AsyncTask Android example
Asked Answered
T

21

711

I was reading about AsyncTask, and I tried the simple program below. But it does not seem to work. How can I make it work?

public class AsyncTaskActivity extends Activity {

    Button btn;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        btn = (Button) findViewById(R.id.button1);
        btn.setOnClickListener((OnClickListener) this);
    }

    public void onClick(View view){
        new LongOperation().execute("");
    }

    private class LongOperation extends AsyncTask<String, Void, String> {
        @Override
        protected String doInBackground(String... params) {
            for(int i=0;i<5;i++) {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
            TextView txt = (TextView) findViewById(R.id.output);
            txt.setText("Executed");
            return null;
        }

        @Override
        protected void onPostExecute(String result) {
        }

        @Override
        protected void onPreExecute() {
        }

        @Override
        protected void onProgressUpdate(Void... values) {
        }
    }
}

I am just trying to change the label after 5 seconds in the background process.

This is my main.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:layout_width="fill_parent"
              android:layout_height="fill_parent"
              android:orientation="vertical" >
    <ProgressBar
        android:id="@+id/progressBar"
        style="?android:attr/progressBarStyleHorizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:indeterminate="false"
        android:max="10"
        android:padding="10dip">
    </ProgressBar>
    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Start Progress" >
    </Button>
    <TextView android:id="@+id/output"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Replace"/>
</LinearLayout>
Turbofan answered 12/3, 2012 at 17:9 Comment(8)
also you can show the progress by calling publishprogress() from the doInBackground() method.Selfinduction
here is asynctask example AsyncTask ExampleCaucasia
here is an asynctask example of downloading image too : android-ios-tutorials.com/182/…Us
AsyncTask - Design Pattern : stackoverflow.com/questions/5058661/…Ectropion
check this example wiki.workassis.com/android-asynctaskWeixel
linked answer, stackoverflow.com/questions/31277518/…Caudad
youtube.com/watch?v=QfQE1ayCzf8 does exactly thisLucindalucine
for those trying to implement concurrency post API 30...AsyncTask is depreciated. See documentation: public abstract class AsyncTaskDraughtboard
B
716

Ok, you are trying to access the GUI via another thread. This, in the main, is not good practice.

The AsyncTask executes everything in doInBackground() inside of another thread, which does not have access to the GUI where your views are.

preExecute() and postExecute() offer you access to the GUI before and after the heavy lifting occurs in this new thread, and you can even pass the result of the long operation to postExecute() to then show any results of processing.

See these lines where you are later updating your TextView:

TextView txt = findViewById(R.id.output);
txt.setText("Executed");

Put them in onPostExecute().

You will then see your TextView text updated after the doInBackground completes.

I noticed that your onClick listener does not check to see which View has been selected. I find the easiest way to do this is via switch statements. I have a complete class edited below with all suggestions to save confusion.

import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.provider.Settings.System;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.view.View.OnClickListener;

public class AsyncTaskActivity extends Activity implements OnClickListener {

    Button btn;
    AsyncTask<?, ?, ?> runningTask;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        btn = findViewById(R.id.button1);

        // Because we implement OnClickListener, we only
        // have to pass "this" (much easier)
        btn.setOnClickListener(this);
    }

    @Override
    public void onClick(View view) {
        // Detect the view that was "clicked"
        switch (view.getId()) {
        case R.id.button1:
            if (runningTask != null)
                runningTask.cancel(true);
            runningTask = new LongOperation();
            runningTask.execute();
            break;
        }
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        // Cancel running task(s) to avoid memory leaks
        if (runningTask != null)
            runningTask.cancel(true);
    }

    private final class LongOperation extends AsyncTask<Void, Void, String> {

        @Override
        protected String doInBackground(Void... params) {
            for (int i = 0; i < 5; i++) {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    // We were cancelled; stop sleeping!
                }
            }
            return "Executed";
        }

        @Override
        protected void onPostExecute(String result) {
            TextView txt = (TextView) findViewById(R.id.output);
            txt.setText("Executed"); // txt.setText(result);
            // You might want to change "executed" for the returned string
            // passed into onPostExecute(), but that is up to you
        }
    }
}
Bienne answered 12/3, 2012 at 17:12 Comment(11)
I am unable to do this <code> btn.setOnClickListener(this); </code> Eclipse gives an error ----- "The method setOnClickListener(View.OnClickListener) in the type View is not applicable for the arguments (AsyncTaskActivity)"Turbofan
I would advise changing the setting of the text to not be static but take the argument from the onPostExecute(String result) method. It would make it clearer for future readers that the argument is populated by the return value of doInBackground(String... params).Uganda
@Eric Tobias - That exact things done in the commented section already. I was following and answering the users question in my full example.Bienne
@Graham Indeed, I saw the comment. However, I think it is best not to refer to the static assignment at all. It is, in my opinion, a bad habit and should be avoided. Not to mention that such a wonderful response as yours has an obligation to teach good practices as well! ;)Uganda
@Graham Yes, lets not go to that dark place please! ;) Thank you for your correspondence. I now do understand your reason and can only agree. Thank you for taking the time to reply.Uganda
As an addendum and google seeder (and coming from someone currently learning this stuff which is how I came across this) : the majority of UI updates you'll do for something where you need progress reported back to the user is in the call back onProgressUpdate which is executed in the main UI thread.Toothlike
This will surely mess up if your activity is rotated or destroyed for any reason...Columbine
@Columbine - Yes, but that is NOT what the OP was asking. The question relates to making an AsyncTask work not what the pit falls or alternatives are of using one.Bienne
Fair enough. Just pointing it out. It might be worth highlighting in the answer (esp as it's very highly up-voted) as this can be an easy way to shoot yourself in the foot.Columbine
@Graham Smith, is it possible to access Views like TextView of another activity ( txt.setText("Executed");) within a separated AsyncTask's class? like to put the asynctask in a separated .java file (not as an inner class).Spermaceti
There are ways of doing that, however I would not use AsyncTask for anything, as it is the path to a dark and horrid places.Bienne
C
850

My full answer is here, but here is an explanatory image to supplement the other answers on this page. For me, understanding where all the variables were going was the most confusing part in the beginning.

enter image description here

Chalky answered 10/4, 2015 at 10:41 Comment(1)
params is an array. (In the example above, it was a String array.) This allows you to pass in multiple parameters of the same type. Then you can access those parameters with params[0], params[1], params[2], etc. In the example, there was only a single String in the params array. If you need to pass in multiple parameters of different types (for example, a String and an int), see this question.Chalky
B
716

Ok, you are trying to access the GUI via another thread. This, in the main, is not good practice.

The AsyncTask executes everything in doInBackground() inside of another thread, which does not have access to the GUI where your views are.

preExecute() and postExecute() offer you access to the GUI before and after the heavy lifting occurs in this new thread, and you can even pass the result of the long operation to postExecute() to then show any results of processing.

See these lines where you are later updating your TextView:

TextView txt = findViewById(R.id.output);
txt.setText("Executed");

Put them in onPostExecute().

You will then see your TextView text updated after the doInBackground completes.

I noticed that your onClick listener does not check to see which View has been selected. I find the easiest way to do this is via switch statements. I have a complete class edited below with all suggestions to save confusion.

import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.provider.Settings.System;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.view.View.OnClickListener;

public class AsyncTaskActivity extends Activity implements OnClickListener {

    Button btn;
    AsyncTask<?, ?, ?> runningTask;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        btn = findViewById(R.id.button1);

        // Because we implement OnClickListener, we only
        // have to pass "this" (much easier)
        btn.setOnClickListener(this);
    }

    @Override
    public void onClick(View view) {
        // Detect the view that was "clicked"
        switch (view.getId()) {
        case R.id.button1:
            if (runningTask != null)
                runningTask.cancel(true);
            runningTask = new LongOperation();
            runningTask.execute();
            break;
        }
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        // Cancel running task(s) to avoid memory leaks
        if (runningTask != null)
            runningTask.cancel(true);
    }

    private final class LongOperation extends AsyncTask<Void, Void, String> {

        @Override
        protected String doInBackground(Void... params) {
            for (int i = 0; i < 5; i++) {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    // We were cancelled; stop sleeping!
                }
            }
            return "Executed";
        }

        @Override
        protected void onPostExecute(String result) {
            TextView txt = (TextView) findViewById(R.id.output);
            txt.setText("Executed"); // txt.setText(result);
            // You might want to change "executed" for the returned string
            // passed into onPostExecute(), but that is up to you
        }
    }
}
Bienne answered 12/3, 2012 at 17:12 Comment(11)
I am unable to do this <code> btn.setOnClickListener(this); </code> Eclipse gives an error ----- "The method setOnClickListener(View.OnClickListener) in the type View is not applicable for the arguments (AsyncTaskActivity)"Turbofan
I would advise changing the setting of the text to not be static but take the argument from the onPostExecute(String result) method. It would make it clearer for future readers that the argument is populated by the return value of doInBackground(String... params).Uganda
@Eric Tobias - That exact things done in the commented section already. I was following and answering the users question in my full example.Bienne
@Graham Indeed, I saw the comment. However, I think it is best not to refer to the static assignment at all. It is, in my opinion, a bad habit and should be avoided. Not to mention that such a wonderful response as yours has an obligation to teach good practices as well! ;)Uganda
@Graham Yes, lets not go to that dark place please! ;) Thank you for your correspondence. I now do understand your reason and can only agree. Thank you for taking the time to reply.Uganda
As an addendum and google seeder (and coming from someone currently learning this stuff which is how I came across this) : the majority of UI updates you'll do for something where you need progress reported back to the user is in the call back onProgressUpdate which is executed in the main UI thread.Toothlike
This will surely mess up if your activity is rotated or destroyed for any reason...Columbine
@Columbine - Yes, but that is NOT what the OP was asking. The question relates to making an AsyncTask work not what the pit falls or alternatives are of using one.Bienne
Fair enough. Just pointing it out. It might be worth highlighting in the answer (esp as it's very highly up-voted) as this can be an easy way to shoot yourself in the foot.Columbine
@Graham Smith, is it possible to access Views like TextView of another activity ( txt.setText("Executed");) within a separated AsyncTask's class? like to put the asynctask in a separated .java file (not as an inner class).Spermaceti
There are ways of doing that, however I would not use AsyncTask for anything, as it is the path to a dark and horrid places.Bienne
H
77

I'm sure it is executing properly, but you're trying to change the UI elements in the background thread and that won't do.

Revise your call and AsyncTask as follows:

Calling Class

Note: I personally suggest using onPostExecute() wherever you execute your AsyncTask thread and not in the class that extends AsyncTask itself. I think it makes the code easier to read especially if you need the AsyncTask in multiple places handling the results slightly different.

new LongThread() {
    @Override public void onPostExecute(String result) {
        TextView txt = (TextView) findViewById(R.id.output);
        txt.setText(result);
    }
}.execute("");

LongThread class (extends AsyncTask):

@Override
protected String doInBackground(String... params) {
    for (int i = 0; i < 5; i++) {
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    return "Executed";
}      
Hilmahilt answered 12/3, 2012 at 17:15 Comment(2)
Thanks for providing an example that decouples the AsyncTask from the ActivityNelsen
yes, finally someone decouples the task and the activity. thank you.And overriding the onPostExecute in the activity is brilliant.Inconclusive
T
64

Concept and code here

I have created a simple example for using AsyncTask of Android. It starts with onPreExecute(), doInBackground(), publishProgress() and finally onProgressUpdate().

In this, doInBackground() works as a background thread, while other works in the UI Thread. You can't access an UI element in doInBackground(). The sequence is the same as I have mentioned.

However, if you need to update any widget from doInBackground, you can publishProgress from doInBackground which will call onProgressUpdate to update your UI widget.

class TestAsync extends AsyncTask<Void, Integer, String> {
    String TAG = getClass().getSimpleName();

    protected void onPreExecute() {
        super.onPreExecute();
        Log.d(TAG + " PreExceute","On pre Exceute......");
    }

    protected String doInBackground(Void...arg0) {
        Log.d(TAG + " DoINBackGround", "On doInBackground...");

        for (int i=0; i<10; i++){
            Integer in = new Integer(i);
            publishProgress(i);
        }
        return "You are at PostExecute";
    }

    protected void onProgressUpdate(Integer...a) {
        super.onProgressUpdate(a);
        Log.d(TAG + " onProgressUpdate", "You are in progress update ... " + a[0]);
    }

    protected void onPostExecute(String result) {
        super.onPostExecute(result);
        Log.d(TAG + " onPostExecute", "" + result);
    }
}

Call it like this in your activity:

new TestAsync().execute();

Developer Reference Here

Turgent answered 16/9, 2013 at 12:2 Comment(1)
classes starts with generally capital letters in Java , that's a notation usually followedTepper
R
22

Move these two lines:

TextView txt = (TextView) findViewById(R.id.output);
txt.setText("Executed");

out of your AsyncTask's doInBackground method and put them in the onPostExecute method. Your AsyncTask should look something like this:

private class LongOperation extends AsyncTask<String, Void, String> {

    @Override
    protected String doInBackground(String... params) {
        try {
            Thread.sleep(5000); // no need for a loop
        } catch (InterruptedException e) {
            Log.e("LongOperation", "Interrupted", e);
            return "Interrupted";
        }
        return "Executed";
    }      

    @Override
    protected void onPostExecute(String result) {               
        TextView txt = (TextView) findViewById(R.id.output);
        txt.setText(result);
    }
}
Rosellaroselle answered 12/3, 2012 at 17:16 Comment(2)
Hey what i am running async task on service i want to return some value to main ui thread.Butterflies
@Butterflies - Check out this discussion. There are two issues: reporting results from an AsyncTask, which my answer addresses; and sending a value from a service to the ui thread, which the other discussion addresses. These issues are independent.Rosellaroselle
M
16

How to memorize the parameters used in AsyncTask?

Don't

If you are new to AsyncTask then it is very common to get confused while writing an AsyncTask. The main culprits are the parameters used in the AsyncTask, i.e., AsyncTask<A, B, C>. Based on the A, B, C (arguments) signature of the methods differs which makes things even more confusing.

Keep it simple!

The key is don't memorize. If you can visualize what your task really needs to do then writing the AsyncTask with the correct signature at the first attempt would be a piece of cake. Just figure out what your Input, Progress, and Output are, and you will be good to go.

So what is an AsyncTask?

AsyncTask is a background task that runs in the background thread. It takes an Input, performs Progress and gives an Output.

I.e., AsyncTask<Input, Progress, Output>.

For example:

Enter image description here

What is the relationship with methods?

Between AsyncTask and doInBackground()

Enter image description here

doInBackground() and onPostExecute(),onProgressUpdate()` are also related

Enter image description here

How to write that in the code?

DownloadTask extends AsyncTask<String, Integer, String>{

    // Always same signature
    @Override
    public void onPreExecute()
    {}

    @Override
    public String doInbackGround(String... parameters)
    {
        // Download code
        int downloadPerc = // Calculate that
        publish(downloadPerc);

        return "Download Success";
    }

    @Override
    public void onPostExecute(String result)
    {
        super.onPostExecute(result);
    }

    @Override
    public void onProgressUpdate(Integer... parameters)
    {
        // Show in spinner, and access UI elements
    }

}

How will you run this Task?

new DownLoadTask().execute("Paradise.mp3");
Misapprehend answered 25/1, 2019 at 6:20 Comment(0)
C
14

Background / Theory

AsyncTask allows you to run a task on a background thread, while publishing results to the UI thread.

The user should always able to interact with the app so it is important to avoid blocking the main (UI) thread with tasks such as downloading content from the web.

This is why we use an AsyncTask.

It offers a straightforward interface by wrapping the UI thread message queue and handler that allow you to send and process runnable objects and messages from other threads.

Implementation

AsyncTask is a generic class. (It takes parameterized types in its constructor.)

It uses these three generic types:

Params - the type of the parameters sent to the task upon execution.

Progress - the type of the progress units published during the background computation.

Result - the type of the result of the background computation.

Not all types are always used by an asynchronous task. To mark a type as unused, simply use the type Void:

private class MyTask extends AsyncTask<Void, Void, Void> { ... }

These three parameters correspond to three primary functions you can override in AsyncTask:

  • doInBackground(Params...)
  • onProgressUpdate(Progress...)
  • onPostExecute(Result)

To execute AsyncTask

  • Call execute() with parameters to be sent to the background task.

What Happens

  1. On main/UI thread, onPreExecute() is called.

    • To initialize something in this thread. (E.g. show a progress bar on the user interface.)
  2. On a background thread, doInBackground(Params...) is called.

    • (Params were passed via execute.)
    • Where the long-running task should happen.
    • Must override at least doInBackground() to use AsyncTask.

    • Call publishProgress(Progress...) to update the user interface with a display of progress (e.g. UI animation or log text printed) while the background computation is still executing.

      • Causes onProgressUpdate() to be called.
  3. On the background thread a result is returned from doInBackground().

    • (This triggers the next step.)
  4. On main/UI thread, onPostExecute() is called with the returned result.

Examples

In both examples the "blocking task" is a download from the web.

  • Example A downloads an image and displays it in an ImageView, while
  • Example B downloads some files.

Example A

The doInBackground() method downloads the image and stores it in an object of type BitMap. The onPostExecute() method takes the bitmap and places it in the ImageView.

class DownloadImageTask extends AsyncTask<String, Void, Bitmap> {
    ImageView bitImage;

    public DownloadImageTask(ImageView bitImage) {
        this.bitImage = bitImage;
    }

    protected Bitmap doInBackground(String... urls) {
        String urldisplay = urls[0];
        Bitmap mBmp = null;
        try {
            InputStream in = new java.net.URL(urldisplay).openStream();
            mBmp = BitmapFactory.decodeStream(in);
        } catch (Exception e) {
            Log.e("Error", e.getMessage());
            e.printStackTrace();
        }
        return mBmp;
    }

    protected void onPostExecute(Bitmap result) {
        bitImage.setImageBitmap(result);
    }
}

Example B

 private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> {
     protected Long doInBackground(URL... urls) {
         int count = urls.length;
         long totalSize = 0;
         for (int i = 0; i < count; i++) {
             totalSize += Downloader.downloadFile(urls[i]);
             publishProgress((int) ((i / (float) count) * 100));
             // Escape early if cancel() is called
             if (isCancelled()) break;
         }
         return totalSize;
     }

     protected void onProgressUpdate(Integer... progress) {
         setProgressPercent(progress[0]);
     }

     protected void onPostExecute(Long result) {
         showDialog("Downloaded " + result + " bytes");
     }
 }

Example B execution

new DownloadFilesTask().execute(url1, url2, url3);
Caudad answered 17/9, 2017 at 16:20 Comment(3)
Very nice.. But I keep getting error about return type clashing - attempting to use incompatible return type. I have tried all sorts of return types, same error.Lucindalucine
Hi @johnktejik, you may want to search for that specific problem. Maybe this is what is happening to you: the-return-type-is-incompatible-with-asynctaskCaudad
Excellent! Consider going into editing?Sellma
W
12

When an asynchronous task is executed, the task goes through four steps:

  1. onPreExecute()
  2. doInBackground(Params...)
  3. onProgressUpdate(Progress...)
  4. onPostExecute(Result)

Below is a demo example:

private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> {

    protected Long doInBackground(URL... urls) {
        int count = urls.length;
        long totalSize = 0;
        for (int i = 0; i < count; i++) {
            totalSize += Downloader.downloadFile(urls[i]);
            publishProgress((int) ((i / (float) count) * 100));

            // Escape early if cancel() is called
            if (isCancelled())
                break;
        }
        return totalSize;
    }

    protected void onProgressUpdate(Integer... progress) {
        setProgressPercent(progress[0]);
    }

    protected void onPostExecute(Long result) {
        showDialog("Downloaded " + result + " bytes");
    }
 }

And once you created, a task is executed very simply:

new DownloadFilesTask().execute(url1, url2, url3);
Wirra answered 23/1, 2014 at 11:20 Comment(1)
execute expects a paremeter as Runnable. It doesn't accept string. What is the type of your url? string or notZygotene
A
11

Shortest example for just doing something asynchronously:

class MyAsyncTask extends android.os.AsyncTask {
    @Override
    protected Object doInBackground(Object[] objects) {
        // Do something asynchronously
        return null;
    }
}

To run it:

(new MyAsyncTask()).execute();
Afterbody answered 12/6, 2014 at 11:17 Comment(0)
S
6

I would recommend making your life easier by using this library for background works:

https://github.com/Arasthel/AsyncJobLibrary

It's this simple...

AsyncJob.doInBackground(new AsyncJob.OnBackgroundJob() {

    @Override
    public void doOnBackground() {
        startRecording();
    }
});
Squalor answered 29/11, 2014 at 23:39 Comment(0)
B
6

When you are in the worker thread, you can not directly manipulate UI elements on Android.

When you are using AsyncTask please understand the callback methods.

For example:

public class MyAyncTask extends AsyncTask<Void, Void, Void>{

    @Override
    protected void onPreExecute() {
        // Here you can show progress bar or something on the similar lines.
        // Since you are in a UI thread here.
        super.onPreExecute();
    }

    @Override
    protected void onPostExecute(Void aVoid) {
        super.onPostExecute(aVoid);
        // After completing execution of given task, control will return here.
        // Hence if you want to populate UI elements with fetched data, do it here.
    }

    @Override
    protected void onProgressUpdate(Void... values) {
        super.onProgressUpdate(values);
        // You can track you progress update here
    }

    @Override
    protected Void doInBackground(Void... params) {
        // Here you are in the worker thread and you are not allowed to access UI thread from here.
        // Here you can perform network operations or any heavy operations you want.
        return null;
    }
}

FYI: To access the UI thread from a worker thread, you either use runOnUiThread() method or post method on your view.

For instance:

runOnUiThread(new Runnable() {
    textView.setText("something.");
});

or
    yourview.post(new Runnable() {
    yourview.setText("something");
});

This will help you know the things better. Hence in you case, you need to set your textview in the onPostExecute() method.

Brawner answered 22/12, 2015 at 18:25 Comment(0)
S
3

Sample Async Task with POST request:

List<NameValuePair> params = new ArrayList<NameValuePair>();
params.add(new BasicNameValuePair("key1", "value1"));
params.add(new BasicNameValuePair("key1", "value2"));
new WEBSERVICEREQUESTOR(URL, params).execute();

class WEBSERVICEREQUESTOR extends AsyncTask<String, Integer, String>
{
    String URL;
    List<NameValuePair> parameters;

    private ProgressDialog pDialog;

    public WEBSERVICEREQUESTOR(String url, List<NameValuePair> params)
    {
        this.URL = url;
        this.parameters = params;
    }

    @Override
    protected void onPreExecute()
    {
        pDialog = new ProgressDialog(LoginActivity.this);
        pDialog.setMessage("Processing Request...");
        pDialog.setIndeterminate(false);
        pDialog.setCancelable(false);
        pDialog.show();
        super.onPreExecute();
    }

    @Override
    protected String doInBackground(String... params)
    {
        try
        {
            DefaultHttpClient httpClient = new DefaultHttpClient();
            HttpEntity httpEntity = null;
            HttpResponse httpResponse = null;

            HttpPost httpPost = new HttpPost(URL);

            if (parameters != null)
            {
                httpPost.setEntity(new UrlEncodedFormEntity(parameters));
            }
            httpResponse = httpClient.execute(httpPost);

            httpEntity = httpResponse.getEntity();
            return EntityUtils.toString(httpEntity);

        }  catch (Exception e)
        {

        }
        return "";
    }

    @Override
    protected void onPostExecute(String result)
    {
        pDialog.dismiss();

        try
        {

        }
        catch (Exception e)
        {

        }
        super.onPostExecute(result);
    }
}
Stace answered 6/2, 2015 at 6:39 Comment(0)
F
3

Update: March 2020

According to Android developer official documentation, AsyncTask is now deprecated.

It's recommended to use kotlin corourines instead. Simply, it allows you to write asynchronous tasks in a sequential style.

Farceuse answered 27/3, 2020 at 22:40 Comment(1)
MF!!!!!!!! FGoogle!!!!!!!!!!!!! FAndroid!!!!!!!!!! All My App Use AsynTask :(Cahra
D
1

Simply:

LongOperation MyTask = new LongOperation();
MyTask.execute();
Doy answered 14/12, 2012 at 12:19 Comment(0)
K
1

You need to declare the button onclicklistener. Once clicked, it calls AsyncTask class DownloadJson.

The process will be shown below:

@Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        btn = (Button) findViewById(R.id.button1);

        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                new DownloadJson().execute();
            }
        });
    }

    // DownloadJSON AsyncTask
    private class DownloadJson extends AsyncTask<Void, Void, Void> {
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
        }

        @Override
        protected Void doInBackground(Void... params) {
            newlist = new ArrayList<HashMap<String, String>>();
            json = jsonParser.makeHttpRequest(json, "POST");
            try {
                newarray = new JSONArray(json);
                    for (int i = 0; i < countdisplay; i++) {
                        HashMap<String, String> eachnew = new HashMap<String, String>();
                        newobject = newarray.getJSONObject(i);
                        eachnew.put("id", newobject.getString("ID"));
                        eachnew.put("name", newobject.getString("Name"));
                        newlist.add(eachnew);
                    }
                }
            } catch (JSONException e) {
                Log.e("Error", e.getMessage());
                e.printStackTrace();
            }
            return null;
        }

        @Override
        protected void onPostExecute(Void args) {
            newlisttemp.addAll(newlist);
            NewAdapterpager newadapterpager = new NewAdapterpager(ProcesssActivitypager.this, newlisttemp);
            newpager.setAdapter(newadapterpager);
        }
    }
Kinaesthesia answered 25/6, 2014 at 4:35 Comment(0)
P
1
private class AsyncTaskDemo extends AsyncTask<Void, Void, Void> {

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        // Showing progress dialog
        progressDialog = new ProgressDialog(this);
        progressDialog.setMessage("Loading...");
        progressDialog.setCancelable(false);
        progressDialog.show();
    }

    @Override
    protected Void doInBackground(Void... arg0) {

        // Do code here

        return null;
    }

    @Override
    protected void onPostExecute(Void result) {
        super.onPostExecute(result);
        // Dismiss the progress dialog
        if (progressDialog.isShowing()) {
            progressDialog.dismiss();
        }
    }

    @Override
    protected void onCancelled() {

        super.onCancelled();
        progressDialog.dismiss();
        Toast toast = Toast.makeText(
                          getActivity(),
                          "An error is occurred due to some problem",
                          Toast.LENGTH_LONG);
        toast.setGravity(Gravity.TOP, 25, 400);
        toast.show();
    }

}
Prune answered 26/8, 2014 at 4:12 Comment(0)
P
1

AsyncTask:

public class MainActivity extends AppCompatActivity {

 private String ApiUrl="your_api";

   @Override
   protected void onCreate(Bundle savedInstanceState) {
     super.onCreate(savedInstanceState);
     setContentView(R.layout.activity_main);
     MyTask myTask=new MyTask();
     try {
         String result=myTask.execute(ApiUrl).get();
         Toast.makeText(getApplicationContext(),result,Toast.LENGTH_SHORT).show();
     } catch (ExecutionException e) {
         e.printStackTrace();
     } catch (InterruptedException e) {
        e.printStackTrace();
     }

  }


   public class MyTask extends AsyncTask<String,Void,String>{


    @Override
    protected String doInBackground(String... strings) {
        String result="";
        HttpURLConnection httpURLConnection=null;
        URL url;

        try {
            url=new URL(strings[0]);
            httpURLConnection=(HttpURLConnection) url.openConnection();
            InputStream inputStream=httpURLConnection.getInputStream();
            InputStreamReader reader=new InputStreamReader(inputStream);
            result=getData(reader);
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return result;
    }

    public String getData(InputStreamReader reader) throws IOException{
        String result="";
        int data=reader.read();
        while (data!=-1){
            char now=(char) data;
            result+=data;
            data=reader.read();
        }
        return result;
    }
  }
}
Pyrophotometer answered 12/2, 2019 at 10:42 Comment(1)
Why is class MyTask inside of class MainActivity? Is that customary?Sellma
F
1

While working with AsyncTask, it is necessary to create a class-successor and in it to register the implementation of methods necessary for us. In this lesson we will look at three methods:

doInBackground - will be executed in a new thread, and here we solve all our difficult tasks. Because a non-primary thread does not have access to the UI.

onPreExecute - executed before doInBackground and has access to the UI

onPostExecute - executed after doInBackground (does not work if AsyncTask was canceled - about this in the next lessons) and has access to the UI.

This is the MyAsyncTask class:

class MyAsyncTask extends AsyncTask<Void, Void, Void> {

  @Override
  protected void onPreExecute() {
    super.onPreExecute();
    tvInfo.setText("Start");
  }

  @Override
  protected Void doInBackground(Void... params) {
    // Your background method
    return null;
  }

  @Override
  protected void onPostExecute(Void result) {
    super.onPostExecute(result);
    tvInfo.setText("Finish");
  }
}

And this is how to call in your Activity or Fragment:

MyAsyncTask myAsyncTask = new MyAsyncTask();
myAsyncTask.execute();
Frizzly answered 28/6, 2019 at 10:9 Comment(1)
Re "about this in the next lessons": What does that refer to? E.g., where is this taken from?Sellma
K
0

Sample AsyncTask example with progress

import android.animation.ObjectAnimator;
import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.animation.AccelerateDecelerateInterpolator;
import android.view.animation.DecelerateInterpolator;
import android.view.animation.LinearInterpolator;
import android.widget.Button;
import android.widget.ProgressBar;
import android.widget.TextView;

public class AsyncTaskActivity extends AppCompatActivity implements View.OnClickListener {

    Button btn;
    ProgressBar progressBar;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        btn = (Button) findViewById(R.id.button1);
        btn.setOnClickListener(this);
        progressBar = (ProgressBar)findViewById(R.id.pbar);
    }

    public void onClick(View view) {
        switch (view.getId()) {
            case R.id.button1:
                new LongOperation().execute("");
                break;
        }
    }

    private class LongOperation extends AsyncTask<String, Integer, String> {

        @Override
        protected String doInBackground(String... params) {
            Log.d("AsyncTask", "doInBackground");
            for (int i = 0; i < 5; i++) {
                try {
                    Log.d("AsyncTask", "task "+(i + 1));
                    publishProgress(i + 1);
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    Thread.interrupted();
                }
            }
            return "Completed";
        }

        @Override
        protected void onPostExecute(String result) {
            Log.d("AsyncTask", "onPostExecute");
            TextView txt = (TextView) findViewById(R.id.output);
            txt.setText(result);
            progressBar.setProgress(0);
        }

        @Override
        protected void onPreExecute() {
            Log.d("AsyncTask", "onPreExecute");
            TextView txt = (TextView) findViewById(R.id.output);
            txt.setText("onPreExecute");
            progressBar.setMax(500);
            progressBar.setProgress(0);
        }

        @Override
        protected void onProgressUpdate(Integer... values) {
            Log.d("AsyncTask", "onProgressUpdate "+values[0]);
            TextView txt = (TextView) findViewById(R.id.output);
            txt.setText("onProgressUpdate "+values[0]);

            ObjectAnimator animation = ObjectAnimator.ofInt(progressBar, "progress", 100 * values[0]);
            animation.setDuration(1000);
            animation.setInterpolator(new LinearInterpolator());
            animation.start();
        }
    }
}
Koa answered 14/11, 2018 at 9:38 Comment(0)
N
0

if you open AsyncTask class you can see below code.

public abstract class AsyncTask<Params, Progress, Result> {
    @WorkerThread
    protected abstract Result doInBackground(Params... params);
    @MainThread
    protected void onPreExecute() {
    }
    @SuppressWarnings({"UnusedDeclaration"})
    @MainThread
    protected void onPostExecute(Result result) {
    }
}

AsyncTask features

  1. AsyncTask is abstract class
  2. AsyncTask is have 3 generic params.
  3. AsyncTask has abstract method of doInBackground, onPreExecute, onPostExecute
  4. doInBackground is WorkerThread (you can't update UI)
  5. onPreExecute is MainThread
  6. onPostExecute is MainThread (you can update UI)

example

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_login);
    mEmailView = (AutoCompleteTextView) findViewById(R.id.email);

    AsyncTask<Void, Void, Post> asyncTask = new AsyncTask<Void, Void, Post>() {
        @Override
        protected Post doInBackground(Void... params) {
            try {
                ApiClient defaultClient = Configuration.getDefaultApiClient();
                String authorization = "eyJhbGciOiJIUzI1NiJ9.eyJ1c2VyX2lkIjoxLCJleHAiOjE1ODIxMzM4MTB9.bA3Byc_SuB6jzqUGAY4Pyt4oBNg0VfDRctZ8-PcPlYg"; // String | JWT token for Authorization
                ApiKeyAuth Bearer = (ApiKeyAuth) defaultClient.getAuthentication("Bearer");
                Bearer.setApiKey(authorization);
                PostApi apiInstance = new PostApi();
                String id = "1"; // String | id
                Integer commentPage = 1; // Integer | Page number for Comment
                Integer commentPer = 10; // Integer | Per page number For Comment
                Post result;
                try {
                    result = apiInstance.apiV1PostsIdGet(id, authorization, commentPage, commentPer);
                } catch (ApiException e) {
                    e.printStackTrace();
                    result = new Post();
                }
                return result;
            } catch (Exception e) {
                e.printStackTrace();
                return new Post();
            }
        }

        @Override
        protected void onPostExecute(Post post) {
            super.onPostExecute(post);
            if (post != null) {
                mEmailView.setText(post.getBody());
                System.out.print(post);
            }
        }
    };
    asyncTask.execute();
}
Nickynico answered 18/2, 2020 at 20:56 Comment(0)
M
-1

Change your code as given below:

@Override
protected void onPostExecute(String result) {

    runOnUiThread(new Runnable() {
        public void run() {
            TextView txt = (TextView) findViewById(R.id.output);
            txt.setText("Executed");
        }
    });
}
Mannie answered 15/5, 2016 at 7:55 Comment(1)
You shouldn't need to use runOnUiThread as onPostExecute always runs on thread 1 (does it not?)Scant

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