在 Kotlin 中回调后返回一个值

Question

在 kotlin 中回调后如何返回值，我尝试使用 Thread.sleep 但它不起作用

   fun searchColorFromAPI(): Colors {
    val service: RetrofitService = ServiceGenerator.createService(RetrofitService::class.java)
    val result: MutableList<String> = arrayListOf()
    val call: Call<Colors?>? = service.unityConverter(result)
    call?.enqueue(object : Callback<Colors?> {
        override fun onResponse(call: Call<Colors?>?, response: Response<Colors?>) {
            //switchProgressVisibility()
            if (response.isSuccessful) {
                val serviceResponse: Colors? = response.body()

                if (serviceResponse != null) {
                    mColors = serviceResponse

                }
                else {
                    //buildToast(getString(R.string.null_response))
                }
            }

            else {
                //buildToast(getString(R.string.response_unsuccessful))
                val errorBody: ResponseBody = response.errorBody()
                Log.e(TAG, errorBody.toString())
            }
        }

        override fun onFailure(call: Call<Colors?>?, t: Throwable?) {
           /* buildToast(getString(R.string.error_calling_service))
            Log.e(TAG, t?.message)*/
        }
    })

    return mColors
}

总是，mColors 在 onFailure 或 onResponse 之前返回，因为它们是异步的。在这段代码在 MainActivity 之前，但我被建议起飞，但现在当我尝试获取 mColors 时，我在执行 onResponse 之前和之后得到空值，请我还在学习 Kotlin 和 Android。

Answer 1

您的问题源于Retrofitcall 是asynchronous，所以只要您调用searchColorFromAPI，它就会返回您mColors，但API 调用可能尚未进行，所以在 API 调用之前获得 mColors 值。

要解决这个问题，你可以这样做

1. 使用callback，这将需要对您当前的设置进行少量修改，但第二个选项比这个更可取。使用callback，您的函数应该看起来像这个。

   /* Now instead of returning a value, your function takes a function (named callback) 
      as parameter. when your api call finishes, you can call the callback function and 
      pass the api response.
    */
   fun searchColorFromAPI(callback: (Colors?) -> Unit) {
       val service: RetrofitService = ServiceGenerator.createService(RetrofitService::class.java)
       val result: MutableList<String> = arrayListOf()
       val call: Call<Colors?>? = service.unityConverter(result)
       call?.enqueue(object : Callback<Colors?> {
           override fun onResponse(call: Call<Colors?>?, response: Response<Colors?>) {
               //switchProgressVisibility()
               if (response.isSuccessful) {
                   val serviceResponse: Colors? = response.body()
                   /** pass API response to callback */
                   callback(serviceResponse)
               }
               else {
                   val errorBody: ResponseBody = response.errorBody()
                   Log.e(TAG, errorBody.toString())
                   callback(null)
               }
           }
   
           override fun onFailure(call: Call<Colors?>?, t: Throwable?) {
               callback(null)
           }
       })
   }
   

并在您的activity 中声明function，如下所示。

// This function will be called when your api call finishes
// and it will give you the api response
fun apiCallback(colors: Colors?){
      if(colors == null){
        // API Call failed
    }
    else{
        // use colors as returned by API
    }
}

现在调用searchColorFromApi 应该是这样的

searchColorFromApi(apiCallback)

1. 使用Live Data，在viewmodel 中声明field，如果您不使用viewmodel，则在具有searchColorFromApi 函数的class 中声明它。

   var colors: MutableLiveData<Colors> = MutableLiveData()
   

并修改你的searchColorFromAPI函数如下

fun searchColorFromAPI() {
        val service: RetrofitService = ServiceGenerator.createService(RetrofitService::class.java)
        val result: MutableList<String> = arrayListOf()
        val call: Call<Colors?>? = service.unityConverter(result)
        call?.enqueue(object : Callback<Colors?> {
            override fun onResponse(call: Call<Colors?>?, response: Response<Colors?>) {
                //switchProgressVisibility()
                if (response.isSuccessful) {
                    val serviceResponse: Colors? = response.body()
                    if (serviceResponse != null) {
                       colors.postValue(response.body)
                    }
                }
                else {
                    colors.postValue(null)
                    val errorBody: ResponseBody = response.errorBody()
                    Log.e(TAG, errorBody.toString())
                }
            }

            override fun onFailure(call: Call<Colors?>?, t: Throwable?) {
                colors.postValue(null)
            }
        })
    }

并在您的activity 中关注

  fun setupObservers(){
       yourApiCallingClass.colors.observe(this, Observer {
        // this code is called when ever value of color field changes
       })
   }

Answer 2

1. 您可以使用实时数据，一旦回调接收到就会更新，调用者片段/活动会观察到相同的实时数据

2. 您可以使用协程从其中包含异步调用的函数返回一个值。

3. 您可以使用对活动/片段的接口回调来触发从改造调用接收到的更新。