Android Retrofit enqueue方法没有返回值

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

就像您已经提到的那样，enqueue 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，因为现在尚未从 API 收到任何响应，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 API 的响应时），同时您返回的列表此时为空。这里您不能直接从该方法返回数据。

您可以使用接口使用回调方法。由于您已经获得列表形式的响应，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，通过成功和失败回调，您可以处理这两种情况。