handler.kt

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

class MainActivity : AppCompatActivity() {

    private val MSG_SHOW_PROGRESS = 1
    private val MSG_SHOW_IMAGE = 2
    private lateinit var handler: Handler

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

        handler = Handler(UIHandler())

        Thread(JsonMessageCreator("https://jsonplaceholder.typicode.com/todos/1")).start()

    }

    fun getJsonFromServer(url: String): String {

        val inputStream: BufferedReader

        val jsonUrl = URL(url)
        val dc = jsonUrl.openConnection()

        dc.connectTimeout = 5000
        dc.readTimeout = 5000

        inputStream = BufferedReader(
            InputStreamReader(
                dc.getInputStream()
            )
        )

        var string = ""
        inputStream.readLines().forEach{
            string += it
        }

        return string
    }

    inner class JsonMessageCreator(val url: String) : Runnable {
        override fun run() {
            handler.obtainMessage(MSG_SHOW_PROGRESS).sendToTarget()

            val data = getJsonFromServer(url)

            handler.obtainMessage(MSG_SHOW_IMAGE, data).sendToTarget()
        }
    }

    inner class UIHandler: Handler.Callback {
        override fun handleMessage(msg: Message?): Boolean {
            when(msg?.what) {
                MSG_SHOW_PROGRESS -> progress_bar.visibility = View.VISIBLE
                MSG_SHOW_IMAGE -> {
                    progress_bar.visibility = View.GONE
                    json_textview.text = msg.obj as CharSequence
                }
            }
            return true
        }
    }
}