如何保留@escaping闭包的返回结果，避免数据丢失？

我需要实现一个接收邮政编码（postalCode）并返回城市、州和国家的函数。目前函数能成功获取结果，在调用的@escaping闭包内可以正常打印数据，但把数据保存到闭包外的变量后，数据就丢失了。

我的代码 #

func getAddress(forPostalCode postalCode: String, completion: @escaping (_ city: String?, _ state: String?, _ country: String?, _ error: Error?) -> Void) {
    let geocoder = CLGeocoder()
    let addressString = "\(postalCode), USA"
    geocoder.geocodeAddressString(addressString) { placemarks, error in
        if let error = error {
            completion(nil, nil, nil, error)
            return
        }
        guard let placemark = placemarks?.first else {
            completion(nil, nil, nil, NSError(domain: "com.example.app", code: 1, userInfo: [NSLocalizedDescriptionKey: "No placemarks found"]))
            return
        }
        guard let city = placemark.locality else {
            completion(nil, nil, nil, NSError(domain: "com.example.app", code: 2, userInfo: [NSLocalizedDescriptionKey: "City not found"]))
            return
        }
        guard let state = placemark.administrativeArea else {
            completion(nil, nil, nil, NSError(domain: "com.example.app", code: 3, userInfo: [NSLocalizedDescriptionKey: "State not found"]))
            return
        }
        guard let country = placemark.country else {
            completion(nil, nil, nil, NSError(domain: "com.example.app", code: 4, userInfo: [NSLocalizedDescriptionKey: "Country not found"]))
            return
        }
        completion(city, state, country, nil)
    }
}

let postalCode = "10001"
var aCity: String = ""
var aState: String = ""
var aCountry: String = ""

getAddress(forPostalCode: postalCode) { city, state, country, error in
    if let error = error {
        print("Error: \(error.localizedDescription)")
        return
    }
    if let city = city, let state = state, let country = country {
        aCity = city
        aState = state
        aCountry = country
        print("Internal: \(aCity), \(aState) in \(aCountry)")
    }
    else {
        print("Error: Unable to retrieve address for postal code \(postalCode)")
    }
}

print("External: \(aCity), \(aState) in \(aCountry)")

运行结果 #

External: ,  in 
Internal: New York, NY in United States

这是因为geocodeAddressString是异步操作，它会在后台线程执行地理编码请求，不会阻塞主线程。代码的实际执行顺序是：

1. 初始化aCity、aState、aCountry为空字符串
2. 调用getAddress，发起异步地理编码请求
3. 直接执行闭包外的print("External: ...")，此时异步请求还未完成，变量仍为初始空值
4. 异步请求完成后，才会进入闭包内部给变量赋值并打印Internal: ...

解决方案 #

所有依赖异步请求结果的操作，必须放在闭包内部执行，或者使用Swift现代的async/await语法处理异步逻辑：

方案1：在闭包内处理结果（最直接）

把需要使用地址数据的逻辑全部移到闭包内部：

getAddress(forPostalCode: postalCode) { city, state, country, error in
    if let error = error {
        print("Error: \(error.localizedDescription)")
        return
    }
    guard let city = city, let state = state, let country = country else {
        print("Error: Unable to retrieve address for postal code \(postalCode)")
        return
    }
    // 在这里执行所有需要用到地址数据的操作
    print("Address: \(city), \(state) in \(country)")
    // 比如更新UI、保存到本地等
    // updateUserInterface(city: city, state: state, country: country)
}

方案2：改用async/await（更简洁的异步写法）

将原函数改造为支持async/await的版本，避免闭包嵌套：

import CoreLocation

func getAddress(forPostalCode postalCode: String) async throws -> (city: String, state: String, country: String) {
    let geocoder = CLGeocoder()
    let addressString = "\(postalCode), USA"
    
    let placemarks = try await geocoder.geocodeAddressString(addressString)
    guard let placemark = placemarks.first else {
        throw NSError(domain: "com.example.app", code: 1, userInfo: [NSLocalizedDescriptionKey: "No placemarks found"])
    }
    guard let city = placemark.locality else {
        throw NSError(domain: "com.example.app", code: 2, userInfo: [NSLocalizedDescriptionKey: "City not found"])
    }
    guard let state = placemark.administrativeArea else {
        throw NSError(domain: "com.example.app", code: 3, userInfo: [NSLocalizedDescriptionKey: "State not found"])
    }
    guard let country = placemark.country else {
        throw NSError(domain: "com.example.app", code: 4, userInfo: [NSLocalizedDescriptionKey: "Country not found"])
    }
    return (city, state, country)
}

// 调用示例（需在异步上下文，比如Task中）
let postalCode = "10001"
Task {
    do {
        let address = try await getAddress(forPostalCode: postalCode)
        print("Address: \(address.city), \(address.state) in \(address.country)")
        // 在这里处理地址数据
    } catch {
        print("Error: \(error.localizedDescription)")
    }
}

内容的提问来源于stack exchange，提问作者indyMac