在 Kotlin 中，在回调后返回一个值

Question

我该如何在Kotlin中在回调后返回一个值？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
}

由于它们是异步的，因此在onFailure或onResponse之前，始终会返回mColors。这段代码以前在MainActivity中，但我被建议移除它，但现在当我尝试获取mColors时，在onResponse执行前后都会得到空值，请注意我仍在学习Kotlin和Android。

- Manoel Henrique

---

Kotlin的lambda函数对于回调很有用。请查看此答案以获取从函数回调的示例。[https://stackoverflow.com/a/59942825/12688463] - Patibandha Parth

Answer 1

你遇到的问题源于Retrofit call是异步的，所以当你调用searchColorFromAPI时，它会立即返回mColors，但API调用可能还没有被执行，因此你会在API调用前获取到mColors的值。

为了解决这个问题，你可以进行以下操作：

1. Use callback, this will require little modification in your current setup, but the 2nd option is preferable over this. Using callback your function should look like this.

   /* 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)
           }
       })
   }
   

在你的activity中声明一个如下的function。

// 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的方法应该像这样

searchColorFromApi(apiCallback)

2. Use Live Data, declare following field in your viewmodel, if you are not using viewmodel then declare it in the class which has searchColorFromApi function.

   var colors: MutableLiveData<Colors> = MutableLiveData()
   

并将您的searchColorFromAPI函数修改为以下内容。

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)
            }
        })
    }

在您的 activity 中，请执行以下操作

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

- mightyWOZ

Answer 2

1. 你可以使用实时数据，该数据在回调接收后更新，同样的实时数据由调用方片段/活动观察。

2. 您可以使用协程从具有异步调用的函数返回值。

3. 您可以使用接口回调到活动/片段以触发来自Retrofit调用的更新。

- Ashok Kumar

---

我同意你的回答，但你并没有向提问者展示如何实现这些，所以我认为它并没有增加太多价值 :) 你应该考虑在每个条目中添加简单的示例。 - a_local_nobody