Android Retrofit enqueue 方法不返回值

Question

fun getSurvey() : MutableList<SurveyMaster>{
        //Retrofit creates implementation for this method getSurveyList()
        val call : Call<List<SurveyMaster>> = retrofitPlaceHolderAPI.getSurveyList("bearer "+customIDToken)

        val surveyList =   mutableListOf<SurveyMaster>()

        //enqueue - executing on the background thread, to execute asynchronously
        call.enqueue(object : Callback<List<SurveyMaster>>{
            override fun onFailure(call: Call<List<SurveyMaster>>, t: Throwable) {
                Log.e("Error", t.message)
                //Log.e("Error", t.localizedMessage)
            }

            override fun onResponse(call: Call<List<SurveyMaster>>, response: Response<List<SurveyMaster>>) {
                if (!response.isSuccessful){
                    Log.e("SurveyList Error", "code: "+response.code())
                    return
                }

                val surveys = response.body()

                for(element in surveys!!){
                    surveyList.add(element)
                }
            }
        })
        return surveyList
    }

Enqueue 方法在后台线程上运行，但是如何在 enqueue 方法中返回列表。上面的代码返回空列表。提前致谢

Answer 1

就像您已经提到的那样，入队将在后台线程上执行此 API 调用，因此您的函数将始终返回空列表，因为目前尚未收到来自 API 的响应，因此您提到的这个循环：-

for(element in surveys!!){
    surveyList.add(element)
}

这个循环将在稍后运行（当收到来自 API 的响应时）同时你已经返回了此时为空的列表。这里你不能直接从方法返回数据。

您可以通过接口使用回调方法。由于您已经以列表的形式获得响应，因此无需创建单独的列表并将数据添加到其中。

fun getSurvey(onGetSurveyListener: OnGetSurveyListener){

    val call : Call<List<SurveyMaster>> = retrofitPlaceHolderAPI.getSurveyList("bearer "+customIDToken)

    val surveyList =   mutableListOf<SurveyMaster>()

    //enqueue - executing on the background thread, to execute asynchronously
    call.enqueue(object : Callback<List<SurveyMaster>>{
        override fun onFailure(call: Call<List<SurveyMaster>>, t: Throwable) {
            Log.e("Error", t.message)
            onGetSurveyListener.onGetSurveyFailure(t.localizedMessage)
            //Log.e("Error", t.localizedMessage)
        }

        override fun onResponse(call: Call<List<SurveyMaster>>, response: Response<List<SurveyMaster>>) {
            if (!response.isSuccessful){
                Log.e("SurveyList Error", "code: "+response.code())
                return
            }
            response.body?.let{list->
               onGetSurveyListener.onGetSurveySuccess(list)
            }
        }
    })
}

interface OnGetSurveyListener {
    fun onGetSurveySuccess(surveyList:List<SurveyMaster>)
    fun onGetSurveyFailure(errorMessage: String)
}

现在有了成功和失败回调，您可以处理这两种情况。

Answer 2

这是一个异步调用，所以 onResponse 在网络调用成功后被调用，并且您在此之前返回列表。您需要使用回调返回数据。像这样修改你的函数：

fun getSurvey(onComplete: (MutableList<SurveyMaster>) -> Unit) {
    //Retrofit creates implementation for this method getSurveyList()
    val call : Call<List<SurveyMaster>> = retrofitPlaceHolderAPI.getSurveyList("bearer "+customIDToken)
    
    val surveyList =   mutableListOf<SurveyMaster>()
    
    //enqueue - executing on the background thread, to execute asynchronously
    call.enqueue(object : Callback<List<SurveyMaster>>{
        override fun onFailure(call: Call<List<SurveyMaster>>, t: Throwable) {
            Log.e("Error", t.message)
            //Log.e("Error", t.localizedMessage)
        }
        
        override fun onResponse(call: Call<List<SurveyMaster>>, response: Response<List<SurveyMaster>>) {
            if (!response.isSuccessful){
                Log.e("SurveyList Error", "code: "+response.code())
                return
            }
            
            val surveys = response.body()
            
            for(element in surveys!!){
                surveyList.add(element)
            }
            onComplete.invoke(surveyList)
        }
    })
}

然后像这样调用你的方法：

    getSurvey { list ->
      // Do something with your list.
    }

根据您的问题，您可能还希望返回一些 onFailure 事件以了解网络请求是否成功。