使用 URLSession 获取数据时“不允许从后台线程发布更改”

Question

我正在尝试从 Unsplash API 获取数据，但收到以下错误：“不允许从后台线程发布更改；请确保在模型更新时从主线程发布值（通过 receive(on:) 等运算符）” ”。

这是模型结构：

// MARK: - UnsplashData
struct UnsplashData: Codable {
    let id: String
    let createdAt, updatedAt, promotedAt: Date
    let width, height: Int
    let color, blurHash: String
    let unsplashDataDescription: String?
    let altDescription: String
    let urls: Urls
    let links: UnsplashDataLinks
    let categories: [String]
    let likes: Int
    let likedByUser: Bool
    let currentUserCollections: [String]
    let sponsorship: JSONNull?
    let user: User
    let exif: Exif
    let location: Location
    let views, downloads: Int

    enum CodingKeys: String, CodingKey {
        case id
        case createdAt = "created_at"
        case updatedAt = "updated_at"
        case promotedAt = "promoted_at"
        case width, height, color
        case blurHash = "blur_hash"
        case unsplashDataDescription = "description"
        case altDescription = "alt_description"
        case urls, links, categories, likes
        case likedByUser = "liked_by_user"
        case currentUserCollections = "current_user_collections"
        case sponsorship, user, exif, location, views, downloads
    }
}

// MARK: - Exif
struct Exif: Codable {
    let make, model, exposureTime, aperture: String
    let focalLength: String
    let iso: Int

    enum CodingKeys: String, CodingKey {
        case make, model
        case exposureTime = "exposure_time"
        case aperture
        case focalLength = "focal_length"
        case iso
    }
}

// MARK: - UnsplashDataLinks
struct UnsplashDataLinks: Codable {
    let linksSelf, html, download, downloadLocation: String

    enum CodingKeys: String, CodingKey {
        case linksSelf = "self"
        case html, download
        case downloadLocation = "download_location"
    }
}

// MARK: - Location
struct Location: Codable {
    let title, name, city, country: String?
    let position: Position
}

// MARK: - Position
struct Position: Codable {
    let latitude, longitude: Double?
}

// MARK: - Urls
struct Urls: Codable {
    let raw, full, regular, small: String
    let thumb: String
}

// MARK: - User
struct User: Codable {
    let id: String
    let updatedAt: Date
    let username, name, firstName, lastName: String
    let twitterUsername: String?
    let portfolioURL: String
    let bio: String?
    let location: String
    let links: UserLinks
    let profileImage: ProfileImage
    let instagramUsername: String
    let totalCollections, totalLikes, totalPhotos: Int
    let acceptedTos: Bool

    enum CodingKeys: String, CodingKey {
        case id
        case updatedAt = "updated_at"
        case username, name
        case firstName = "first_name"
        case lastName = "last_name"
        case twitterUsername = "twitter_username"
        case portfolioURL = "portfolio_url"
        case bio, location, links
        case profileImage = "profile_image"
        case instagramUsername = "instagram_username"
        case totalCollections = "total_collections"
        case totalLikes = "total_likes"
        case totalPhotos = "total_photos"
        case acceptedTos = "accepted_tos"
    }
}

// MARK: - UserLinks
struct UserLinks: Codable {
    let linksSelf, html, photos, likes: String
    let portfolio, following, followers: String

    enum CodingKeys: String, CodingKey {
        case linksSelf = "self"
        case html, photos, likes, portfolio, following, followers
    }
}

// MARK: - ProfileImage
struct ProfileImage: Codable {
    let small, medium, large: String
}

// MARK: - Encode/decode helpers

class JSONNull: Codable, Hashable {

    public static func == (lhs: JSONNull, rhs: JSONNull) -> Bool {
        return true
    }

    public var hashValue: Int {
        return 0
    }

    public func hash(into hasher: inout Hasher) {
        // No-op
    }

    public init() {}

    public required init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        if !container.decodeNil() {
            throw DecodingError.typeMismatch(JSONNull.self, DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Wrong type for JSONNull"))
        }
    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encodeNil()
    }
}

这是我的 ObservableObject：

class UnsplashAPI: ObservableObject {
    enum State {
        case loading
        case loaded(UnsplashData)
    }

    @Published var state = State.loading

    let url = URL(string: "https://api.unsplash.com/")!

    func request() {
        guard var components = URLComponents(url: url.appendingPathComponent("photos/random"),
                                             resolvingAgainstBaseURL: true)
        else {
            fatalError("Couldn't append path component")
        }

        components.queryItems = [
            URLQueryItem(name: "client_id", value: "vMDQ3Vzix8FN6MJL5Qpl3y0F7GdQsTtOjBe_L-IG2ro")
        ]

        let request = URLRequest(url: components.url!)

        let urlSession = URLSession(configuration: URLSessionConfiguration.default)
        urlSession.dataTask(with: request) { data, urlResponse, error in
            if let data = data {
                let decoder = JSONDecoder()
                let dateFormatter = DateFormatter()
                dateFormatter.locale = Locale(identifier: "en_US_POSIX")
                dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ssZ"
                decoder.dateDecodingStrategy = .formatted(dateFormatter)
                do {
                    let response = try decoder.decode(UnsplashData.self, from: data)
                    self.state = .loaded(response) //error here
                } catch {
                    print(error)
                    fatalError("Couldn't decode")
                }
            } else if let error = error {
                print(error.localizedDescription)
            } else {
                fatalError("Didn't receive data")
            }
        }.resume()
    }
}

最后这是我使用 Postman 请求的示例响应：

{
    "id": "HWx5PYGudcI",
    "created_at": "2020-12-08T22:11:11-05:00",
    "updated_at": "2020-12-26T23:19:29-05:00",
    "promoted_at": "2020-12-09T03:14:06-05:00",
    "width": 4000,
    "height": 6000,
    "color": "#8ca6a6",
    "blur_hash": "LAD,r_D*_M?^%ER4%$-oyYp0m+WE",
    "description": null,
    "alt_description": "boy in gray crew neck shirt",
    "urls": {
        "raw": "https://images.unsplash.com/photo-1607483421673-181fb79394b3?ixid=MXwxMTU5MTR8MHwxfHJhbmRvbXx8fHx8fHx8&ixlib=rb-1.2.1",
        "full": "https://images.unsplash.com/photo-1607483421673-181fb79394b3?crop=entropy&cs=srgb&fm=jpg&ixid=MXwxMTU5MTR8MHwxfHJhbmRvbXx8fHx8fHx8&ixlib=rb-1.2.1&q=85",
        "regular": "https://images.unsplash.com/photo-1607483421673-181fb79394b3?crop=entropy&cs=tinysrgb&fit=max&fm=jpg&ixid=MXwxMTU5MTR8MHwxfHJhbmRvbXx8fHx8fHx8&ixlib=rb-1.2.1&q=80&w=1080",
        "small": "https://images.unsplash.com/photo-1607483421673-181fb79394b3?crop=entropy&cs=tinysrgb&fit=max&fm=jpg&ixid=MXwxMTU5MTR8MHwxfHJhbmRvbXx8fHx8fHx8&ixlib=rb-1.2.1&q=80&w=400",
        "thumb": "https://images.unsplash.com/photo-1607483421673-181fb79394b3?crop=entropy&cs=tinysrgb&fit=max&fm=jpg&ixid=MXwxMTU5MTR8MHwxfHJhbmRvbXx8fHx8fHx8&ixlib=rb-1.2.1&q=80&w=200"
    },
    "links": {
        "self": "https://api.unsplash.com/photos/HWx5PYGudcI",
        "html": "https://unsplash.com/photos/HWx5PYGudcI",
        "download": "https://unsplash.com/photos/HWx5PYGudcI/download",
        "download_location": "https://api.unsplash.com/photos/HWx5PYGudcI/download"
    },
    "categories": [],
    "likes": 51,
    "liked_by_user": false,
    "current_user_collections": [],
    "sponsorship": null,
    "user": {
        "id": "3Bj-zCFL4-g",
        "updated_at": "2020-12-26T14:58:36-05:00",
        "username": "owensito",
        "name": "Owen Vangioni",
        "first_name": "Owen",
        "last_name": "Vangioni",
        "twitter_username": null,
        "portfolio_url": null,
        "bio": "Capturing magical moments...\nInstagram: @owensitens                        18 years",
        "location": "Argentina ",
        "links": {
            "self": "https://api.unsplash.com/users/owensito",
            "html": "https://unsplash.com/@owensito",
            "photos": "https://api.unsplash.com/users/owensito/photos",
            "likes": "https://api.unsplash.com/users/owensito/likes",
            "portfolio": "https://api.unsplash.com/users/owensito/portfolio",
            "following": "https://api.unsplash.com/users/owensito/following",
            "followers": "https://api.unsplash.com/users/owensito/followers"
        },
        "profile_image": {
            "small": "https://images.unsplash.com/profile-1583211530737-0c1a46227535image?ixlib=rb-1.2.1&q=80&fm=jpg&crop=faces&cs=tinysrgb&fit=crop&h=32&w=32",
            "medium": "https://images.unsplash.com/profile-1583211530737-0c1a46227535image?ixlib=rb-1.2.1&q=80&fm=jpg&crop=faces&cs=tinysrgb&fit=crop&h=64&w=64",
            "large": "https://images.unsplash.com/profile-1583211530737-0c1a46227535image?ixlib=rb-1.2.1&q=80&fm=jpg&crop=faces&cs=tinysrgb&fit=crop&h=128&w=128"
        },
        "instagram_username": "owensitens",
        "total_collections": 1,
        "total_likes": 13,
        "total_photos": 135,
        "accepted_tos": true
    },
    "exif": {
        "make": "NIKON CORPORATION",
        "model": "NIKON D3300",
        "exposure_time": "1/320",
        "aperture": "5.3",
        "focal_length": "45.0",
        "iso": 200
    },
    "location": {
        "title": null,
        "name": null,
        "city": null,
        "country": null,
        "position": {
            "latitude": null,
            "longitude": null
        }
    },
    "views": 536045,
    "downloads": 1267
}

Answer 1

您需要将线程切换到主线程，您可以从该主线程（并且只能从主线程！）在 iOS 中进行 UI 更改。要修复该错误，您需要使用GCD并将更改状态的行包装在async闭包块中。

DispatchQueue.main.async {
    self.state = .loaded(response) // error should not be triggered anymore
}