Protocols with associated type and factory pattern?
My data access layer consists of a generic Repository protocol
protocol Repository {
associatedtype T
func getAll() -> Promise<[T]>
}
and its concrete implementation:
class FirebaseRepository<T: Model>: Repository {
func getAll() -> Promise<[T]> {
fatalError()
}
}
Basically, Repository can be RestRepository, FirebaseRepository, PlistRepositry etc. Repository is used by the business logic:
/// My business logic
class ModelService<T: Repository> {
private let repository: T
public init(repository: T) {
self.repository = repository
}
}
The problem comes when I'm trying to apply a factory pattern to a repository. Here's what I came in first:
/// Returns a concrete Repository implementation
class RepositoryFactory {
func makeRepository<T: Model>(type: T.Type) -> Repository {
return FirebaseRepository<T>()
}
}
and this definitely gets a compiler error:
Protocol 'Repository' can only be used as a generic constraint because it has Self or associated type requirements
The only viable option I came to is this:
func makeRepository<T: Model>(type: T.Type, U: Repository) -> U {
return FirebaseRepository<T>() as! U
}
but as you understand, the force optional unwrapping is not acceptable in the production code.
How to make protocols with associated types work with factory design pattern?
swift generics design-patterns swift-protocols
asked Nov 11 at 13:38
Andrey Gordeev
15.2k662100
add a comment |
My data access layer consists of a generic Repository protocol
protocol Repository {
associatedtype T
func getAll() -> Promise<[T]>
}
and its concrete implementation:
class FirebaseRepository<T: Model>: Repository {
func getAll() -> Promise<[T]> {
fatalError()
}
}
Basically, Repository can be RestRepository, FirebaseRepository, PlistRepositry etc. Repository is used by the business logic:
/// My business logic
class ModelService<T: Repository> {
private let repository: T
public init(repository: T) {
self.repository = repository
}
}
The problem comes when I'm trying to apply a factory pattern to a repository. Here's what I came in first:
/// Returns a concrete Repository implementation
class RepositoryFactory {
func makeRepository<T: Model>(type: T.Type) -> Repository {
return FirebaseRepository<T>()
}
}
and this definitely gets a compiler error:
Protocol 'Repository' can only be used as a generic constraint because it has Self or associated type requirements
The only viable option I came to is this:
func makeRepository<T: Model>(type: T.Type, U: Repository) -> U {
return FirebaseRepository<T>() as! U
}
but as you understand, the force optional unwrapping is not acceptable in the production code.
How to make protocols with associated types work with factory design pattern?
swift generics design-patterns swift-protocols
asked Nov 11 at 13:38
Andrey Gordeev
15.2k662100
add a comment |
My data access layer consists of a generic Repository protocol
protocol Repository {
associatedtype T
func getAll() -> Promise<[T]>
}
and its concrete implementation:
class FirebaseRepository<T: Model>: Repository {
func getAll() -> Promise<[T]> {
fatalError()
}
}
Basically, Repository can be RestRepository, FirebaseRepository, PlistRepositry etc. Repository is used by the business logic:
/// My business logic
class ModelService<T: Repository> {
private let repository: T
public init(repository: T) {
self.repository = repository
}
}
The problem comes when I'm trying to apply a factory pattern to a repository. Here's what I came in first:
/// Returns a concrete Repository implementation
class RepositoryFactory {
func makeRepository<T: Model>(type: T.Type) -> Repository {
return FirebaseRepository<T>()
}
}
and this definitely gets a compiler error:
Protocol 'Repository' can only be used as a generic constraint because it has Self or associated type requirements
The only viable option I came to is this:
func makeRepository<T: Model>(type: T.Type, U: Repository) -> U {
return FirebaseRepository<T>() as! U
}
but as you understand, the force optional unwrapping is not acceptable in the production code.
How to make protocols with associated types work with factory design pattern?
swift generics design-patterns swift-protocols
asked Nov 11 at 13:38
Andrey Gordeev
15.2k662100
My data access layer consists of a generic Repository protocol
protocol Repository {
associatedtype T
func getAll() -> Promise<[T]>
}
and its concrete implementation:
class FirebaseRepository<T: Model>: Repository {
func getAll() -> Promise<[T]> {
fatalError()
}
}
Basically, Repository can be RestRepository, FirebaseRepository, PlistRepositry etc. Repository is used by the business logic:
/// My business logic
class ModelService<T: Repository> {
private let repository: T
public init(repository: T) {
self.repository = repository
}
}
The problem comes when I'm trying to apply a factory pattern to a repository. Here's what I came in first:
/// Returns a concrete Repository implementation
class RepositoryFactory {
func makeRepository<T: Model>(type: T.Type) -> Repository {
return FirebaseRepository<T>()
}
}
and this definitely gets a compiler error:
Protocol 'Repository' can only be used as a generic constraint because it has Self or associated type requirements
The only viable option I came to is this:
func makeRepository<T: Model>(type: T.Type, U: Repository) -> U {
return FirebaseRepository<T>() as! U
}
but as you understand, the force optional unwrapping is not acceptable in the production code.
How to make protocols with associated types work with factory design pattern?
swift generics design-patterns swift-protocols
swift generics design-patterns swift-protocols
asked Nov 11 at 13:38
Andrey Gordeev
15.2k662100
asked Nov 11 at 13:38
Andrey Gordeev
15.2k662100
asked Nov 11 at 13:38
Andrey Gordeev
15.2k662100
asked Nov 11 at 13:38
Andrey Gordeev
15.2k662100
asked Nov 11 at 13:38
Andrey Gordeev
15.2k662100
15.2k662100
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
You can use Type erasure. Here is an example:
protocol CustomProtocol {
associatedtype AssociatedType
func foo(argument: AssociatedType)
func bar() -> AssociatedType
}
If you want to use CustomProtocol directly, you will receive your error:
let array = [CustomProtocol]()
Protocol 'CustomProtocol' can only be used as a generic constraint because it has Self or associated type requirements
So you can make the same trick, as Swift does with their sequences:
public struct AnyCustomProtocol<T>: CustomProtocol {
func bar() -> T {
fatalError("Needs implementation")
}
func foo(argument: T) {
}
}
let array = [AnyCustomProtocol<Any>]() // works fine
Your problem solution in this case will look somehow like this:
class Promise<T> {
}
protocol Model {
}
protocol Repository {
associatedtype T
func getAll() -> Promise<[T]>
}
class FirebaseRepository<T: Model>: AnyRepository<T> {
override func getAll() -> Promise<[T]> {
fatalError()
}
}
class AnyRepository<T>: Repository {
func getAll() -> Promise<[T]> {
fatalError()
}
}
class RepositoryFactory {
func makeRepository<T: Model>(type: T.Type) -> AnyRepository<T> {
return FirebaseRepository<T>()
}
}
__
For further reading you can check this and Official docs on Generics
answered Nov 11 at 14:11
fewlinesofcode
2,026518
Thank you very much for your reply. The approach you've suggested works good, though I'd expect more clear solution.AnyRepositoryhas to have method stubs, which doesn't look clean. I suppose it's related to Swift's lacking of expressiveness. Abstract class sounds a better fit for solving this problem, but we don't have such in Swift yet :-/
– Andrey Gordeev
Nov 11 at 15:03
1
@AndreyGordeev, yes. But this is the most "Correct" solution of your problem, IMO. There is a proposal for abstract classes and methods in Swift. So let's just wait :)
– fewlinesofcode
Nov 11 at 15:07
add a comment |
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
You can use Type erasure. Here is an example:
protocol CustomProtocol {
associatedtype AssociatedType
func foo(argument: AssociatedType)
func bar() -> AssociatedType
}
If you want to use CustomProtocol directly, you will receive your error:
let array = [CustomProtocol]()
Protocol 'CustomProtocol' can only be used as a generic constraint because it has Self or associated type requirements
So you can make the same trick, as Swift does with their sequences:
public struct AnyCustomProtocol<T>: CustomProtocol {
func bar() -> T {
fatalError("Needs implementation")
}
func foo(argument: T) {
}
}
let array = [AnyCustomProtocol<Any>]() // works fine
Your problem solution in this case will look somehow like this:
class Promise<T> {
}
protocol Model {
}
protocol Repository {
associatedtype T
func getAll() -> Promise<[T]>
}
class FirebaseRepository<T: Model>: AnyRepository<T> {
override func getAll() -> Promise<[T]> {
fatalError()
}
}
class AnyRepository<T>: Repository {
func getAll() -> Promise<[T]> {
fatalError()
}
}
class RepositoryFactory {
func makeRepository<T: Model>(type: T.Type) -> AnyRepository<T> {
return FirebaseRepository<T>()
}
}
__
For further reading you can check this and Official docs on Generics
answered Nov 11 at 14:11
fewlinesofcode
2,026518
Thank you very much for your reply. The approach you've suggested works good, though I'd expect more clear solution.AnyRepositoryhas to have method stubs, which doesn't look clean. I suppose it's related to Swift's lacking of expressiveness. Abstract class sounds a better fit for solving this problem, but we don't have such in Swift yet :-/
– Andrey Gordeev
Nov 11 at 15:03
1
@AndreyGordeev, yes. But this is the most "Correct" solution of your problem, IMO. There is a proposal for abstract classes and methods in Swift. So let's just wait :)
– fewlinesofcode
Nov 11 at 15:07
add a comment |
You can use Type erasure. Here is an example:
protocol CustomProtocol {
associatedtype AssociatedType
func foo(argument: AssociatedType)
func bar() -> AssociatedType
}
If you want to use CustomProtocol directly, you will receive your error:
let array = [CustomProtocol]()
Protocol 'CustomProtocol' can only be used as a generic constraint because it has Self or associated type requirements
So you can make the same trick, as Swift does with their sequences:
public struct AnyCustomProtocol<T>: CustomProtocol {
func bar() -> T {
fatalError("Needs implementation")
}
func foo(argument: T) {
}
}
let array = [AnyCustomProtocol<Any>]() // works fine
Your problem solution in this case will look somehow like this:
class Promise<T> {
}
protocol Model {
}
protocol Repository {
associatedtype T
func getAll() -> Promise<[T]>
}
class FirebaseRepository<T: Model>: AnyRepository<T> {
override func getAll() -> Promise<[T]> {
fatalError()
}
}
class AnyRepository<T>: Repository {
func getAll() -> Promise<[T]> {
fatalError()
}
}
class RepositoryFactory {
func makeRepository<T: Model>(type: T.Type) -> AnyRepository<T> {
return FirebaseRepository<T>()
}
}
__
For further reading you can check this and Official docs on Generics
answered Nov 11 at 14:11
fewlinesofcode
2,026518
Thank you very much for your reply. The approach you've suggested works good, though I'd expect more clear solution.AnyRepositoryhas to have method stubs, which doesn't look clean. I suppose it's related to Swift's lacking of expressiveness. Abstract class sounds a better fit for solving this problem, but we don't have such in Swift yet :-/
– Andrey Gordeev
Nov 11 at 15:03
1
@AndreyGordeev, yes. But this is the most "Correct" solution of your problem, IMO. There is a proposal for abstract classes and methods in Swift. So let's just wait :)
– fewlinesofcode
Nov 11 at 15:07
add a comment |
You can use Type erasure. Here is an example:
protocol CustomProtocol {
associatedtype AssociatedType
func foo(argument: AssociatedType)
func bar() -> AssociatedType
}
If you want to use CustomProtocol directly, you will receive your error:
let array = [CustomProtocol]()
Protocol 'CustomProtocol' can only be used as a generic constraint because it has Self or associated type requirements
So you can make the same trick, as Swift does with their sequences:
public struct AnyCustomProtocol<T>: CustomProtocol {
func bar() -> T {
fatalError("Needs implementation")
}
func foo(argument: T) {
}
}
let array = [AnyCustomProtocol<Any>]() // works fine
Your problem solution in this case will look somehow like this:
class Promise<T> {
}
protocol Model {
}
protocol Repository {
associatedtype T
func getAll() -> Promise<[T]>
}
class FirebaseRepository<T: Model>: AnyRepository<T> {
override func getAll() -> Promise<[T]> {
fatalError()
}
}
class AnyRepository<T>: Repository {
func getAll() -> Promise<[T]> {
fatalError()
}
}
class RepositoryFactory {
func makeRepository<T: Model>(type: T.Type) -> AnyRepository<T> {
return FirebaseRepository<T>()
}
}
__
For further reading you can check this and Official docs on Generics
answered Nov 11 at 14:11
fewlinesofcode
2,026518
You can use Type erasure. Here is an example:
protocol CustomProtocol {
associatedtype AssociatedType
func foo(argument: AssociatedType)
func bar() -> AssociatedType
}
If you want to use CustomProtocol directly, you will receive your error:
let array = [CustomProtocol]()
Protocol 'CustomProtocol' can only be used as a generic constraint because it has Self or associated type requirements
So you can make the same trick, as Swift does with their sequences:
public struct AnyCustomProtocol<T>: CustomProtocol {
func bar() -> T {
fatalError("Needs implementation")
}
func foo(argument: T) {
}
}
let array = [AnyCustomProtocol<Any>]() // works fine
Your problem solution in this case will look somehow like this:
class Promise<T> {
}
protocol Model {
}
protocol Repository {
associatedtype T
func getAll() -> Promise<[T]>
}
class FirebaseRepository<T: Model>: AnyRepository<T> {
override func getAll() -> Promise<[T]> {
fatalError()
}
}
class AnyRepository<T>: Repository {
func getAll() -> Promise<[T]> {
fatalError()
}
}
class RepositoryFactory {
func makeRepository<T: Model>(type: T.Type) -> AnyRepository<T> {
return FirebaseRepository<T>()
}
}
__
For further reading you can check this and Official docs on Generics
answered Nov 11 at 14:11
fewlinesofcode
2,026518
edited Nov 11 at 14:23
answered Nov 11 at 14:11
fewlinesofcode
2,026518
answered Nov 11 at 14:11
fewlinesofcode
2,026518
answered Nov 11 at 14:11
fewlinesofcode
2,026518
2,026518
Thank you very much for your reply. The approach you've suggested works good, though I'd expect more clear solution.AnyRepositoryhas to have method stubs, which doesn't look clean. I suppose it's related to Swift's lacking of expressiveness. Abstract class sounds a better fit for solving this problem, but we don't have such in Swift yet :-/
– Andrey Gordeev
Nov 11 at 15:03
1
@AndreyGordeev, yes. But this is the most "Correct" solution of your problem, IMO. There is a proposal for abstract classes and methods in Swift. So let's just wait :)
– fewlinesofcode
Nov 11 at 15:07
add a comment |
Thank you very much for your reply. The approach you've suggested works good, though I'd expect more clear solution.AnyRepositoryhas to have method stubs, which doesn't look clean. I suppose it's related to Swift's lacking of expressiveness. Abstract class sounds a better fit for solving this problem, but we don't have such in Swift yet :-/
– Andrey Gordeev
Nov 11 at 15:03
1
@AndreyGordeev, yes. But this is the most "Correct" solution of your problem, IMO. There is a proposal for abstract classes and methods in Swift. So let's just wait :)
– fewlinesofcode
Nov 11 at 15:07
Thank you very much for your reply. The approach you've suggested works good, though I'd expect more clear solution.
AnyRepository has to have method stubs, which doesn't look clean. I suppose it's related to Swift's lacking of expressiveness. Abstract class sounds a better fit for solving this problem, but we don't have such in Swift yet :-/– Andrey Gordeev
Nov 11 at 15:03
Thank you very much for your reply. The approach you've suggested works good, though I'd expect more clear solution.
AnyRepository has to have method stubs, which doesn't look clean. I suppose it's related to Swift's lacking of expressiveness. Abstract class sounds a better fit for solving this problem, but we don't have such in Swift yet :-/– Andrey Gordeev
Nov 11 at 15:03
1
1
@AndreyGordeev, yes. But this is the most "Correct" solution of your problem, IMO. There is a proposal for abstract classes and methods in Swift. So let's just wait :)
– fewlinesofcode
Nov 11 at 15:07
@AndreyGordeev, yes. But this is the most "Correct" solution of your problem, IMO. There is a proposal for abstract classes and methods in Swift. So let's just wait :)
– fewlinesofcode
Nov 11 at 15:07
add a comment |
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