Swift中POST请求参数无法传递变量的问题排查与解决

调用按国家获取城市数据的API时，遇到一个奇怪的问题：直接在请求体里写"country": "Nigeria"这类硬编码值，请求完全正常；但改用全局变量selectedCountryString处理后传入，返回结果为空。

初始有问题的代码 #

let myCountry = selectedCountryString.lowercased().trimmingCharacters(in: .whitespacesAndNewlines)
guard let url = URL(string: "https://countriesnow.space/api/v0.1/countries/population/cities/filter") else {
    return
}

var request = URLRequest(url: url)
request.httpMethod = "POST"
request.setValue("application/json", forHTTPHeaderField: "Content-Type")   
let body: [String: Any] = [
    "limit": 10,
    "order": "dsc",
    "orderBy": "value",
    "country": "\(myCountry)"
]

request.httpBody = try? JSONSerialization.data(withJSONObject: body, options: .fragmentsAllowed)
let task = URLSession.shared.dataTask(with: request) {data, _, error in
    guard let data = data, error == nil else {
        return
    }
    do{                
        let response = try JSONDecoder().decode(CitiesPopModel.self, from: data)
        onCompletion(response)
    } 
    catch {
        print("Error country -> \(myCountry)")              
    }
}       
task.resume()

修改后可正常运行的代码 #

改用Alamofire实现后，变量能正常传递参数，请求返回预期结果：

func callCitiesByPopAPI(completion: @escaping (CitiesPopModel?, Error?) -> ()) {
     let url = "https://countriesnow.space/api/v0.1/countries/population/cities/filter"
     
     let parameters: [String: Any] = [
        "limit": 20,
        "order": "dsc",
        "orderBy": "value",
        "country": selectedCountryString
     ]
     
     AF.request(url, method: .post, parameters: parameters, encoding: JSONEncoding.default).responseDecodable(of: CitiesPopModel.self) { response in
         if let error = response.error {
             completion(nil, error)
             return
         }
         if let result = response.value {
             completion(result, nil)
             print("City pop model result is \(result)")
             return
         }
     }
 }

问题排查总结 #

初始代码的问题大概率出在这几点：

• 对国家名称做了lowercased()处理，而该API要求国家名称严格匹配大小写（比如"Nigeria"和"nigeria"不被识别为同一个国家）
• JSONSerialization使用.fragmentsAllowed选项可能导致JSON格式异常，Alamofire的JSONEncoding.default会自动处理标准的JSON序列化
• 字符串插值"\(myCountry)"可能引入了额外不可见字符，Alamofire对参数的处理更严谨

内容的提问来源于stack exchange，提问作者THEKRYS