If I have a series of protocols like so:
protocol Customer {
var name:String { get set }
var age: Int { get set }
var startDate:Date { get set }
var meals:Array<String> { get set }
var market:Int { get set }
}
protocol Vegan:Customer {
}
protocol Vegetarian:Customer {
}
protocol Paleo:Customer {
}
and extension like so:
extension Customer where Self:Vegan, Self:Vegetarian {
func getMeals() -> Array<String> {
return ["VeganMeal1", "VeganMeal2", "VeganMeal3", "VeganMeal4"]
}
}
extension Customer where Self:Vegetarian {
func getMeals() -> Array<String> {
return ["VegetarianMeal1", "VegetarianMeal2", "VegetarianMeal3", "VegetarianMeal4"]
}
}
extension Customer where Self:Paleo {
func getMeals() -> Array<String> {
return ["PaleoMeal1", "PaleoMeal2", "PaleoMeal3", "PaleoMeal4"]
}
}
and this struct
struct aCustomer:Customer, Vegan {
var name:String
var age: Int
var startDate:Date
var meals:Array<String>
var market:Int
}
when I create a new object based on that struct
var newCustomer = aCustomer(name:"William", age:40, startDate:Date(), meals:[], market:1)
how come I can't access the getMeals function in the extensions? I get an error stating getMeals is an ambiguous reference.
extension Customer where Self:Vegan, Self:Vegetarian {
extends Customer in the case where the adopter or Customer also both adopts Vegan and Vegetarian. Your aCustomer (a struct type starting with a small letter?? the horror, the horror) does not do that, so the extension doesn't apply to it.
If the goal is to inject the same code either when the adopter adopts Vegan or when the adopter adopts Vegetarian, use two extensions, or, if you don't like the repetition, have them both adopt some "higher" protocol that is extended with the desired code to be injected.
Related
I have defined the following protocols:
protocol ListableArrayElement: Identifiable, Equatable {
associatedtype T = Hashable
var id:T { get }
}
protocol Listable: ObservableObject, RandomAccessCollection where Element == ArrayElement {
associatedtype ArrayElement: ListableArrayElement
var _elements: [ArrayElement] { get set }
var count: Int { get }
var isEmpty: Bool { get }
var endIndex: Int { get }
var startIndex: Int { get }
subscript(position: Int) -> ArrayElement { get set }
func index(after i: Int) -> Int
func append(_ element: ArrayElement)
func insert(_ newElement: ArrayElement, at i: Int)
func remove(at index: Int)
func removeAll()
func index(of element: ArrayElement) -> Int?
}
protocol FavouritesArray: Listable, Codable where Element: Codable {
// MARK: - PROPERTIES
var _elements: [Element] { get set }
var key: String { get set }
init()
init(key: String)
}
There is an associated extension to FavouritesArray that provides conforming types with the ability to add/remove elements, and persist/load the array to/from UserDefaults via the key. All well and good. (Also note the listable protocol helps me avoid writing some boiler plate code for ViewModels that have an array of 'something' at their heart.)
Now I also want to write a generic SwiftUI view that can build a menu, using the FavouriteArray functions. I am struggling to understand how to express the type signature:
I'm passing instances of types conforming to FavouritesArray via EnvironmentObject, and thus want to write something like:
struct FavouritesArrayView<Favourites: FavouritesArray, Favourite>: View
where Favourite == FavouritesArray.Element {
#EnvironmentObject var favourites: Favourites
#ObservedObject var favourite: Favourite
// Other properties here
var body: some View {
// Layout code here
}
}
This gives the compiler error: Associated type 'Element' can only be used with a concrete type or generic parameter base
Any tips on how to achieve this?
First of all you need to declare that Favourite conforms to ObservableObject and then the where condition should use your associated type and not the protocol it conforms to, where Favourites.Element == Favourite
struct FavouritesArrayView<Favourites: FavouritesArray, Favourite: ObservableObject>: View where Favourites.Element == Favourite
TL;DR
How can I conform to the supscript function of a protocol in my implementation?
Protocol:
protocol DataStore {
//...
subscript<T>(id: T.ID) -> T where T: Identifiable { get set }
}
Neither
subscript<T>(id: T.ID) -> T where T : Identifiable {
get { Project() }//just to return anything for the time being…
set {}
}
nor
subscript(id: Task.ID) -> Task {
get { Project() }//just to return anything for the time being…
set {}
}
work...
The details:
I have developed a habit of creating specific data stores for my models. They all have the same functionality. A specific example could look like this:
final class ProjectDataStore: ObservableObject {
{
static let shared = ProjectDataStore()
let persistence = AtPersistLocally.shared // that's from a personal package I made that handles saving and loading locally as json files
#Published var projects: [Project] {
didSet { //save }
}
private init(projects: [Project]? = nil) {
//load from persistence
}
subscript(id: Project.ID) -> Project? {
get { //return project with id }
set { //set project at id }
}
func getBinding(by id: Project.ID) -> Binding<Project> {
//return Binding
}
func getProjectBy(taskId: Task.ID) -> Project {
//return instance
}
private func getIndex(by id: Project.ID) -> Int? {
//return index in array
}
private func load() -> [Project] {
//load from persistence
}
private func save() {
//save from persistence
}
}
While this works as expected, I'd like to be able to introduce a protocol that I could when adding new functionality / models to have a blueprint on what's necessary for my DataStore.
Here is my first attempt:
protocol DataStore {
static var shared: Self { get }
}
extension DataStore {
var persistence: AtPersistLocally {
get {
AtPersistLocally.shared
}
}
}
To also conform to ObservableObject, I introduce a typealias
typealias ObservableObjectDataStore = DataStore & ObservableObject
and change my model to conform to this new typealias:
final class ProjectDataStore: ObservableObjectDataStore {
//...
}
With this, I already have a static instance and access to persistence available.
Next step of course is to move more and more properties to the protocol–which is what I am struggling with right now.
Let's look at superscript first of all: I guess I understand what needs to be added to the protocol:
protocol DataStore {
//...
subscript<T>(id: T.ID) -> T where T: Identifiable { get set }
}
My problem now is that I don't know how to go about conforming to this subscript now while also getting access to a concrete model from the generic T from the implementation. This attempt…
final class ProjectDataStore: ObservableObjectDataStore {
//...
subscript<T>(id: T.ID) -> T where T : Identifiable {
get { Project() }//just to return anything for the time being…
set {}
}
}
…leads to the error message Cannot convert return expression of type 'Task' to return type 'T'.
If I go with…
final class ProjectDataStore: ObservableObjectDataStore {
//...
subscript(id: Task.ID) -> Task {
get { Project() }//just to return anything for the time being…
set {}
}
}
…the error message changes to Type 'TaskDataStore' does not conform to protocol 'DataStore'.
So I guess basically what I am asking is: how can I conform to my protocol's generic superscript in my implementation of ProjectDataStore?
I have a feeling that I am not too far of, but a critical info is obviously missing…
subscript<T>(id: T.ID) -> T where T: Identifiable { get set }
This says that the caller may pick any Identifiable T, and this method promise return a value of that type. This can't be implemented (other than calling fatalError() and crashing). Identifiable doesn't even promise that the type has an init. Your protocol is impossible.
What you probably meant to write is, which says that a given DataStore will return some specific Identifiable type (not "whatever type the caller requests"):
protocol DataStore {
associatedType Item: Identifiable
subscript(id: Item.ID) -> Item { get set }
}
But I expect this would be much better implemented without a protocol, and just use a generic struct:
struct DataStore<Item: Identifiable> { ... }
I need to declare an array like this:
var cups: [Cup<Drink>] = []
The Cup is a struct and the Drink is a protocol, but I got the following error:
Value of protocol type 'Drink' cannot conform to 'Drink'; only struct/enum/class types can conform to protocols
I know the protocol type 'Drink' can be erased by a AnyDrink struct, the flowing code is an example.
But in fact, the Type-Erasure will become extremely complicated when associatetype, Self and static-method (in the case of non-final class adopts Drink, such as the protocol Equatable with static method ==) are used in Drink.
Is there a better way to declare the cups array?
Or: Is there any easy way to make Type-Erasure? It should be a builtin feature.
protocol Drink {
...
}
struct AnyDrink: Drink {
let drink: Drink
...
}
struct Water: Drink {
...
}
struct Coffee: Drink {
...
}
struct Tea: Drink {
...
}
struct Cup<T: Drink> {
private(set) var drink: T?
mutating func bottomUp() {
drink = nil
}
}
struct Waiter {
var cups: [Cup<AnyDrink>] = []
mutating func makeACupOfSth(_ cup: Cup<AnyDrink>) {
cups.append(cup)
}
mutating func pleaseGiveMeACupOfSthToDrink() -> Cup<AnyDrink> {
return cups.removeFirst()
}
static func excuse(_ customer: Customer) -> Waiter {
return Waiter()
}
}
struct Customer {
var me: Self { self }
func drink() {
var waiter = Waiter.excuse(me)
var cup = waiter.pleaseGiveMeACupOfSthToDrink()
cup.bottomUp()
}
}
For question Is there a better way to declare the cups array?:
You can use another protocol called DrinkGeneric like this and implement it by Cup Struct:
protocol DrinkGeneric {
func sample()
func typOfDrink() -> Drink.Type
}
struct Cup<T: Drink>: DrinkGeneric {
public var drink: T?
mutating func bottomUp() {
drink = nil
}
public func typeOfDrink() -> Drink.Type {
return type(of: drink!)
}
func sample() {
print("sample")
}
}
Then create an array with type DrinkGeneric like this:
var cups: [DrinkGeneric] = [Cup(drink: Water()), Cup(drink: Tea())]
For check type:
if cups[0].typeOfDrink() is Water.Type {
// Any work
}
How can I perform conformance check against protocol with AssociatedType. Xcode shows error:
Protocol 'MyListener' can only be used as a generic constraint because
it has Self or associated type requirements
My ultimate goal is to extract "MyListener.section" from an array of weakObjects, where the handler matches the function argument.
Note. The NSPointerArray of weakObjects is suppose to capture different types of MyListeners.
public class MyHandler<O,E> {
var source = [O]()
var dest = [E]()
}
public protocol MyListener:class {
var section: Int {get}
associatedtype O
associatedtype E
var handler: MyHandler<O,E>? { get }
}
public class MyAnnouncer {
private let mapWeakObjects: NSPointerArray = NSPointerArray.weakObjects()
public func add<L: MyListener>(listener: L) {
let pointer = Unmanaged.passUnretained(listener).toOpaque()
mapWeakObjects.addPointer(pointer)
}
public func search<O, E> (h:MyHandler<O,E>) -> [Int] {
_ = mapWeakObjects.allObjects.filter { listener in
if listener is MyListener { // Compilation failed
}
if let _ = listener as? MyListener { //Compilation error
}
if listener is MyListener.Type { //Compilation failed
}
}
return [] // ultimate goal is to extract corresponding [MyListener.section].
}
}
Unfortunately, Swift doesn't support protocols with AssociatedType to conformance.
You should try to use Type Erasure. One of the way is to implement type erasure by creating new AnyType class.
Here is another way to release type erasure (example from the internet)
protocol SpecialValue { /* some code*/ }
protocol TypeErasedSpecialController {
var typeErasedCurrentValue: SpecialValue? { get }
}
protocol SpecialController : TypeErasedSpecialController {
associatedtype SpecialValueType : SpecialValue
var currentValue: SpecialValueType? { get }
}
extension SpecialController {
var typeErasedCurrentValue: SpecialValue? { return currentValue }
}
extension String : SpecialValue {}
struct S : SpecialController {
var currentValue: String?
}
var x: Any = S(currentValue: "Hello World!")
if let sc = x as? TypeErasedSpecialController { // Now we can perform conformance
print(sc.typeErasedCurrentValue)
}
I'm trying to write a protocol that allows me to version models in my app. In order to do that I wrote the following VersionManager.
class VersionManager<Type: Decodable> {
private var database: Database
init(database: Database) {
self.database = database
}
var versions: [Type] {
return []
}
}
After that I wrote a protocol that I can add to models:
protocol Versionable {
}
extension Versionable {
private var manager: VersionManager<Restaurant> {
return VersionManager<Restaurant>(database: Database.shared)
}
public var versions: [Restaurant] {
return manager.versions
}
}
Now, the problem I'm facing is that I tried passing the type dynamically instead of hardcoded, like I have now for Restaurant.
So I tried changing the protocol to this:
protocol Versionable {
var kind: Decodable.Type { get }
}
Then I wanted to pass kind to VersionManager. However, when I try that Xcode throws this error: Expected '>' to complete generic argument list.
Is there any other way to do this?
If you want to use generics inside a protocol, you need to use an associatedtype
protocol Versionable {
associatedtype Model: Decodable
}
extension Versionable {
private var manager: VersionManager<Model> {
return VersionManager<Model>(database: Database.shared)
}
public var versions: [Model] {
return manager.versions
}
}
The model that is going to implement the Versionable protocol will have to resolve this type:
struct SomeModel: Versionable {
typealias Model = Int
}
SomeModel().versions // [Int]
I'm guessing Restaurant in your example refers to the model that implements Versionable. In that case, you can just use the Self reference inside your protocol extension:
protocol Versionable: Decodable {
}
extension Versionable {
private var manager: VersionManager<Self> {
return VersionManager<Self>(database: Database.shared)
}
public var versions: [Self] {
return manager.versions
}
}
struct SomeModel: Versionable {}
SomeModel().versions // [SomeModel]
Please note that the Versionable protocol now requires the Decodable conformance because of the VersionManager<Type: Decodable> generic constraint.