Android AsyncTask Example in Kotlin for background processing of task

The AsyncTask help us to perform some operation in background and update the results or status of the work to main thread. Here in this tutorial of  "Android AsyncTask Example in Kotlin"  we will learn about how to implement the AsyncTask using Kotlin code in Android for performing the background operation.

Prerequisite

• We required AndroidStudio 3.0 or later


• At the time of wiring this tutorial we have Kotlin Pulgin version 1.2.10.  You can check out the "Getting started with Android Development in Kotlin" if you have not setup the plugin yet

Why and where to use the AsyncTask?

Android system will perform the task on main thread in sequential order one by one. So what happen in the case if we have to run some process for long time, like downloading the file from internet or backend, or any other task which may took long time? It will hold the main thread and user may fill bad user experience.

Android enforce the application to respond within 5 seconds. If an activity don't respond within that period the android system will show application not responding warning to user.  The usual way to get out of these kind problem is to run such process in background using thread and handler.

So AsyncTask class provide the same functionality to perform any such task in background and synchronize the child thread with main thread.

It is highly recommended to use the AsyncTask  for sort time long running background operation (a few seconds).  If your requirement to  keep the thread running for long time you should use the other API provided by java.util.concurrent like Executor, ThreadPoolExecutor and FutureTask.

AsyncTask's Type

AsyncTask use three parameter as given below.

Params, It is the type of parameter which will be send to task for execution

Progress, Second parameter are used for type of progress published by  doInBackground method.

result, It is return type of the parameter which will be returned to main thread after background processing finished.

AsyncTask common method's and its use

When we execute the AsyncTask in background the whole process gone through four stage.

onPreExecute(), This method run on UI thread and used for intialized and setup the basic UI of main thread.

doInBackground(), After the onPreExecute method this run and perform the all background computation


onProgressUpdate(), this method execute when publishProgress method are called from doInBackground method and run on main UI thread.

onPostExecute(), It is the final method of the AsyncTask which run post completion of the task and run on UI thread.



Using above diagram you can understand the basic follow of the Async Task. So far we have discuss about the AsyncTask common functionality. Now we are going to implement the same using the demo application.

AsyncTask implementation in Kotlin

Just create a simple project using android studio and select the Kotlin as main language.  It will create the MainActivity.kt class and other required component for our project.

Overview of our app " Android AsyncTask Example in Kotlin "

In this demo app we will use the three component TextView, Button and a progress bar to show the progress. To show the full steps we have created the textview as auto-scroll, so that it will show the complete steps in textbox. In this example when we tab on button it will start the AsyncTask .

Layout of our kotlin Demo App

Let us create or modify the MainActivity.xml layout file as given below.

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.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="com.nplix.kotlinasynctask.MainActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="280dp"
        android:text="Hello World!"
        android:id="@+id/statusText"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        android:gravity="bottom"/>
    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/btn_async"
        android:text="Start Async Task"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="@id/statusText"/>

    <ProgressBar
        android:id="@+id/progressBar"
        style="?android:attr/progressBarStyleHorizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="@id/btn_async"
        android:visibility="gone"/>

</android.support.constraint.ConstraintLayout>

Basic Variable declaration of MainActivity.kt

Define the variable as given below in our main class.

    private var btn: Button?=null ;
    private var statusText: TextView ?=null;
    private var statusProgress: ProgressBar?=null;
    private var stringBuilder:StringBuilder?=null

Next initialize the all variable as given below.

btn= findViewById<Button>(R.id.btn_async);
        statusText=findViewById<TextView>(R.id.statusText)
        statusText?.movementMethod=ScrollingMovementMethod()
        statusProgress=findViewById(R.id.progressBar)
        progressBar.max=2;
        progressBar.progress = 0 
        stringBuilder = StringBuilder("Async task Demo\n---------------------")

AsyncTask Class

Now it's time to create the MyAsyncTask class by extending the AsyncTask. Here in this example we are creating the MyAsyncTask class as internal class of Main. So that we can use the all variable defined in main class. So create or modify the MyAsyncTask class as given below.

inner class MyAsyncTask : AsyncTask<String, Int, Int>(){

The all three parameter that we have defined in class structure is type of String, Int and Int.

First parameter is params for input type to method and we have defined it as string type.

Second parameter is for progress update,  which will be used by onProgressUpdate method.

Third and  last parameter we have defined as Int, it is for result type , It define the type which will be returned after final processing.

Now time to override the onPreExecute() method and setup the basic UI or initialization of other related task.

override fun onPreExecute() {

        super.onPreExecute()

        stringBuilder?.append("Async task started... \n In PreExecute Method \n")
        statusText?.text = "${stringBuilder.toString()}"
        progressBar.visibility=View.VISIBLE
        progressBar.progress = 0
        Log.d("Kotlin","On PreExecute Method")

    }

Override the onProgressUpdate and onPostExecute method as given below.

override fun onPreExecute() {

        super.onPreExecute()

        stringBuilder?.append("Async task started... \n In PreExecute Method \n")
        statusText?.text = "${stringBuilder.toString()}"
        progressBar.visibility=View.VISIBLE
        progressBar.progress = 0
        Log.d("Kotlin","On PreExecute Method")

    }
// Update the final status by overriding the OnPostExecute method.
    override fun onPostExecute(result: Int?) {
        super.onPostExecute(result)
        stringBuilder?.append("Task Completed.\n")
        statusText?.text = "${stringBuilder.toString()}"
        Log.d("Kotlin","On Post Execute and size of String is:$result")
    }

At last now modify the our doInBackground method as given below.

override fun doInBackground(vararg params: String?): Int {
        val count:Int=params.size
        var index=0
        while (index<count){

            Log.d("Kotlin","In doInBackground Method and Total parameter passed is :$count " +
                    "and processing $index with value: ${params[index]}")
//Publish the post so that it can be posted to onProgressUpdate method 
//to update the main thread          
  publishProgress(index+1)
     //Sleeping for 1 seconds
            Thread.sleep(1000)
            index++
        }

        return count;

    }

Canceling a Task

For cancelling a background task you can call the task.cancel(boolean) method. We have to implement a validation check in doInBackground() method which will check if task is canceled suing isCanceled() method. It return true if task has been canceled and break the processing of background task.

Full code of Main and AsyncTask class

package com.nplix.kotlinasynctask

import android.os.AsyncTask
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.support.v4.widget.AutoScrollHelper
import android.text.method.ScrollingMovementMethod
import android.util.Log
import android.view.View
import android.widget.Button
import android.widget.ProgressBar
import android.widget.TextView
import kotlinx.android.synthetic.main.activity_main.*


class MainActivity : AppCompatActivity() {
    private var btn: Button?=null ;
    private var statusText: TextView ?=null;
    private var statusProgress: ProgressBar?=null;
    private var stringBuilder:StringBuilder?=null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        //Get the UI element from layout and initialize the UI element
        btn= findViewById<Button>(R.id.btn_async);
        statusText=findViewById<TextView>(R.id.statusText)
        statusText?.movementMethod=ScrollingMovementMethod()

        statusProgress=findViewById(R.id.progressBar)
        progressBar.max=2;
        progressBar.progress = 0
        stringBuilder = StringBuilder("Async task Demo\n---------------------")
        stringBuilder?.append("\nWaiting to start the Async Jobs\n")
        statusText?.text = "${stringBuilder.toString()}"
       // Set on Click listener on start async button
        btn?.setOnClickListener(View.OnClickListener {

            // Declare the AsyncTask and start the execution
          var  task:MyAsyncTask = MyAsyncTask()
            task.execute("www.nplix.com","www.debugandroid.com")

        })

    }
// Create inner class by extending the AsyncTask
inner class MyAsyncTask : AsyncTask<String, Int, Int>(){

//Override the doInBackground method
    override fun doInBackground(vararg params: String?): Int {
        val count:Int=params.size
        var index=0
        while (index<count){

            Log.d("Kotlin","In doInBackground Method and Total parameter passed is :$count " +
                    "and processing $index with value: ${params[index]}")
// Publish the progress         
   publishProgress(index+1)
     //Sleeping for 1 seconds
            Thread.sleep(1000)
            index++
        }

        return count;

    }
// Override the onProgressUpdate method to post the update on main thread
    override fun onProgressUpdate(vararg values: Int?) {
        super.onProgressUpdate(*values)
        if(values[0]!=null) {
            statusProgress?.progress = values[0] as Int
            stringBuilder?.append("Publish post called with ${values[0]}\n")
            statusText?.text = stringBuilder.toString()
        }

    }


// Setup the intial UI before execution of the background task
    override fun onPreExecute() {

        super.onPreExecute()

        stringBuilder?.append("Async task started... \n In PreExecute Method \n")
        statusText?.text = "${stringBuilder.toString()}"
        progressBar.visibility=View.VISIBLE
        progressBar.progress = 0
        Log.d("Kotlin","On PreExecute Method")

    }
// Update the final status by overriding the OnPostExecute method.
    override fun onPostExecute(result: Int?) {
        super.onPostExecute(result)
        stringBuilder?.append("Task Completed.\n")
        statusText?.text = "${stringBuilder.toString()}"
        Log.d("Kotlin","On Post Execute and size of String is:$result")
    }

}


}

 

After completion of the code you will get your app working like as given below.



You may notice that textbox will automatically scroll to bottom when you click on the start background jobs, If you don't know how to created automatic scrolling text box refer the our How to create Auto Scrollable TextView?

Conclusion

In short we can say that whole process of creating the background operation using asynctask is the very simple. The doinBackground method is responsible for all background processing .  Rest other three method are just supporting method to synchronized the background processing thread with main thread.