类不返回可变映射对象

Question

我正在使用 Kotlin 制作一个带有 API 的基本货币转换器应用程序。为了下载信息，我创建了一个名为 DownloadTaskClass 的新类，它扩展了已弃用的 AsyncTask 类(我还没有学习 java.util.concurrent 包的其他类(。

DownloadTaskClass 包含一个将 MutableMap 返回到 MainActivity 的方法。但是，我不明白为什么此方法不返回 MutableMap 对象。以下是课程。

DownloadTaskClass.kt

package com.example.kotlincurrency
import android.os.AsyncTask
import org.json.JSONObject
import java.io.InputStream
import java.io.InputStreamReader
import java.lang.Exception
import java.net.HttpURLConnection
import java.net.URL
class DownloadTaskClass: AsyncTask<String, Void, String>() {
private var data: Int = 0
private var result: String = ""
private var mutableMap: MutableMap<String, Any> = mutableMapOf()
private lateinit var url: URL
private lateinit var inputStream: InputStream
private lateinit var inputStreamReader: InputStreamReader
private lateinit var httpURLConnection: HttpURLConnection
override fun doInBackground(vararg urls: String): String? {
url = URL(urls[0])
httpURLConnection = url.openConnection() as HttpURLConnection
inputStream = httpURLConnection.inputStream
inputStreamReader = InputStreamReader(inputStream)
data = inputStreamReader.read()
while (data != -1){
val current: Char = data.toChar()
result += current
data = inputStreamReader.read()
}
return result
}
override fun onPostExecute(result: String?) {
super.onPostExecute(result)
try {
val jsonObject = JSONObject(result)
mutableMap["date"] = jsonObject.getString("date")

val rates: String = jsonObject.getString("rates")
val currencyJSONObject = JSONObject(rates)
mutableMap["USD"] = currencyJSONObject.getString("USD")
mutableMap["GBP"] = currencyJSONObject.getString("GBP")
mutableMap["EUR"] = currencyJSONObject.getString("EUR")
mutableMap["CHF"] = currencyJSONObject.getString("CHF")
mutableMap["SEK"] = currencyJSONObject.getString("SEK")
mutableMap["CAD"] = currencyJSONObject.getString("CAD")
}
catch (e: Exception){
e.printStackTrace()
}
}
fun returnMapMethod(): MutableMap<String, Any>{
return mutableMap
}
}

MainActivity.kt

package com.example.kotlincurrency
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import androidx.appcompat.app.ActionBar
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
setActionBar()
downloadInitInfo()
}
private fun setActionBar() {
supportActionBar!!.displayOptions = ActionBar.DISPLAY_SHOW_CUSTOM
supportActionBar!!.setDisplayShowCustomEnabled(true)
supportActionBar!!.setCustomView(R.layout.layout_action_bar)
}
private fun downloadInitInfo(){
val downloadTaskClass = DownloadTaskClass()
downloadTaskClass.execute("https://api.exchangeratesapi.io/latest?base=INR")
println(downloadTaskClass.returnMapMethod())
}
}

Answer 1

您在doInBackground()方法中发出异步请求，可能需要一些时间才能调用onPostExecute()并填充数据mutableMap。调用returnMapMethod()方法时mutableMap对象尚未填充数据。您需要使用一些侦听器在加载数据时收到通知。

下面是使用LiveData观察数据的示例：

class DownloadTaskClass: AsyncTask<String, Void, String>() {

private var data: MutableLiveData<MutableMap<String, Any>> = MutableLiveData()
// ...

override fun onPostExecute(result: String?) {
// ...
data.value = mutableMap
}
fun returnMapMethod(): LiveData<out Map<String, Any>> {
return data
}
}

在您的活动中：

private fun downloadInitInfo(){
val downloadTaskClass = DownloadTaskClass()
downloadTaskClass.execute("https://api.exchangeratesapi.io/latest?base=INR")
downloadTaskClass.returnMapMethod().observe(this, Observer { 
println(it) // "it" contains the `mutableMap` data
})
}