[swift-users] Referencing different objects implementing a protocol where the protocol has 'associatedtype'

Palfi, Andras andras.palfi at sap.com
Sun Sep 18 08:21:50 CDT 2016

Hi All,

Using protocols are quite important in case of “plugin” concept to implement delegation: allow others to implement small components which will be run in specific cases.
To support one component a delegate or closure is sufficient. For the delegate we can use the protocol as referenced type unless it has associated type. If it uses associated type we can still use Generics to reference the object and use protocol only for constraints.
Allowing more components to subscribe causes the problem that we cannot reference them:

·         We cannot use generics as there can be any arbitrary number and type of objects so we have to use protocols to reference them

·         When there is an associatedtype in the protocol it is not possible to use to reference objects

For example:
We have a generic class which holds a collection of some type of objects – the generic parameter is a type of an item.
This class exposes a kind of delegate (specified by a protocol) and arbitrary number of implementers can subscribe. These delegates will be called in a specific case: for example when a new item is added to the collection.
The defined protocol uses associatedtype since the specific type of element is unkown.
The problem that the code cannot compile because of the error: "protocol 'SomeProtocol' can only be used as a generic constraint because it has Self or associated type requirements"

So we know the specific element we want to store but still cannot specify it. We cannot use generic for the ‘delegates’ as it should contain different implementations of SomeProtocol.

protocol SomeProtocol {
    associatedtype ElementType

    func someMethod(withElement: ElementType) -> Void

class SomeProtocolImplInt : SomeProtocol {
    typealias ElementType = Int

    var sum : Int = 0

    func someMethod(withElement element: Int) {
        sum += element

class Container<SomeElementType>  {
    typealias ElementType = SomeElementType

    var elements = Array<SomeElementType>()

    var delegates = Array<SomeProtocol where SomeProtocol.ElementType==SomeElementType>() // ‘where’ could solve the problem but not allowed here

    func subscribe<D : SomeProtocol>(delegate : D) where D.ElementType==SomeElementType { // would be better without generics as this generates as many methods as many different types used to call it

    func add(element: SomeElementType) {

        for delegate in delegates {
            delegate.someMethod(withElement: element)

// usage:
var container = Container<Int>()
let calculator = SomeProtocolImplInt()
container.subscribe(delegate: calculator)
container.add(element: 1)

Use 'Thunks'. These are type eraser structs, which implements the same protocol. For each method they referencing (capturing) the original methods and properties of the object. So 'thunks' are technically proxy objects.

struct DirtyThunk<SomeElementType> : SomeProtocol {

    private let _someMethod : (SomeElementType) -> Void

    init<D : SomeProtocol>(delegate : D) where D.ElementType==SomeElementType {
        _someMethod = delegate.someMethod

    func someMethod(withElement: SomeElementType) {

the subscribe method will be modified:
func subscribe<D : SomeProtocol>(delegate : D) where D.ElementType==SomeElementType { // would be better without generics as this generates as many methods as many different types used to call it
        let thunk = DirtyThunk(delegate)

This solution works - however we can never retrieve the original object any more as it is not referenced. The implementation of the “thunks” are also painful a bit. The methods are captured only by the name of the methods without the parameters so leads the problem if different methods have the same prefix.

I tried to solve using ‘Any’ to reference the delegates but then cannot cast to a proper type to call them.

Do one know any better solution?


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