SwiftUI中如何仅在视图首次显示时执行Task内的API调用？

我已经实现了缓存机制，只需要在视图首次显示时调用API，但现在发现每次导航返回该视图时，API调用都会重复触发。我可以用if判断缓存来规避，但觉得这是临时方案。作为SwiftUI新手，想找到类似UIKit中viewDidLoad的正规实现方式。

相关代码如下：

final class ViewModel: ObservableObject {
    
    private let apiManager: APIManager
    
    @Published var image: UIImage? = nil
    
    init(apiManager: APIManager = APIManager()) {
        self.apiManager = apiManager
    }

    func fetchEmojis() async {
        do {
            _ = try await apiManager.downloadAsyncAwait(.emojis, type: emojis.self)
        } catch {
            print("API fetchEmojis issues: \(error)")
        }
    }
}

struct ContentView: View {
    
    @StateObject var viewModel = ViewModel()
    
    var body: some View {
        NavigationView {
            VStack {
                NavigationLink(destination: AvatarsGridView()) {
                    Text("Avatar List")
                }
                Text("")
                    .padding(0)
            }
            .task {
                print("start")
                await viewModel.fetchEmojis()
                print("finish")
            }
        }
    }
}

APIManager代码（非核心）：

typealias emojis = [String: String]

enum EndpointEnum {
    private var baseURL: String { return "https://api.github.com" }
    
    case emojis
    case userAvatar(String)
    case appleRepositories(Int)
    
    private var fullPath: String {
        var endpoint:String
        switch self {
        case .emojis:
            endpoint = "/emojis"
        case .userAvatar(let userName):
            endpoint = "/users/\(userName)"
        case .appleRepositories(let page):
            endpoint = "/users/apple/repos?page=\(page)"
        }
        return baseURL + endpoint
    }
    
    var url: URL {
        guard let url = URL(string: fullPath) else {
            preconditionFailure("The url used in \(EndpointEnum.self) is not valid")
        }
        return url
    }
}

enum APIManagerError: Error {
    case httpStatus
    case decoding
}

final class APIManager {
    
    private let session: URLSession
    private let decoder = JSONDecoder()

    init(session: URLSession = URLSession.shared) {
        self.session = session
        self.coreDataManager = coreDataManager
    }
    
    func downloadAsyncAwait<T: Codable>(_ endpointEnum: EndpointEnum, type: T.Type) async throws -> T? {
        
        let url = endpointEnum.url
        
        let request = URLRequest(url: url)
        do {
            let (data, response) = try await session.data(for: request, delegate: nil)
            guard let response = response as? HTTPURLResponse,
                  response.statusCode >= 200 && response.statusCode < 300 else {

                throw APIManagerError.httpStatus
            }
            
            let result = try decoder.decode(type, from: data)            
            return result
        } catch  {
            throw APIManagerError.decoding
        }
    }
}

解决方案 #

SwiftUI中没有直接对应UIKit viewDidLoad的API，但可以通过以下几种正规方式实现仅首次加载时调用API：

方式一：利用@StateObject的初始化特性（推荐） #

@StateObject修饰的ViewModel只会在视图首次创建时初始化一次，因此可以在ViewModel的初始化方法中启动异步任务，确保API调用仅执行一次。

修改后的ViewModel代码：

final class ViewModel: ObservableObject {
    
    private let apiManager: APIManager
    @Published var image: UIImage? = nil
    
    init(apiManager: APIManager = APIManager()) {
        self.apiManager = apiManager
        // 初始化时启动异步任务，仅执行一次
        Task {
            await fetchEmojis()
        }
    }

    func fetchEmojis() async {
        do {
            _ = try await apiManager.downloadAsyncAwait(.emojis, type: emojis.self)
        } catch {
            print("API fetchEmojis issues: \(error)")
        }
    }
}

对应的ContentView需移除.task修饰符：

struct ContentView: View {
    
    @StateObject var viewModel = ViewModel()
    
    var body: some View {
        NavigationView {
            VStack {
                NavigationLink(destination: AvatarsGridView()) {
                    Text("Avatar List")
                }
                Text("")
                    .padding(0)
            }
        }
    }
}

方式二：给.task添加固定ID #

如果希望保留视图层的任务触发逻辑，可以给.task传入一个固定不变的ID，任务只会在ID首次出现时执行，后续视图刷新或导航返回都不会重复触发。

修改后的ContentView：

struct ContentView: View {
    
    @StateObject var viewModel = ViewModel()
    
    var body: some View {
        NavigationView {
            VStack {
                NavigationLink(destination: AvatarsGridView()) {
                    Text("Avatar List")
                }
                Text("")
                    .padding(0)
            }
            // 固定ID确保任务仅执行一次
            .task(id: "initial_emoji_fetch") {
                print("start")
                await viewModel.fetchEmojis()
                print("finish")
            }
        }
    }
}

方式三：ViewModel内部管理加载状态 #

在ViewModel中添加标记记录是否已完成API调用，确保fetchEmojis方法仅执行一次，同时支持加载失败后的重试逻辑。

修改后的ViewModel：

final class ViewModel: ObservableObject {
    
    private let apiManager: APIManager
    @Published var image: UIImage? = nil
    // 标记是否已成功加载过数据
    private var hasFetchedEmojis = false
    
    init(apiManager: APIManager = APIManager()) {
        self.apiManager = apiManager
    }

    func fetchEmojis() async {
        guard !hasFetchedEmojis else { return }
        hasFetchedEmojis = true
        
        do {
            _ = try await apiManager.downloadAsyncAwait(.emojis, type: emojis.self)
        } catch {
            print("API fetchEmojis issues: \(error)")
            // 加载失败时重置标记，允许后续重试
            hasFetchedEmojis = false
        }
    }
}

内容的提问来源于stack exchange，提问作者bruno