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Identifiable Protocol

Introduction

This proposal introduces an Identifiable protocol, a general concept that is broadly useful— for diff algorithms, user interface libraries, and other generic code—to correlate snapshots of the state of an entity in order to identify changes. It is a fundamental notion that deserves representation in the standard library.

Swift-evolution thread: Move SwiftUI's Identifiable and related types into the standard library

Motivation

There are many use cases for identifying distinct values as belonging to a single logical entity. Consider a Contact record:

struct Contact {
    var id: Int
    var name: String
}

let john = Contact(id: 1000, name: "John Appleseed")
var johnny = john
johnny.name = "Johnny Appleseed"

Snapshots of a Contact, like john and johnny, refer to the same logical person, even though that person may change their name over time and at any moment, may share any number of other details with distinct persons. Being able to determine that two such snapshots belong to the same logical entity is a broadly useful capability.

Representing such identity as simply the ObjectIdentifier of a class instance (or using === directly) sometimes works, but there are cases, such as when the instances are persistent or distributed across processes, where it simply doesn't, and even when it does work, allocating class instances to represent identity of value types is needlessly costly.

Diffing

User interfaces often involve collections of elements, each of which represents an entity. Consider a list of favorite contacts:

struct FavoriteContactList: View {
    var favorites: [Contact]

    var body: some View {
        List(favorites) { contact in
            FavoriteCell(contact)
        }
    }
}

In order to provide a high quality user experience when updating such a user interface with new content it is necessary to distinguish between the identity of the represented entity and the representation of the state of the entity that is presented to the user. Content in an interface representing an entity whose state has changed but identity has not should be updated in place (rather than resorting to removing the old content and inserting the new content).

A user interface component is capable of making such a distinction if its represented entities are Identifiable:

struct List {
    init<Data: Collection, RowContent: View>(
        _ data: Data,
        @ViewBuilder rowContent: @escaping (Data.Element) -> RowContent
    ) where Data.Element: Identifiable
}

Identifiable supports diff algorithms that are able to report entity insertions, moves and removals. These algorithms are also able to detect changes to the state of an entity that is represented in both collections. This can include changes to the state of an entity that also moves in the collection.

While diffs are often applied to the user interface layer of a program the diff algorithm does not necessarily need to run in the user interface layer. It can be desirable to compute a diff in the model layer. For example, the model layer updates may be processed in the background and the diff can be computed before moving back to the main thread to apply the changes to the UI. There may also be more than one simultaneous presentation of the same data in the UI, in which case computing the diff in the UI layer is redundant.

Model layer code that performs these computations often has no dependencies outside the standard library itself. It is unlikely to accept a dependency on a UI framework that defines its own Identifiable protocol. If Identifiable doesn't move to the standard library Swift programmers will need to continue using their own variation of this protocol and will need to ensure it is able co-exist with other similar definitions found in other frameworks higher up the dependency stack. Unfortunately none these variations are likey to be compatible with one another.

Proposed solution

The proposed solution is to define a new Identifiable protocol:

/// A class of types whose instances hold the value of an entity with stable identity.
protocol Identifiable {

    /// A type representing the stable identity of the entity associated with `self`.
    associatedtype ID: Hashable

    /// The stable identity of the entity associated with `self`.
    var id: ID { get }
}

This protocol will be used by diff algorithms, user interface libraries and other generic code to correlate snapshots of the state of an entity in order to identify changes to that state from one snapshot to another.

An example conformance follows:

struct Contact: Identifiable {
    var id: Int
    var name: String
}

There are a variety of considerations (value or reference semantics, persisted, distributed, performance, convenience, etc.) to weigh when choosing the appropriate representation of identity for an entity. ID is an associatedtype because no single concrete type of identifier is appropriate in all cases.

id was chosen as the name of the requirement over the unabbreviated form because it is a frequently used term of art that will allow easy conformance.

Detailed design

Object identifiability

In order to make it as convenient as possible to conform to Identifiable, a default id is provided for all class instances:

extension Identifiable where Self: AnyObject {
    var id: ObjectIdentifier {
        return ObjectIdentifier(self)
    }
}

Then, a class whose instances are identified by their object identities need not explicitly provide an id:

final class Contact: Identifiable {
    var name: String

    init(name: String) {
        self.name = name
    }
}

Note, a class may provide a custom implementation of id:

final class Contact: Identifiable {
    let id: Int
    let name: String

    init(id: Int, name: String) {
        self.id = id
        self.name = name
    }
}

Source compatibility

This is a purely additive change.

Effect on ABI stability

This is a purely additive change.

Effect on API resilience

This has no impact on API resilience which is not already captured by other language features.

Alternatives considered

Per-use identification

Instead of constraining a collection's elements to an Identifiable protocol, generic code could take an additional parameter that projects the identity of an entity from its representation:

struct FavoriteContactList: View {
    var favorites: [Contact]

    var body: some View {
        List(favorites, id: \.id) { contact in
            FavoriteCell(contact)
        }
    }
}

struct List {
    public init<Data: Collection, ID: Hashable, RowContent: View>(
        _ data: Data,
        id: KeyPath<Data, ID>,
        @ViewBuilder rowContent: @escaping (Data.Element) -> RowContent
    )
}

This is undesirable because a type generally has a single, canonical identity, but this approach unnecessarily re-defines an entity's identity at every use site, which is error-prone.

Furthermore, this isn't a practical alternative because there is evidence that if Swift doesn't define an Identifiable concept, libraries will opt to define their own rather than take an identifier at the use-site.

Concrete conformances

The purpose of Identifiable is to distinguish the identity of an entity from the state of an entity. Concrete types like UUID, Int, and String are commonly used as identifiers, however they do not have an identifier, so they should not conform to Identifiable.

Future directions

Collection diffing

Today there is a collection diffing convenience for Equatable elements:

extension BidirectionalCollection where Element: Equatable {
  func difference<C: BidirectionalCollection>(
    from other: C
  ) -> CollectionDifference<Element> where C.Element == Self.Element
}

It may be desirable to add a similar convenience for Identifiable elements (and prefer use of Identifiable to Equatable when a type conforms to both). This is omitted from the immediate proposal in order to keep it focused.

Conditional conformances

It may be desirable to provide the conditional conformance Optional: Identifiable where Wrapped: Identifiable. This is omitted from the immediate proposal in order to keep it focused.

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