgramLine]
The swift4's Codable protocol is extremely useful. It provide default implementation functions if the conformation is rightly defined.
E.g. this is totally fine:
struct Good: Codable {
var foo: String // Non-optional
var bar: Int? // Optional
}
but this one will raise compile error with the request to create protocol conforming
struct Bad: Codable {
var foo: UIButton // Non-optional raise compile error for not conforming Codable Protocol
var bar: UIView? // optional is okay (not compile error because when decode failed, it fallback to nil)
var codable: SomeCodable // if the property is also Codable, then it's fine too!
}
So, the question is: Can I write a protocol that can require its conformance to follow itself (like properties need to conform same protocol)?
If yes, how? If no, why?
Also, I'd also like to know how defining CodingKeys inside the struct can change the encode/decode behaviour? Can I make something like that in my protocol as well?
Martin is correct you cannot make this on your own without touching the compiler.
First let's take a look at this basic example where I explain how coding keys are used.
struct CodableStruct: Codable {
let primitive: Int // No issues yet
enum CodingKeys: String, CodingKey {
case primitive
// This is the default coding key (i.e the JSON has structure ["primitive": 37]
// You can change this key to anything you need
//
// ex case primitive = "any_thing_you_want"
// JSON has to have structure ["any_thing_you_want": 37]
}
}
Changing the codingKey just changes the key the code will use when looking to "decode" that value from your JSON.
Now let's talk about the compiler. Let's say we create another struct
struct NotCodableStruct {
let number: Double
}
This struct does not conform to Codable. If we go and add this into our previous struct we have:
struct CodableStruct: Codable {
let primative: Int
let notCodable: NotCodableStruct // doesn't compile because this doesn't conform to codable
enum CodingKeys: String, CodingKey {
case primative
case notCodable
}
}
Since NotCodableStruct does not conform to Codable the compiler complains. In other words all variables in a struct or object that conforms to Codable must also conform to Codable. See the below screenshot for more information.
Of course if you make NotCodableStruct conform to Codable everyone will be happy again. Since there is no way for you to enforce the requirement that all variables conform to Codable you cannot make a similar protocol.
I was going to reflect some JSON data from web service into swift struct. So I created a protocol which conforms to decodable protocol and planed to create some structs to conform it. This is the protocol I had created:
protocol XFNovelApiResponse: Decodable {
var data: Decodable {get}
var error: NovelApiError {get}
}
struct NovelApiError: Decodable {
let msg: String
let errorCode: String
}
It was compiled. But when I started to write my struct I got an error. The struct's code is here:
struct XFNovelGetNovelsApiResponse: XFNovelApiResponse {
let data: NovelsData
let error: NovelApiError
struct NovelsData: Decodable {
}
}
The error says type 'XFNovelGetNovelsApiResponse' does not conform to protocol 'XFNovelApiResponse'. I know the 'data' property should be implemented in wrong way. How can I fix it? Thanks.
You are asking to describe the kind of type that data can hold, rather than the actual type. That means it needs to be an associatedtype:
protocol XFNovelApiResponse: Decodable {
associatedtype DataType: Decodable
var data: DataType {get}
var error: NovelApiError {get}
}
Note that protocols with associated types can generate a lot of complexity, so you should carefully consider if this protocol is really necessary, or if XFNovelApiResponse could, for example, be generic instead. It depends on what other types implement this protocol.
For example, another implementation of a similar set of data structures without protocols would be:
struct XFNovelApiResponse<DataType: Decodable>: Decodable {
var data: DataType
var error: NovelApiError
}
struct NovelsData: Decodable {
}
struct NovelApiError: Decodable {
let msg: String
let errorCode: String
}
let novels = XFNovelApiResponse(data: NovelsData(),
error: NovelApiError(msg: "", errorCode: ""))
Alternately, you can implement this with classes and subclasses, which allow inheritance. Structs do not inherit from protocols, they conform to protocols. If you really mean inheritance, classes are the right tool. (But I expect generics are the better solution here.)
I'm trying to add an extension to Observable.
The code looks like this:
extension Observable where Element == ApiResponse<ItemsContainer<T>>, T:Codable
I'm receiving the following exception: Use of undeclared type T.
So apparently this doesn't work.
The only thing missing is to constrain the generic inside ItemsContainer to conform to Codable.
Could be as simple as a syntactical issue or maybe I'm just not good enough with generics. Any help is appreciated!
Edit: To give the idea - ApiResponse and ItemsContainer look like this
public struct ApiResponse<ApiModel> {
public let data: ApiModel?
}
struct ItemsContainer<Items>: Codable where Items: Codable {
let items: [Items]
}
Issue
You cannot constraint extensions to a Model Type which holds generic values, without specifying the Model Type of the generic value.
You only constrain protocols based on their associatedtypes or generics based on their generic type, on the extension signature. Therefore T is not recognized, because none of the protocols or generic declare it.
Solution
So by keeping in mind what I said above, a Model Type needs to be fully defined on the extension context. But wait that does not satisfy our requirement, we want it to be generic!
Then we do not need a Model Type, we need a protocol!
We have two Model Types (ApiResponse and ItemsContainer) which we need to know the generic type, therefore we need two protocols for each of them.
ApiResponse
Let's create one named ApiResponseProtocol
public protocol ApiResponseProtocol {
associatedtype Model
var data: Model? { get }
}
Cool, the associatedtype Model will play our role as the generic value for the ApiModel on the object. Let's make ApiResponse conform to ApiResponseProtocol
public struct ApiResponse<ApiModel>: ApiResponseProtocol {
public let data: ApiModel?
}
Generic ApiModel here can be defined as Model from the protocol.
ItemsContainer
Next steps would be the same for the ItemsContainer
public protocol ItemsContainerProtocol {
associatedtype Item
var items: [Item] { get }
}
public struct ItemsContainer<Items>: Codable, ItemsContainerProtocol where Items: Codable {
public let items: [Items]
}
Extension
Now since we can access each of the generic types from the protocol (associatedtypes), the output would become something like this:
// This would be for example ApiResponse<ItemsContainer<Model>> where Model is a Model Type conforming to Codable
extension Observable where Element: ApiResponseProtocol, Element.Model: ItemsContainerProtocol, Element.Model.Item: Codable {}
I know this question has been asked before but I have no idea how to solve this current problem. I have defined a protocol MultipleChoiceQuestionable with an associatedtype property:
protocol Questionable {
var text: String {get set}
var givenAnswer: String? {get set}
}
protocol MultipleChoiceQuestionable: Questionable {
associatedtype Value
var answers: Value { get }
}
struct OpenQuestion: Questionable {
var text: String
var givenAnswer: String?
}
struct MultipleChoiceQuestion: MultipleChoiceQuestionable {
typealias Value = [String]
var text: String
var givenAnswer: String?
var answers: Value
}
struct NestedMultipleChoiceQuestion: MultipleChoiceQuestionable {
typealias Value = [MultipleChoiceQuestion]
var text: String
var answers: Value
var givenAnswer: String?
}
Types which conform to this protocol are saved in an array as Questionable like so:
// This array contains OpenQuestion, MultipleChoiceQuestion and NestedMultipleChoiceQuestion
private var questions: [Questionable] = QuestionBuilder.createQuestions()
Somewhere in my code I want to do something like:
let question = questions[index]
if let question = question as? MultipleChoiceQuestionable {
// Do something with the answers
question.answers = .....
}
This is not possible because Xcode warns me: Protocol MultipleChoiceQuestionable can only be used as a generic constraint. I've been searching around on how to solve this issue since generics are quite new for me. Apparently Swift doesn't know the type of the associatedtype during compile time which is the reason this error is thrown. I've read about using type erasure but I don't know if that solves my problem. Maybe I should use generic properties instead or perhaps my protocols are defined wrong?
If the action you want to apply to your sub-protocol objects does not rely on the associated type (i.e. neither has the a generic parameter nor returns the generic type) you can introduce an auxiliary protocol which just exposes the properties/methods you need, let your type conform to that protocol, and declare the question in terms of that protocol.
For example, if you just want to know some info about the question:
protocol MultipleChoiceInfo {
var numberOfAnswers: Int { get }
}
extension MultipleChoiceQuestion: MultipleChoiceInfo {
var numberOfAnswers: Int { return answers.count }
}
// do the same for the other multiple-choice types
Then you can access the questions through the new protocol like this:
let question = questions[index]
if let info = question as? MultipleChoiceInfo {
print(info.numberOfAnswers)
}
As I said, if you cannot provide an abstract (non-generic) interface then this won't work.
EDIT
If you need to process the generic data inside your questions you can extract the logic depending on the concrete generic type into another "processing" type which provides an interface to your questions. Each question type then dispatches its data to the processor interface:
protocol MultipleChoiceProcessor {
func process(stringAnswers: [String])
func process(nestedAnswers: [MultipleChoiceQuestion])
}
protocol MultipleChoiceProxy {
func apply(processor: MultipleChoiceProcessor)
}
extension MultipleChoiceQuestion: MultipleChoiceProxy {
func apply(processor: MultipleChoiceProcessor) {
processor.process(stringAnswers: answers)
}
}
Just create a type conforming to MultipleChoiceProcessor and do the type-check dance again:
if let proxy = question as? MultipleChoiceProxy {
proxy.apply(processor:myProcessor)
}
As an aside, if you don't have more protocols and structs in your real application, you might also just ditch the protocol stuff altogether... for this kind of problem it seems a bit over-engineered.