Swift - 结构类型不符合协议

Question

I am learning Swift, but I am bit stuck with the basics.. Downloaded a sample project from Apple but still not see what is the difference between these two:

My struct:

import Foundation
import SwiftUI
import CoreLocation

struct Store: Hashable, Codable, Identifiable {
    var id: Int
    var name: String
    var categories: [Category]

    private var logoName: String
    var logo: Image {
        Image(logoName)
    }

    private var coordinates: Coordinates
    var locationCoordinate: CLLocationCoordinate2D {
        CLLocationCoordinate2D(
            latitude: coordinates.latitude,
            longitude: coordinates.longitude)
    }

    struct Coordinates: Hashable, Codable {
        var latitude: Double
        var longitude: Double
    }
}

Sample:

import Foundation
import SwiftUI
import CoreLocation

struct Landmark: Hashable, Codable, Identifiable {
    var id: Int
    var name: String
    var park: String
    var state: String
    var description: String
    var isFavorite: Bool

    private var imageName: String
    var image: Image {
        Image(imageName)
    }

    private var coordinates: Coordinates
    var locationCoordinate: CLLocationCoordinate2D {
        CLLocationCoordinate2D(
            latitude: coordinates.latitude,
            longitude: coordinates.longitude)
    }

    struct Coordinates: Hashable, Codable {
        var latitude: Double
        var longitude: Double
    }
}

The first is failing with:

"Type 'Store' does not conform to protocol 'Equatable'"
"Type 'Store' does not conform to protocol 'Hashable'"

The second one is working properly.

What is the difference? Is there a settings somewhere in Xcode? :)

(The sample is from: https://developer.apple.com/tutorials/swiftui/handling-user-input )

Answer 1

Store has a property that is an array of Category . I would guess that Category does not conform to Equatable or Hashable , and so Swift cannot synthesize conformance.

Landmark contains properties that all conform to Equatable and Hashable , so it can synthesize conformance for Landmark .

There are two solutions to make Store conform.

1. Make Category conform to both protocols. Then Swift could synthesize conformance for Store .

2. Explicitly implement the conformance for Hashable and Equatable for Store by implementing static func == (lhs: Store, rhs: Store) -> Bool and func hash(into: inout Hasher)

Depending on the implementation of Category , the simplest solution just might be:

extension Category: Hashable, Equatable { }