SwiftUI之Identifiable协议

A class of types whose instances hold the value of an entity with stable identity.

@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
public protocol Identifiable {

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

    /// The stable identity of the entity associated with this instance.
    var id: Self.ID { get }
}

/// Use the `Identifiable` protocol to provide a stable notion of identity to a 
/// class or value type. For example, you could define a `User` type with an `id`
/// property that is stable across your app and your app's database storage. 
/// You could use the `id` property to identify a particular user even if other 
/// data fields change, such as the user's name.
///
/// `Identifiable` leaves the duration and scope of the identity unspecified.
/// Identities can have any of the following characteristics:
///
/// - Guaranteed always unique, like UUIDs.
/// - Persistently unique per environment, like database record keys.
/// - Unique for the lifetime of a process, like global incrementing integers.
/// - Unique for the lifetime of an object, like object identifiers.
/// - Unique within the current collection, like collection indices.
///
/// It's up to both the conformer and the receiver of the protocol to document
/// the nature of the identity.
///
/// Conforming to the Identifiable Protocol
/// =======================================
///
/// `Identifiable` provides a default implementation for class types (using
/// `ObjectIdentifier`), which is only guaranteed to remain unique for the
/// lifetime of an object. If an object has a stronger notion of identity, it
/// may be appropriate to provide a custom implementation.

struct ExpenseItem {
    let id: UUID()
    let name: String
    let type: String
    let amount: Int
}

ForEach(expenses.items, id: \.id) { item in
    Text(item.name)
}

struct ExpenseItem: Identifiable {
    // 协议要求必须添加
    let id = UUID()
    let name: String
    let type: String
    let amount: Int
}

ForEach(expenses.items) { item in
    Text(item.name)
}