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Are onPreExecute () and onPostExecute () executed in the user interface thread or in the thread from which AsyncTask is launched?

I wrote AsyncTask for short background operations in android for quite a while and had a very simple question. If I started AsyncTask from a separate thread, and not from the main UI thread, my onPreExecute() and onPostExecute will still be called in the UI thread or the thread that I started AsyncTask . I'm curious because I was not able to show the popup inside the onPreExecute() method when I started it from some other thread.

EDIT 2

 I tried writing this simple activity to try: public class MainActivity extends AppCompatActivity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); new Thread(new Runnable() { @Override public void run() { final TestAsyncTask task = new TestAsyncTask(); task.execute(); } }).start(); } private class TestAsyncTask extends AsyncTask<Void, Void, Void> { @Override protected void onPreExecute() { super.onPreExecute(); } @Override protected Void doInBackground(Void... voids) { return null; } @Override protected void onPostExecute(Void aVoid) { super.onPostExecute(aVoid); Toast.makeText(MainActivity.this, "Yo!", Toast.LENGTH_LONG).show(); } } }

This is normal. But when I run the application with the following code:

 public class MainActivity extends AppCompatActivity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); new Thread(new Runnable() { @Override public void run() { final TestAsyncTask task = new TestAsyncTask(); task.execute(); } }).start(); } private class TestAsyncTask extends AsyncTask<Void, Void, Void> { @Override protected void onPreExecute() { super.onPreExecute(); Toast.makeText(MainActivity.this, "Yo!", Toast.LENGTH_LONG).show(); } @Override protected Void doInBackground(Void... voids) { return null; } } }

The following error failed:

 Caused by: java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()

as one of the lines in the stack trace.

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3 answers

While the docs say that these callbacks are executed in the main thread - this is not true. onPreExecute() is executed synchronously with executeOnExecutor() i.e. in a thread starting with AsyncTask .

onPostExecute() always starts in the main thread. (it is called from finish() , and this happens inside the Handler , which uses the looper of the main thread).

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You should do UI updates, and also show or pop up in AsyncTask onPostExecute (), which runs in the UI thread. The AsyncTask doinBackground () method works in another thread.

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Assignment of official documents:

 onPostExecute Added in API level 3 void onPostExecute (Result result) Runs on the UI thread after doInBackground(Params...). The specified result is the value returned by doInBackground(Params...). onPreExecute Added in API level 3 void onPreExecute () Runs on the UI thread before doInBackground(Params...).

you can find it here https://developer.android.com/reference/android/os/AsyncTask.html#onPostExecute(Result)

Make a background thread in the background while OnPreExecute and OnPostExecute are running on the main Ui branch.

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