使用 URLSession 获取数据时“不允许从后台线程发布更改”

分类：编程问答

如何解决使用 URLSession 获取数据时“不允许从后台线程发布更改”

我正在尝试从 Unsplash API 获取数据，但是我收到以下错误：“不允许从后台线程发布更改；确保从主线程发布值（通过诸如 receive(on:) 之类的运算符）关于模型更新。”

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,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,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,photos,likes: String
    let portfolio,following,followers: String

    enum CodingKeys: String,likes,portfolio,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()
    }
}

最后是我使用 Postman 请求的响应示例：

{
    "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
}

解决方法

您需要将线程切换到您被允许（并且只能从它！）在 iOS 中进行 UI 更改的 main thread。要修复该错误，您需要使用 GCD 并在 async 闭包块中简单地换行您更改状态的行。

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

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