android kotlin - Coroutines async

MainActivity.kt

package com.example.coroutine

import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import kotlinx.android.synthetic.main.activity_main.*
import kotlinx.coroutines.*
import java.io.BufferedReader
import java.io.InputStreamReader
import java.net.URL


class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        // url to get html
        val url:URL = URL("https://developer.android.com/")

        button.setOnClickListener {
            // creates a coroutine and returns its future result
            // as an implementation of deferred

            // async task to get html as string from url
            val result: Deferred = GlobalScope.async {
                url.getHtml()
            }


            // global scope is used to launch top-level coroutines which are
            // operating on the whole application lifetime
            GlobalScope.launch(Dispatchers.Main) {
                // show html as string when available
                textView.text = result.await()
            }
        }
    }
}


// extension function to get html from url
fun URL.getHtml():String {
    val reader = BufferedReader(InputStreamReader(this.openStream()))
    var input: String? = ""
    val stringBuffer = StringBuffer()
    while (reader.readLine().also { input = it } != null) {
        stringBuffer.append(input)
    }
    reader.close()
    return stringBuffer.toString ()
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity"
android:background="#EDEAE0">
<com.google.android.material.button.MaterialButton
android:id="@+id/button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"        android:text="Get HTML"        android:backgroundTint="#665D1E"
app:layout_constraintStart_toStartOf="parent"        app:layout_constraintTop_toTopOf="parent" />
<com.google.android.material.textview.MaterialTextView
android:id="@+id/textView"        style="@style/TextAppearance.MaterialComponents.Headline6"
android:layout_width="0dp"        android:layout_height="wrap_content"
android:padding="8dp"        android:layout_marginTop="16dp"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toBottomOf="@+id/button"
tools:text="Hello World!" />
</androidx.constraintlayout.widget.ConstraintLayout>

build.gradle (app) [dependencies]

// kotlin coroutineimplementation 'org.jetbrains.kotlinx:kotlinx-coroutines-core:1.3.7'implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-android:1.3.7'