I have two structs with the same fields. What is the best way to merge them.
struct Type1{
var variable1:String?
var variable2:Double?
var variable3:String?
var notImporant:String?
}
struct Type2{
var variable1A:String?
var variable2A:String?
var variable3A:String!
}
What is the best way to convert type2 to type1? I am getting a return from an API and parsing it using codable but there are two different structs and I need to get one struct. The data is the same, it is just mapped differently in terms of types. Some of the structs have more info and others have less.
Just make a copy constructor in both structs like so:
struct Type2 {
var variable1A:String?
var variable2A:String?
var variable3A:String!
init(_ otherType: Type1) {
variable1A = otherType.variable1
variable2A = otherType.variable2
variable3A = otherType.variable3
}
}
You cannot cast two unrelated structs. What you can do is define a common Protocol for the two of them, and use them in places where you don't care which underlying object it is.
protocol CommonProtocol {
var variable1: String? { get }
var variable3: String? { get }
}
struct Type1: CommonProtocol {
var variable1:String?
var variable2:Double?
var variable3:String?
var notImporant:String?
}
struct Type2: CommonProtocol {
var variable1A:String?
var variable2A:String?
var variable3A:String!
}
Then, in whichever place you're currently stuck with a type1 instead of a type2, have that function just accept a CommonProtocol instead, and you can use either.
Note that, while both of your types have a variable2, one of them is a Double? while the other is a String?. There are a few different ways to approach that, which I leave to you. I just left it out of the protocol.
On another note, it's Swift standard to capitalize the names of structs (Type1, Type2). In certain instances, you can run into problems if you don't, so I suggest you do.
Related
I’m trying to create a struct which acts as a storage for the results being returned from a web API. This API returns different JSON results which are modeled as a set of structs.
These results need to be stored in an array inside of a storage class which will need to be generic as it should be able to store arrays of any of the types returned. I’m struggling, however, with adding generic data to an array… and this is where you guys might come in.
This is the storage class:
class FooStorage<F: Fooable> {
private var storage: [F] = []
func add<F: Fooable>(_ someFoo: F) {
storage.append(someFoo)
}
}
These are two sample structs modeling what the API mentioned will return:
struct FooA: Fooable, Decodable {
var foo: String
}
struct FooB: Fooable, Decodable {
var foo: String
var bar: String
}
And finally, this is the protocol I created in order to specify that all of these structs are results of the same API:
protocol Fooable {}
The compiler error I get is this:
No exact matches in call to instance method append
And it is thrown on the storage.append(_:) method of the FooStorage class. Tried to add Equatable and Hashable conformance to the FooX protocols but to no avail. Seems I need some enlightenment here… thanks in advance!
In your add function, remove the generic part & let it use the class generic:
class FooStorage<F: Fooable> {
private var storage: [F] = []
func add(_ someFoo: F) {
storage.append(someFoo)
}
}
I have numerous pages where users may input information. They may input the fields with dates, numbers, or text.
I am trying to receive all changes in Combine and get their outputs as Encodable so I can easily upload the results to the network.
A String is Encodable, so I thought this would be easy but I cannot get this to work in Combine. I get a compiler error:
Cannot convert return expression of type 'Publishers.Map<Published.Publisher, Encodable>' to return type 'Published.Publisher'
There is a workaround where I add another property in SampleTextHandler that is #Published var userTextEncodable: Encodable but that's not what I want to do.
import Combine
protocol FieldResponseModifying {
var id: String { get }
var output: Published<Encodable>.Publisher { get }
}
struct SampleTextWrapper {
var output: Published<Encodable>.Publisher {
// Cannot convert return expression of type 'Publishers.Map<Published<String>.Publisher, Encodable>' to return type 'Published<Encodable>.Publisher'
handler.$userTextOutput.map { $0 as Encodable}
}
let id = UUID().uuidString
let handler = SampleTextHandler()
}
class SampleTextHandler {
#Published var userTextOutput = ""
init () { }
}
Combine uses generics heavily. For example, the type returned by your use of map is Publishers.Map<Published<Value>.Publisher, Encodable>. So you could declare your property like this:
var output: Publishers.Map<Published<Encodable>.Publisher, Encodable> {
handler.$userTextOutput.map { $0 as Encodable}
}
But now your property's type depends closely on how it is implemented. If you change the implementation, you'll have to change the type.
Instead, you should almost certainly use the “type eraser” AnyPublisher, like this:
var output: AnyPublisher<Encodable, Never> {
handler.$userTextOutput
.map { $0 as Encodable }
.eraseToAnyPublisher()
}
You're probably going to run into another issue down the line, due to your use of the Encodable existential. When you hit that, you'll want to post another question.
I have an app that stores User (UserModel) Friend list. if a friend clicks one user, its type is the same type (UserModel). In Swift it wouldnt allow using the model recursively, giving me this error:
"Value type 'OwnerModel' cannot have a stored property that recursively contains it"
import Foundation
struct OwnerModel: Codable {
var ownerId: Int
var ownerEmail: String
var ownerUserName: String
var ownerCommonName: String
var ownerBirthDate: String
var ownerCountry: String
var ownerBdayReminderId: Int
var ownerIsVerified: Bool
var ownerIsOnline: Bool
var ownerIsEventGreeted: Bool
var ownerIsBirthdayGreeted: Bool
var ownerAllowGreeting: Bool
var ownerFriends: OwnerModel
}
Is there a way I can reuse the OwnerModel under ownerFriends?
This can't work because structs are value types. So each OwnerModel would have to have a OwnerModel inside it, which would have to have an OwnerModel inside it, which would have to have an OwnerModel inside it.... This can never resolve. Since you've marked this Codable, try to write the JSON you expect to encode this to.
That said, ownerFriends seems plural, which would suggest [OwnerModel], and that's not a problem, since you could have zero of them:
struct OwnerModel: Codable {
...
var ownerFriends: [OwnerModel]
}
Remember again, however, that structs are value types. So each OwnerModel is just a value. It's not a reference to any other object. If you want to refer to other owners, you may want to store IDs rather than the actual object (or use classes in order to create references).
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.
I want to configure an object with multiple presentables; I've created protocols for them; so that I can combine them into a concrete presentable.
protocol Presentable {
}
protocol TextPresentable: Presentable {
var text:String { get }
}
protocol ImagePresentable: Presentable {
var image:String { get }
var images:[String] { get }
}
The concrete struct:
struct ConcretePresentable: TextPresentable, ImagePresentable {
var text:String { return "Text" }
var image:String { return "Image" }
var images:[String] { return ["A", "B"] }
}
The next thing would be to have a presenter, and I would check if the passed in presenter is actually valid:
typealias TextAndImagePresentable = protocol<TextPresentable, ImagePresentable>
struct ConcretePresenter {
func configureWithPresentable(presentable: Presentable) {
guard let textAndImagePresentable = presentable as? TextAndImagePresentable else {
return
}
print(textAndImagePresentable.text)
print(textAndImagePresentable.image)
print(textAndImagePresentable.images)
}
}
To configure:
let concretePresentable = ConcretePresentable()
let concretePresenter = ConcretePresenter()
concretePresenter.configureWithPresentable(concretePresentable)
It goes fine if I run this in a playground, with all of the code in the same place. However, once I put this in a project and split it up into multiple files (ConcretePresenter.swift, ConcretePresentable.swift holding the concrete structs and Presentable.swift which holds the protocols), I get a EXC_BAD_ACCESS.
Why does that happen?
As a disclaimer, I don't necessarily find this answer very satisfying, but it does work.
So, once I noticed that the text & image properties were being returned in each others places (the value for text is being returned by the image property and vice versa), I figured the problem had something to do with what Swift is doing with managing pointers here.
So, out of curiosity, I wanted to add a truly scalar value to the protocols. I added a value property as an Int to the TextPresentable protocol:
protocol Presentable {}
protocol TextPresentable: Presentable {
var text:String { get }
var value: Int { get }
}
protocol ImagePresentable: Presentable {
var image:String { get }
var images:[String] { get }
}
And then I set up the concrete implementation to return some known value. Here, we're returning 0.
struct ConcretePresentable: TextPresentable, ImagePresentable {
var text:String { return "SomeText" }
var value: Int { return 0 }
var image:String { return "SomeImage" }
var images:[String] { return ["A", "B"] }
}
After running this code, we still get the same crash, but I notice that value, which really shouldn't have a problem printing 0 is instead printing some very large number: 4331676336. This isn't right at all.
I also changed images from an array to a dictionary to see if the error persists--it does. It seems the crash is related to collections and not specific to arrays.
From here, I tried some other things.
I tried making ConcretePresentable a class rather than a struct.
class ConcretePresentable: TextPresentable, ImagePresentable
That resulted in the same behavior.
I tried making ConcretePresentable conform to the typealias rather than the protocols independently:
struct ConcretePresentable: TextAndImagePresentable
That resulted in the same behavior.
I tried doing both of the aforementioned at once:
class ConcretePresentable: TextAndImagePresentable
Yet still the same behavior.
I did come up with one way to make it work though. Make a protocol that conforms to the two protocols in your typealias and make ConcretePresentable conform to that:
protocol TextAndImagePresentable: TextPresentable, ImagePresentable {}
struct ConcretePresentable: TextAndImagePresentable {
// ...
}
The problem here is that if you don't explicitly make ConcretePresentable conform to the protocol, it will fail the guard let even if it does conform to TextPresentable and ImagePresentable.
I asked about this on Swift Users and filled a bug.
Confirmed to be a bug in the compiler:
https://bugs.swift.org/browse/SR-4477
Fixed by Joe Groff now:
Joe Groff added a comment - 2 hours ago
Merged. Should be fixed in future snapshots.
struct uses value semantics and so properties are copied. Swift should have reported this as an error since you are trying to inherit from two protocols which derive from the same base protocol. In classes this will work but in struct it wont because of value semantics for struct. In case you decide to add a variable to Presentable protocol Swift would be confused which ones to bring into the struct. From TextPresentable or ImagePresentable
You should use #protocol Presentable : class and then convert ConcretePresentable to class to fix this.
protocol Presentable : class {
}
class ConcretePresenter {...
The problem is in the cast of the Presentable to TextAndImagePresentable. The guard let succeed, but creates an invalid value (I don't know exactly why).
One way to check it, is look to the console on the execution of the commands:
print(textAndImagePresentable.text)
print(textAndImagePresentable.image)
print(textAndImagePresentable.images)
That will print:
Image
Text
Program ended with exit code: 9
One way to avoid it is to change your method signature to avoid the casting:
func configureWithPresentable(presentable: TextAndImagePresentable) {
print(presentable.text)
print(presentable.image)
print(presentable.images)
}
And, in my opinion, since nothing will happen if the presentable do not conform to both protocols, makes more sense to delimiter it on the method signature.