是否可以强制SwiftUI代码同步执行？异步网络请求提前返回如何解决

你当前的代码逻辑存在本质问题：你使用静态存储属性的初始化闭包发起异步网络请求，闭包执行时网络请求还未完成，就已经返回了空的viewModel.modelData，且该闭包只会执行一次，后续请求拿到结果也不会更新master属性的值。
绝对不要强制把异步网络请求转为同步执行，尤其在主线程操作会直接卡住UI，严重时会触发系统 watchdog 直接杀死应用。

方案1：回调闭包方案（兼容所有iOS版本） #

第一步：修正ViewModel的已知bug #

你当前的getData方法中用JSONEncoder编码JSON字符串属于逻辑错误，会把原有JSON串外层多包一层引号，导致服务端解析失败，修正后代码如下：

class ViewModel<T: Codable> : ObservableObject {
    @Published var modelData : T?
 
    func getData(url: URL, encoded: String, completion: (@escaping ()->()) ) {
        var request = URLRequest(url: url)
        request.setValue("application/json", forHTTPHeaderField: "Content-Type")
        request.httpMethod = "POST"
        // 直接将JSON字符串转为UTF8数据即可，不需要用JSONEncoder二次编码
        request.httpBody = encoded.data(using: .utf8)
        
        URLSession.shared.dataTask(with: request) { data1, response, error in
            if let error = error {
                print("Request error: ", error)
                return
            }
            guard let data1 = data1 else { return }
            DispatchQueue.main.async {
                let decoder = JSONDecoder()
                if let data = try? decoder.decode(T.self, from: data1) {
                    self.modelData = data
                    completion()
                    print("Success!")
                } else {
                    print("Does not decode correctly")
                }
            }
        }.resume()
    }
}

第二步：改造MasterControllerStruct，废弃同步属性改用回调方法 #

struct MasterControllerStruct {
    static var isMasterPopulated = false
    static var objMaster : Master.MasterDetails?
    
    // 替换原同步master属性，用带回调的方法获取数据
    static func getMaster(completion: @escaping (Master.MasterDetails?) -> Void) {
        // 已有缓存直接返回
        if isMasterPopulated {
            completion(objMaster)
            return
        }
        // 无缓存发起网络请求
        let viewModel = ViewModel<Master>()
        let url = URL(string: "URL Goes Here")!
        let encoded = """
        {
            <JSON Parameters go here>
        }
        """
        viewModel.getData(url: url, encoded: encoded) {
            objMaster = viewModel.modelData?.Data
            isMasterPopulated = true
            completion(objMaster)
        }
    }
}

调用示例 #

MasterControllerStruct.getMaster { master in
    // 拿到数据后执行后续逻辑，UI操作需要切回主线程
    DispatchQueue.main.async {
        // 刷新UI等操作
    }
}

方案2：async/await方案（iOS 15+/macOS 12+ 支持） #

如果你的项目不需要兼容旧系统，用Swift Concurrency语法可以让代码更简洁易读：

改造ViewModel为async版本 #

class ViewModel<T: Codable> : ObservableObject {
    @Published var modelData : T?
 
    func getData(url: URL, encoded: String) async throws -> T? {
        var request = URLRequest(url: url)
        request.setValue("application/json", forHTTPHeaderField: "Content-Type")
        request.httpMethod = "POST"
        request.httpBody = encoded.data(using: .utf8)
        
        let (data, _) = try await URLSession.shared.data(for: request)
        let result = try JSONDecoder().decode(T.self, from: data)
        DispatchQueue.main.async {
            self.modelData = result
        }
        return result
    }
}

改造MasterControllerStruct #

struct MasterControllerStruct {
    static var isMasterPopulated = false
    static var objMaster : Master.MasterDetails?
    
    static func getMaster() async -> Master.MasterDetails? {
        if isMasterPopulated {
            return objMaster
        }
        do {
            let viewModel = ViewModel<Master>()
            let url = URL(string: "URL Goes Here")!
            let encoded = """
            {
                <JSON Parameters go here>
            }
            """
            let result = try await viewModel.getData(url: url, encoded: encoded)
            objMaster = result?.Data
            isMasterPopulated = true
            return objMaster
        } catch {
            print("请求失败: \(error.localizedDescription)")
            return nil
        }
    }
}

调用示例 #

Task {
    let master = await MasterControllerStruct.getMaster()
    // UI操作切回主线程
    await MainActor.run {
        // 刷新UI等操作
    }
}

内容的提问来源于stack exchange，提问作者bridgenbarbu