How To Do AsyncTask In Kotlin

Introduction

In this article, we will learn how to perform AsyncTask in Android with Kotlin. AsyncTask is used to perform some asynchronous tasks without blocking the main thread. It is very useful in Android development. Similar to Java, the same code can be used in Kotlin with some minor modifications.

Coding Part

I have divided the coding part into 3 steps as shown in the following.

Creating a new project with Kotlin's support.
Setting up the project with
Implementing Async tasks with Kotlin.

Without any delay, we will start coding for Kotlin AsyncTask.

Step 1 - Creating a new project with Kotlin

Open Android Studio and select Create a new project.
Name the project as your wish and tick the Kotlin checkbox support.
Then Select your Activity type (For Example Navigation Drawer Activity, Empty Activity, etc.).
Then click “Finish” button to create a new project in Android Studio.

Step 2 - Setting up the project with AndroidManifest

Now, we will add internet permission in the Android Manifest file. Because we will see the example with the URL Connection. Just open your AndroidManifest.xml file and add the following line.

<uses-permission android:name="android.permission.INTERNET"/>

Step 3 - Implementing Async tasks with Kotlin

We will start coding. Here, I have used the following simple API to retrieve android version details.

http://demosmushtaq.16mb.com/api/post_sample.php?version_index=14

I have included a Button and TextView in my layout (activity_main.xml) Here, Button and TextView is used to call the API and display the result from the API
1. <?xml version="1.0" encoding="utf-8"?>
2. <android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
3. xmlns:app="http://schemas.android.com/apk/res-auto"
4. xmlns:tools="http://schemas.android.com/tools"
5. android:layout_width="match_parent"
6. android:layout_height="match_parent"
7. tools:context=".MainActivity"
8. tools:layout_editor_absoluteX="0dp"
9. tools:layout_editor_absoluteY="81dp">
11. <TextView
12. android:id="@+id/my_text"
13. android:layout_width="wrap_content"
14. android:layout_height="wrap_content"
15. android:layout_marginEnd="8dp"
16. android:layout_marginStart="8dp"
17. android:layout_marginTop="72dp"
18. android:text="AsyncTask Example"
19. android:textSize="18sp"
20. android:textStyle="bold"
21. app:layout_constraintEnd_toEndOf="parent"
22. app:layout_constraintStart_toStartOf="parent"
23. app:layout_constraintTop_toTopOf="parent" />
25. <ProgressBar
26. android:id="@+id/MyprogressBar"
27. style="?android:attr/progressBarStyle"
28. android:layout_width="wrap_content"
29. android:layout_height="wrap_content"
30. android:layout_marginEnd="8dp"
31. android:layout_marginStart="8dp"
32. android:layout_marginTop="32dp"
33. android:visibility="invisible"
34. app:layout_constraintEnd_toEndOf="parent"
35. app:layout_constraintStart_toStartOf="parent"
36. app:layout_constraintTop_toBottomOf="@+id/my_text" />
38. <Button
39. android:id="@+id/CallBtn"
40. android:layout_width="wrap_content"
41. android:layout_height="wrap_content"
42. android:layout_marginEnd="8dp"
43. android:layout_marginStart="8dp"
44. android:layout_marginTop="36dp"
45. android:text="Execute AsyncTask"
46. app:layout_constraintEnd_toEndOf="parent"
47. app:layout_constraintStart_toStartOf="parent"
48. app:layout_constraintTop_toBottomOf="@+id/MyprogressBar" />
50. </android.support.constraint.ConstraintLayout>

Then open your Activity file (in my case kt) and create a class with AsynTask as a parent.
- AsyncTask has three main methods.
  1. onPreExecute – State before task performance
  2. doInBackground – State for task performance
  3. onPostExecute – State after task performance

Add the following code with your AsyncTask class.
1. class AsyncTaskExample(private var activity: MainActivity?) : AsyncTask<String, String, String>() {
3. override fun onPreExecute() {
4. super.onPreExecute()
5. activity?.MyprogressBar?.visibility = View.VISIBLE
6. }
8. override fun doInBackground(vararg p0: String?): String {
10. var result = ""
11. try {
12. val url = URL("http://demosmushtaq.16mb.com/api/post_sample.php?version_index=14")
13. val httpURLConnection = url.openConnection() as HttpURLConnection
15. httpURLConnection.readTimeout = 8000
16. httpURLConnection.connectTimeout = 8000
17. httpURLConnection.doOutput = true
18. httpURLConnection.connect()
20. val responseCode: Int = httpURLConnection.responseCode
21. Log.d(activity?.tag, "responseCode - " + responseCode)
23. if (responseCode == 200) {
24. val inStream: InputStream = httpURLConnection.inputStream
25. val isReader = InputStreamReader(inStream)
26. val bReader = BufferedReader(isReader)
27. var tempStr: String?
29. try {
31. while (true) {
32. tempStr = bReader.readLine()
33. if (tempStr == null) {
34. break
35. }
36. result += tempStr
37. }
38. } catch (Ex: Exception) {
39. Log.e(activity?.tag, "Error in convertToString " + Ex.printStackTrace())
40. }
41. }
42. } catch (ex: Exception) {
43. Log.d("", "Error in doInBackground " + ex.message)
44. }
45. return result
46. }
48. override fun onPostExecute(result: String?) {
49. super.onPostExecute(result)
50. activity?.MyprogressBar?.visibility = View.INVISIBLE
51. if (result == "") {
52. activity?.my_text?.text = activity?.getString(R.string.network_error)
53. } else {
54. var parsedResult = ""
55. var jsonObject: JSONObject? = JSONObject(result)
56. jsonObject = jsonObject?.getJSONObject("data")
57. parsedResult += "Code Name : " + (jsonObject?.get("code_name")) + "\n"
58. parsedResult += "Version Number : " + (jsonObject?.get("version_number")) + "\n"
59. parsedResult += "API Level : " + (jsonObject?.get("api_level"))
60. activity?.my_text?.text = parsedResult
61. }
62. }
63. }

Here I have used

“HttpURLConnection” to access the API Link.

“BufferedReader” to read the response output of the API Link.

“JSONObject” to parse the response from the server.

Class can be called in the following method.
1. AsyncTaskExample(this).execute()

Full Code of MainActivity

You can find the full code implementation of MainActivty.kt in the following.

class MainActivity : AppCompatActivity() {
private val tag: String = "MainActivity"
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
CallBtn.setOnClickListener {
AsyncTaskExample(this).execute()
}
}
@SuppressLint("StaticFieldLeak")
class AsyncTaskExample(private var activity: MainActivity?) : AsyncTask<String, String, String>() {
override fun onPreExecute() {
super.onPreExecute()
activity?.MyprogressBar?.visibility = View.VISIBLE
}
override fun doInBackground(vararg p0: String?): String {
var result = ""
try {
val url = URL("http://demosmushtaq.16mb.com/api/post_sample.php?version_index=14")
val httpURLConnection = url.openConnection() as HttpURLConnection
httpURLConnection.readTimeout = 8000
httpURLConnection.connectTimeout = 8000
httpURLConnection.doOutput = true
httpURLConnection.connect()
val responseCode: Int = httpURLConnection.responseCode
Log.d(activity?.tag, "responseCode - " + responseCode)
if (responseCode == 200) {
val inStream: InputStream = httpURLConnection.inputStream
val isReader = InputStreamReader(inStream)
val bReader = BufferedReader(isReader)
var tempStr: String?
try {
while (true) {
tempStr = bReader.readLine()
if (tempStr == null) {
break
}
result += tempStr
}
} catch (Ex: Exception) {
Log.e(activity?.tag, "Error in convertToString " + Ex.printStackTrace())
}
}
} catch (ex: Exception) {
Log.d("", "Error in doInBackground " + ex.message)
}
return result
}
override fun onPostExecute(result: String?) {
super.onPostExecute(result)
activity?.MyprogressBar?.visibility = View.INVISIBLE
if (result == "") {
activity?.my_text?.text = activity?.getString(R.string.network_error)
} else {
var parsedResult = ""
var jsonObject: JSONObject? = JSONObject(result)
jsonObject = jsonObject?.getJSONObject("data")
parsedResult += "Code Name : " + (jsonObject?.get("code_name")) + "\n"
parsedResult += "Version Number : " + (jsonObject?.get("version_number")) + "\n"
parsedResult += "API Level : " + (jsonObject?.get("api_level"))
activity?.my_text?.text = parsedResult
}
}
}
}

Download Code

You can download the sample code from the following GitHub link.