MainActivity.kt

package com.example.top10

import android.content.Context
import android.os.AsyncTask
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.os.PersistableBundle
import android.util.Log
import android.view.*
import android.widget.ArrayAdapter
import android.widget.ListView
import android.widget.TextView
import kotlinx.android.synthetic.main.activity_main.*
import org.xmlpull.v1.XmlPullParser
import org.xmlpull.v1.XmlPullParserFactory
import java.lang.Exception
import java.net.URL
import kotlin.properties.Delegates

private const val TAG = "MainActivity"

class ResultEntry() {
    var title: String = ""
    var description: String = ""
    var link: String = ""
}

private const val BREAKING_NEWS = "https://www.nasa.gov/rss/dyn/breaking_news.rss"
private const val EDUCATION_NEWS = "https://www.nasa.gov/rss/dyn/educationnews.rss"
private const val SOLAR_SYSTEM_NEWS = "https://www.nasa.gov/rss/dyn/solar_system.rss"
private const val STATE_URL = ""

class MainActivity : AppCompatActivity() {

    private var url = BREAKING_NEWS
    private var downloadData: DownloadData? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        Log.d(TAG,"onCreate called!")
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        if(savedInstanceState != null) {
            url = savedInstanceState.getString(STATE_URL, url)
        }
        downloadURL(url)
    }

    override fun onDestroy() {
        Log.d(TAG,"onDestroy called!")
        super.onDestroy()
    }

    override fun onRestoreInstanceState(savedInstanceState: Bundle) {
        Log.d(TAG,"onRestoreInstanceState called!")
        super.onRestoreInstanceState(savedInstanceState)
        url = savedInstanceState.getString(STATE_URL,url)
    }

    override fun onStart() {
        Log.d(TAG,"onStart called!")
        super.onStart()
    }

    override fun onResume() {
        Log.d(TAG,"onResume called!")
        super.onResume()
    }

    override fun onPause() {
        Log.d(TAG,"onPause called!")
        super.onPause()
    }

    override fun onStop() {
        Log.d(TAG,"onStop called!")
        super.onStop()
    }

    override fun onRestart() {
        Log.d(TAG,"onRestart called!")
        super.onRestart()
    }

    override fun onSaveInstanceState(outState: Bundle, outPersistentState: PersistableBundle) {
        Log.d(TAG,"onSaveInstanceState called!")
        super.onSaveInstanceState(outState, outPersistentState)
        outState?.putString(STATE_URL, url)
    }

    override fun onCreateOptionsMenu(menu: Menu?): Boolean {
        menuInflater.inflate(R.menu.top_menu, menu)
        return true
    }

    private fun downloadURL(url:String) {
        downloadData = DownloadData(this, xmlListView)
        downloadData?.execute(url)
    }

    override fun onOptionsItemSelected(item: MenuItem): Boolean {
        this.url = when(item.itemId) {
            R.id.breakingNews ->
                BREAKING_NEWS
            R.id.educationNews ->
                EDUCATION_NEWS
            R.id.solarSystemNews ->
                SOLAR_SYSTEM_NEWS
            else ->
                return super.onOptionsItemSelected(item)
        }
        downloadURL(url)
        return true
    }
}

class DownloadData(context: Context,listView: ListView) : AsyncTask<String, Void, String>() { // handles the downloading of xml data. Extends AsyncTask class. Enables data to be fetched asynchronously
    private val TAG = "DownloadData"
    private var propContext: Context by Delegates.notNull()
    private var propListView: ListView by Delegates.notNull()
    //
    init {
        propContext = context
        propListView = listView
    }

    override fun doInBackground(vararg url: String?): String {
        val rssFeed = URL(url[0]).readText()
        if (rssFeed.isEmpty()) {
            Log.e(TAG, "doInBackground: Cant download XML")
        }
        return rssFeed
    }

    override fun onPostExecute(result: String?) {
        super.onPostExecute(result)
        Log.d(TAG, "onPostExecute: parameter is $result")
        val parseResults = ParseResults()
        if (result != null) {
            parseResults.parse(result)
        }
        val feedAdapter = FeedAdapter(propContext, R.layout.list_record, parseResults.results)
        propListView.adapter = feedAdapter
    }
}

    class ParseResults() {

        val results = ArrayList<ResultEntry>()
        // takes the xml data string and converts it i to data

        fun parse(xmlData: String): Boolean {
            var status = true
            var inEntry = false
            var textValue = ""

            try {

                val factory = XmlPullParserFactory.newInstance()
                factory.isNamespaceAware = true
                val xpp = factory.newPullParser()
                xpp.setInput(xmlData.reader())
                var eventType = xpp.eventType
                var currentRecord = ResultEntry()

                while (eventType != XmlPullParser.END_DOCUMENT) {
                    val tagName = xpp.name?.toLowerCase()

                    when (eventType) {
                        XmlPullParser.START_TAG -> {
                            if (tagName == "item") {
                                inEntry = true
                            }
                        }
                        XmlPullParser.TEXT -> textValue = xpp.text
                        XmlPullParser.END_TAG -> {
                            if (inEntry) {
                                when (tagName) {
                                    "item" -> {
                                        results.add(currentRecord)
                                        inEntry = false
                                        currentRecord = ResultEntry()
                                    }

                                    "title" -> currentRecord.title = textValue
                                    "description" -> currentRecord.description = textValue
                                    "link" -> currentRecord.link = textValue
                                }
                            }
                        }
                    }
                    eventType = xpp.next()
                }

            } catch (e: Exception) {
                status = false
            }
            return status
        }
    }


class ViewHolder(v: View) {
    val title: TextView = v.findViewById(R.id.title)
    val description: TextView = v.findViewById(R.id.description)
    val link: TextView = v.findViewById(R.id.link)
}

class FeedAdapter(context: Context, private val resource : Int, private val results:
List<ResultEntry>): ArrayAdapter<ResultEntry>(context,resource) {

    private val inflater = LayoutInflater.from(context)

    override fun getCount(): Int {
        return results.size
    }

    override fun getView(position: Int, convertView: View?, parent: ViewGroup): View {
        val view:View
        val viewHolder:ViewHolder
        if(convertView == null) {
            view = inflater.inflate(resource,parent,false)
            viewHolder = ViewHolder(view)
            view.tag = viewHolder
        }else{
            // reused the view
            view = convertView
            viewHolder = view.tag as ViewHolder
        }

        val currentApp = results[position]

        viewHolder.title.text = currentApp.title
        viewHolder.description.text = currentApp.description
        viewHolder.link.text = currentApp.link

        return view
    }
}