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Swift 如何返回JSON数据

我正在try 返回JSON数据，但我不知道如何操作.我收到这样的错误信息:"无法将类型为‘[PLANT]’的返回表达式转换为返回类型‘PLANT’".我有点理解这个错误消息，它意味着我无法将一种类型转换为另一种类型.但不确定如何解决这个问题.请看我的代码如下:

import UIKit
import SwiftUI

struct PlantResponse: Codable {
    let data : [Plant]
}

struct Plant: Codable {
    let id: Int
    let commonName: String         // camelCase
    let slug: String
    let scientificName: String     // camelCase
    let year: Int
    let bibliography: String
    let author: String
    let status: String
    let rank: String
    let familyCommonName: String   // camelCase
    let family: String
    let genusId: Int               // camelCase
    let genus: String
    let imageUrl: String           // camelCase
    let synonyms: [String]         // an array of strings
    let links: Links               // a custom type, defined below
    let Plant: [Plant]
}

extension Plant {
    struct Links: Codable {
        let `self`: String
        let plant: String
        let genus: String
    }
}

struct ContentView: View {
    @State var plant: Plant?
    
    var body: some View {
        VStack(alignment: .leading) {
            if let plant {
                Text("There is data")
                    .font(.title)
            } else {
                Text("No data available")
            }
        }
        .padding(20.0)
        .task {
            do {
                plant = try await fetchPlantsFromAPI()
            } catch {
                plant = nil
            }
        }
    }
}

func fetchPlantsFromAPI() async throws -> Plant {
    let url = URL(string: "https://trefle.io/api/v1/plants? token=d321f518jTdU1t-doZQif3jpzzW9V0mk3nLnDssF1vY&filter[common_name]=beach%20strawberry")!
    do {
   
        let (data, _) = try await URLSession.shared.data(from: url)

        let decoder = JSONDecoder()
            decoder.keyDecodingStrategy = .convertFromSnakeCase
        let decoded = try! decoder.decode(PlantResponse.self, from: data)
        print(decoded.data)
        return decoded.data //Cannot convert return expression of type '[Plant]' to return type 'Plant'
    } catch {
        throw error
    }
}

#Preview {
    ContentView()
}

任何帮助都将不胜感激.

推荐答案

错误是指出API返回的数组为Plant，即[Plant]，因此请相应更改方法签名:

func fetchPlantsFromAPI() async throws -> [Plant] {…}

然后将@State变量改为数组:

struct ContentView: View {
    @State var plants: [Plant] = []

    var body: some View {
        List(plants) { plant in
            Text(plant.commonName)
        }
        .padding(20.0)
        .task {
            do {
                plants = try await fetchPlantsFromAPI()
            } catch {
                print(error)
                plants = []
            }
        }
    }
}

我宣布Plant是Identifiable和Sendable:

struct Plant: Codable, Identifiable, Sendable {…}

测试这个，我看到了一些额外的问题:

  1. 我会使用URLComponents来确保查询正确转义:

    func fetchPlantsFromAPI() async throws -> [Plant] {
    guard var components = URLComponents(string: "https://trefle.io/api/v1/plants") else {
        throw URLError(.badURL)
    }

    components.queryItems = [
        URLQueryItem(name: "token", value: "…"),
        URLQueryItem(name: "filter[common_name]", value: "beach strawberry")
    ]

    guard let url = components.url else {
        throw URLError(.badURL)
    }

    let (data, _) = try await URLSession.shared.data(from: url)

    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .convertFromSnakeCase
    let decoded = try decoder.decode(PlantResponse.self, from: data)
    return decoded.data
}

  2. 我会避免try!.正如你在上面修改过的例子中看到的，try就足够了.如果出现编码错误，您不希望应用程序崩溃.

  3. 如果您在catch中所做的一切只是重新抛出错误，那么在do-try-catch中没有意义.同样，我已经从上面的第一点中删除了这一点.

  4. 您已将Plant属性添加到Plant类型.你应该删除:

    struct Plant: Codable, Identifiable, Sendable {
    let id: Int
    let commonName: String         // camelCase
    let slug: String
    let scientificName: String     // camelCase
    let year: Int
    let bibliography: String
    let author: String
    let status: String
    let rank: String
    let familyCommonName: String   // camelCase
    let family: String
    let genusId: Int               // camelCase
    let genus: String
    let imageUrl: String           // camelCase
    let synonyms: [String]         // an array of strings
    let links: Links               // a custom type, defined below
//    let Plant: [Plant]
}

    如果我们看一下JSON，植物数组中没有Plant键:

    {
    "data": [
        {
            "id": 263319,
            "common_name": "Beach strawberry",
            "slug": "fragaria-chiloensis",
            "scientific_name": "Fragaria chiloensis",
            "year": 1768,
            "bibliography": "Gard. Dict. ed. 8 : n.° 4 (1768)",
            "author": "(L.) Mill.",
            "status": "accepted",
            "rank": "species",
            "family_common_name": "Rose family",
            "genus_id": 12147,
            "image_url": "https://bs.plantnet.org/image/o/8ee87e6f94833055db1c7df5fc07761852b7b1eb",
            "synonyms": [
                "Fragaria vesca var. chiloensis",
                "Potentilla chiloensis"
            ],
            "genus": "Fragaria",
            "family": "Rosaceae",
            "links": {
                "self": "/api/v1/species/fragaria-chiloensis",
                "plant": "/api/v1/plants/fragaria-chiloensis",
                "genus": "/api/v1/genus/fragaria"
            }
        }
    ],
    "links": {
        "self": "/api/v1/plants?filter%5Bcommon_name%5D=beach+strawberry",
        "first": "/api/v1/plants?filter%5Bcommon_name%5D=beach+strawberry\u0026page=1",
        "last": "/api/v1/plants?filter%5Bcommon_name%5D=beach+strawberry\u0026page=1"
    },
    "meta": {
        "total": 1
    }
}

  5. 我会解码API错误，这样我们就可以优雅地处理它们.例如，

    struct ErrorResponse: Codable {
    let error: Bool
    let messages: String
}

enum PlantError: Error {
    case apiError(Int, String)
    case invalidResponse(URLResponse)
}

func fetchPlantsFromAPI() async throws -> [Plant] {
    guard var components = URLComponents(string: "https://trefle.io/api/v1/plants") else {
        throw URLError(.badURL)
    }

    components.queryItems = [
        URLQueryItem(name: "token", value: "…"),
        URLQueryItem(name: "filter[common_name]", value: "beach strawberry")
    ]

    guard let url = components.url else {
        throw URLError(.badURL)
    }

    let (data, response) = try await URLSession.shared.data(from: url)

    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .convertFromSnakeCase

    guard let httpResponse = response as? HTTPURLResponse else {
        throw PlantError.invalidResponse(response)
    }

    guard 200...299 ~= httpResponse.statusCode else {
        let errorObject = try decoder.decode(ErrorResponse.self, from: data)
        throw PlantError.apiError(httpResponse.statusCode, errorObject.messages)
    }

    return try decoder
        .decode(PlantResponse.self, from: data)
        .data
}

  6. FWIW，我倾向于使API代码泛型，这样它就可以与任何端点一起工作(这样我们就不必到处重复这些代码).也许:

    struct SuccessResponse<T: Decodable>: Decodable {
    let data: T
}

struct ErrorResponse: Decodable {
    let error: Bool
    let messages: String
}

struct Plant: Codable, Identifiable, Sendable {
    let id: Int
    let commonName: String         // camelCase
    let slug: String
    let scientificName: String     // camelCase
    let year: Int
    let bibliography: String
    let author: String
    let status: String
    let rank: String
    let familyCommonName: String   // camelCase
    let family: String
    let genusId: Int               // camelCase
    let genus: String
    let imageUrl: String           // camelCase
    let synonyms: [String]         // an array of strings
    let links: Plant.Links         // a custom type, defined below
}

extension Plant {
    struct Links: Codable {
        let `self`: String
        let plant: String
        let genus: String
    }
}

enum ApiError: Error {
    case apiError(Int, String)
    case invalidResponse(URLResponse)
}

func urlForPlantSearch(_ string: String) throws -> URL {
    guard var components = URLComponents(string: "https://trefle.io/api/v1/plants") else {
        throw URLError(.badURL)
    }

    components.queryItems = [
        URLQueryItem(name: "token", value: apiToken),
        URLQueryItem(name: "filter[common_name]", value: string)
    ]

    guard let url = components.url else {
        throw URLError(.badURL)
    }

    return url
}

func fetchPlantsFromAPI() async throws -> [Plant] {
    let url = try urlForPlantSearch("beach strawberry")

    return try await fetchFromAPI(url: url)
}

// Make this generic so we can reuse this code with any query

private func fetchFromAPI<T: Decodable>(url: URL) async throws -> T {
    let (data, response) = try await URLSession.shared.data(from: url)

    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .convertFromSnakeCase

    guard let httpResponse = response as? HTTPURLResponse else {
        throw ApiError.invalidResponse(response)
    }

    guard 200...299 ~= httpResponse.statusCode else {
        let errorObject = try decoder.decode(ErrorResponse.self, from: data)
        throw ApiError.apiError(httpResponse.statusCode, errorObject.messages)
    }

    return try decoder
        .decode(SuccessResponse<T>.self, from: data)
        .data
}

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