LiveData 没有 Return 更新的字符串

Question

我正在尝试学习 LiveData。我编写了一个示例代码来异步下载一些 API 数据并将其 return 到 MainActivity 以供记录。我没有使用 ViewModel，因为我还没有学习它。

这是 DownloadInfoClass.kt 的代码，我将在其中放置 LiveData 对象：

package com.example.kotlincurrency

import androidx.lifecycle.MutableLiveData
import kotlinx.coroutines.CoroutineScope
import kotlinx.coroutines.Dispatchers.IO
import kotlinx.coroutines.Dispatchers.Main
import kotlinx.coroutines.launch
import kotlinx.coroutines.withContext
import java.io.InputStreamReader
import java.lang.Exception
import java.net.HttpURLConnection
import java.net.URL

class DownloadParseInfoClass {

    private var mutableLiveData: MutableLiveData<String> = MutableLiveData()
    private var result: String = ""

    fun downloadMethod(urls: String) = CoroutineScope(Main).launch {
        result = infoDownloadMethod(urls)
    }

    private suspend fun infoDownloadMethod(urls: String): String{
        var result = ""

        withContext(IO){
            try {
                val url = URL(urls)
                val httpURLConnection = url.openConnection() as HttpURLConnection
                val inputStream = httpURLConnection.inputStream
                val inputStreamReader = InputStreamReader(inputStream)

                var data: Int = inputStreamReader.read()
                while (data != -1){
                    val current = data.toChar().toString()
                    result += current
                    data = inputStreamReader.read()
                }
            }

            catch (e: Exception){
                e.printStackTrace()
            }
        }

        return result
    }

    fun getMutableLiveData(): MutableLiveData<String>{
        mutableLiveData.value = result
        return mutableLiveData
    }

}

在 MainActivity.kt class 中，我将 Observer 放在 OnCreate 方法中。这是代码：

override fun onCreate(savedInstanceState: Bundle?){
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        setActionBarMethod()
        initViews()

        val downloadParseInfoClass = DownloadParseInfoClass()
        downloadParseInfoClass.downloadMethod("https://api.exchangeratesapi.io/latest?base=INR")
        downloadParseInfoClass.getMutableLiveData().observe(this, Observer {
            Log.i("Result", it)
        })
    }

我不明白为什么它不记录数据。可能是因为我没有使用 ViewModel 吗？我的意思是，我浏览过所有寻求解决方案的博客，每个博客都使用了带有 LiveData 的 ViewModel。

Answer 1

您需要 post 从输入流读取新值后。

while (data != -1){
     val current = data.toChar().toString()
     result += current
     data = inputStreamReader.read()
} 
mutableLiveData.postValue(result)

LiveData 没有 Return 更新的字符串

LiveData Doesn't Return The Updated String

android

kotlin

android-livedata

mutablelivedata