Develop Reference

How does AsyncTask work one process to another one?

I'm currently studying android on my own and pretty new to java. I'm wondering how AsyncTask works like this: onPreExecute() -> doInBackground() -> onPostExecute(). When I look at others define their AsynTask, it seems like only method is declared in their code with no calls upon the method. I can't figure out how doInBackground() comes after onPreExecute() with no code that links both like:
onPreExecute(){ ~~~~~ call doInBackground()}
My point is that when AsyncTask.execute() is called, onPreExecute() is called, then doInBackground(), finally onPostExecute(). I couldn't find any code in library that actually connects these together. All I could find is this:
#MainThread
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
#MainThread
public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
Params... params) {
if (mStatus != Status.PENDING) {
switch (mStatus) {
case RUNNING:
throw new IllegalStateException("Cannot execute task:"
+ " the task is already running.");
case FINISHED:
throw new IllegalStateException("Cannot execute task:"
+ " the task has already been executed "
+ "(a task can be executed only once)");
}
}
mStatus = Status.RUNNING;
onPreExecute();
mWorker.mParams = params;
exec.execute(mFuture);
return this;
}
Here when AsyncTask.execute() is called, onPreExecute() is called. But without any connection to doInBackground the task works just fine. I feel like I'm missing some fundamental logic or process of java or android. Plz, help me with this unsolved question in mind. Sample code is shown below. Thank you in advance.
#Override
protected void onPreExecute() {
super.onPreExecute();
mLoadingIndicator.setVisibility(View.VISIBLE);
}
#Override
protected String[] doInBackground(String... params) {
/* If there's no zip code, there's nothing to look up. */
if (params.length == 0) {
return null;
}
String location = params[0];
URL weatherRequestUrl = NetworkUtils.buildUrl(location);
try {
String jsonWeatherResponse = NetworkUtils
.getResponseFromHttpUrl(weatherRequestUrl);
String[] simpleJsonWeatherData = OpenWeatherJsonUtils
.getSimpleWeatherStringsFromJson(MainActivity.this, jsonWeatherResponse);
return simpleJsonWeatherData;
} catch (Exception e) {
e.printStackTrace();
return null;
}
}
#Override
protected void onPostExecute(String[] weatherData) {
// COMPLETED (19) As soon as the data is finished loading, hide the loading indicator
mLoadingIndicator.setVisibility(View.INVISIBLE);
if (weatherData != null) {
// COMPLETED (11) If the weather data was not null, make sure the data view is visible
showWeatherDataView();
/*
* Iterate through the array and append the Strings to the TextView. The reason why we add
* the "\n\n\n" after the String is to give visual separation between each String in the
* TextView. Later, we'll learn about a better way to display lists of data.
*/
for (String weatherString : weatherData) {
mWeatherTextView.append((weatherString) + "\n\n\n");
}
} else {
// COMPLETED (10) If the weather data was null, show the error message
showErrorMessage();
}

I guess you shouldn't waste time on AsyncTask since it is deprecated.
Instead you should focus on coroutines, recommended by google here , or some other state of the art framework to achive what you want (e.g. rx java)

Yes, you are correct. The logic is onPreExecute() -> doInBackground() -> onPostExecute()
Synchronous VS asynchronous
You can read this article for a better understanding even though it's using Javascript to explain it.

How to get logcat -v time in an Android App?

I need to get logcat -v time from a device and save it to a text file in a SD Card. The problem is that my application freezes when I press the button to do it.
I understand that it might happen because logcat -v time keeps getting the log of all actions of the device, nonstop. And I need all this information. But I don't know how to code it correctly. Would anyone could help me, please?
Thanks in advance!
Here is my code:
btn_comeca.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
try {
Process process = Runtime.getRuntime().exec("logcat -v");
BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(process.getInputStream()));
StringBuilder log=new StringBuilder();
String line = "";
while ((line = bufferedReader.readLine()) != null) {
log.append(line);
}
try {
if (ContextCompat.checkSelfPermission(MainActivity.this, Manifest.permission.WRITE_EXTERNAL_STORAGE)!= PackageManager.PERMISSION_GRANTED) {
//Verifica permissão de gravar/ler
if (ActivityCompat.shouldShowRequestPermissionRationale(MainActivity.this, Manifest.permission.WRITE_EXTERNAL_STORAGE)) {
} else {
ActivityCompat.requestPermissions(MainActivity.this, new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, MODE_ENABLE_WRITE_AHEAD_LOGGING);
}
}
File caminho_arquivo = new File("/sdcard/ARQUIVOFINAL.txt");
caminho_arquivo.createNewFile();
FileOutputStream fileOutputStream = new FileOutputStream(caminho_arquivo);
OutputStreamWriter outputStreamWriter = new OutputStreamWriter(fileOutputStream);
TextView tv = (TextView)findViewById(R.id.texttext);
String texto = "";
//tv.setText(log.toString());
texto = log.toString();
//outputStreamWriter.append(tv.getText());
outputStreamWriter.append(texto);
outputStreamWriter.close();
fileOutputStream.close();
Toast.makeText(getBaseContext(), "Text File Saved!", Toast.LENGTH_SHORT).show();
} catch (Exception e) {
Toast.makeText(getBaseContext(), e.getMessage(), Toast.LENGTH_SHORT).show();
}
}
catch (IOException e) {}
}

The problem may be: your logic works synchronously. You should never execute work like that in main thread, since it can lead to ANR (Android Not Responding).
There is plenty of ways to fix that, though. Since you are not using frameworks for multithreading (e.g. RxJava, you can read about it, if you are interested), you can use AsyncTask. You can place this in your listener.
EDIT: However, take a look at MVVM or MVP pattern and multithreading frameworks later. These things are widely used in production for executing different kinds of asynchronous tasks.
new AsyncTask<Void, Void, Void>(){
#Override
protected Void doInBackground(Void... params) {
// Place your logic here
return;
}
}.execute();

Normal invocation of logcat won't every exit by itself.
Therefore your code will be forever stuck in your while loop.
As per https://developer.android.com/studio/command-line/logcat
Use the -d "Dump the log to the screen and exits." commandline option.
This is what I use in a similar method to display the logcat output in to a TextView

Android: Why won't this code run in onCreate, or how can I make it work without a thread?

I have this piece of code that runs alright when I put it in Eclipse, but for some reason it does not want to execute when I put it in an activity's onCreate method in Android studio.
Here is the code:
public class ItemListView extends AppCompatActivity{
String itemURL;
int listSize;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_item_list_view);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
Bundle itemData = getIntent().getExtras();
if(itemData==null){
return;
}
//Gets URL from search bar
itemURL = itemData.getString("itemURL");
try {
Document doc = Jsoup.connect("https://www.amazon.com/s/ref=nb_sb_ss_c_1_6?url=search-alias%3Daps&field-keywords=rx+390&sprefix=rx+390%2Caps%2C166&crid=2MTUBA4KGNY06").get();
String link = doc.select("h2#s-result-count").first().text();
System.out.println(link);
System.out.println(link.substring(1));
if (link.substring(1, 2).equals("-")) {
System.out.println("run1");
listSize = Integer.parseInt(link.substring(2, 3));
System.out.println(listSize);
try {
listSize = Integer.parseInt(link.substring(2, 4));
System.out.println(listSize);
} catch (Exception e) {
}
} else {
System.out.println("run2");
listSize = Integer.parseInt(link.substring(0, 1));
System.out.println(listSize);
try {
listSize = Integer.parseInt(link.substring(0, 2));
System.out.println(listSize);
} catch (Exception e) {
}
}
} catch (Exception e) {
}
System.out.println("listSize: " +listSize);
...
}
}
I need listSize to create a variable array depending on the URL, but when I print the value to make sure it's working it always gives me 0. I have tried running the code in a separate Java Class with AsyncTask and it works, but by the time the code executes in onPostExecute, it's too late since the class above has already tried to initialize the array.
Any advice would be appreciated, thanks!

What you need is a callback to allow you to initialize variable onPostExecute:
interface OnCallCompleteCallBack {
void onCallComplete(int listSize);
}
In your AsyncTask do this:
public class MyTask extends AsyncTask < ... > {
// Maintain a ref to callback
OnCallCompleteCallBack callback;
MyTask(OnCallCompleteCallBack callback) {
this.callback = callback;
}
...
#Override
protected void onPostExecute(Void aVoid) {
if (callback != null) {
callback.onCallComplete(listSize);
}
}
}
Make your Activity implement OnCallCompleteCallBack and start the AsyncTask like this:
new MyTask(this).execute();
You can then use the value inside your activity's implementation of onCallComplete()

Before I answer, just a few observations:
Naming your activity ItemListView seems wrong (an Activity is not a View).
You should never perform networks calls on the UI thread.
If you want to log, you should use Log instead of System.out.println().
And now to the answer. You should execute the code that fetches the data in an AsyncTask (as you mentions it works). Once the data is fetched, you should update array and at that point update the UI in onPostExecute().
Android works pretty well using the MVC (Model-View–Controller) pattern and your problem is a classic case where you need to update the model using data from the server and update the views accordingly. The activity represents controller in this case.

Please go through the topic in android developer site Android Developer, Read the section "Introducing Network Operations on a Separate Thread" - To avoid creating an unresponsive UI, don't perform network operations on the UI thread. By default, Android 3.0 (API level 11) and higher requires you to perform network operations on a thread other than the main UI thread; if you don't, a NetworkOnMainThreadException is thrown.
Thanks Ashish

inside oncreate method can't read the content of php file?

I created an android app. I try to get content from a PHP file. I wrote the logic in below getCont() method, but it doesn't work. It always returns null in Android. When I do the same with Java it returns the PHP content. How to solve this, so no exception is thrown.
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
String dbcont = getCont();
tone = (TextView)findViewById(R.id.textView1);
tone.setText("usean :-)"+dbcont);
}
public String getCont() {
String mit = null;
try {
URL oracle = new URL("http://localhost/grace/conn.php");
BufferedReader in = new BufferedReader(
new InputStreamReader(oracle.openStream()));
StringBuilder sb = new StringBuilder();
String inputLine;
while ((inputLine = in.readLine()) != null) {
sb.append(inputLine);
}
mit = sb.toString();
System.out.println(mit);
}
catch(Exception e){
e.printStackTrace();
}
return mit;
}

You're saying you're not getting an exception? You're opening a URL connection on the UI Thread. This is restricted on android. I'm surprised the app isn't crashing.
http://developer.android.com/reference/android/os/NetworkOnMainThreadException.html
You have to use a separate thread to perform network functions. Read this
http://developer.android.com/reference/android/os/AsyncTask.html
You need to create a class that extends ASyncTask, then in the doInBackground() method, you can open the connection to fetch your PHP file.
You can then return the results which will be passed on to the onPostExecute() method. That will allow you to pass the data from the connection (i.e. the PHP file) back to your UI thread.
Let me know if this helps.

Is it possible to run the "AsyncTask" method inside another method "AsyncTask"?

I have an activity that when started makes a call to a "json" for get data categories of songs, after that I make a call to the method "AsyncTask" for the list of songs that category from another "JSON "the problem is that when I start the activity, this is locked , after 2 seconds, the activity opens the layout and I can see the categories on the action bar and not because the songs are looking for in the background.
main activity (onCreate):
java.io.InputStream source = null;
source = retrieveStream(UrlApi.URL_BASE + UrlApi.URL_STORE + _bundle.getString("_id") + UrlApi.CATEGORY_SONG);
Log.i("URL - KARAOKE", UrlApi.URL_BASE + UrlApi.URL_STORE + _bundle.getString("_id") + UrlApi.CATEGORY_SONG);
Reader reader = new InputStreamReader(source);
Type happyCollection = new TypeToken<Collection<String>>() {}.getType();
_karaoke_category_response = new Gson().fromJson(reader, happyCollection);
if(_karaoke_category_response.size() < 1){
finish();
Toast.makeText(getApplicationContext(), "Local sin karaokes", Toast.LENGTH_SHORT).show();
}else{
Log.i("Category - response", _karaoke_category_response.toString());
_karaoke_category_adapter = new ArrayAdapter<String>(getSupportActionBar().getThemedContext(), R.layout.spinner_item,_karaoke_category_response);
getSupportActionBar().setListNavigationCallbacks(_karaoke_category_adapter, this);
}
The follow code is of search the songs of that categori and set it
class AsyncKaraoke extends AsyncTask<Void, Void, Void> {
String category;
public AsyncKaraoke(String category) {
this.category = category;
}
protected void onPreExecute(){
super.onPreExecute();
setSupportProgressBarIndeterminateVisibility(true);
}
#Override
protected Void doInBackground(Void... params) {
java.io.InputStream source = null;
try {
source = retrieveStream(UrlApi.URL_BASE + UrlApi.URL_STORE + _bundle.getString("_id") + UrlApi.KARAOKE_URL + UrlApi.FILTER_CATEGORY + URLEncoder.encode(category, "UTF-8"));
Log.i("URL - KARAOKE", UrlApi.URL_BASE + UrlApi.URL_STORE + _bundle.getString("_id") + UrlApi.KARAOKE_URL + UrlApi.FILTER_CATEGORY + URLEncoder.encode(category, "UTF-8"));
} catch (UnsupportedEncodingException e) {
e.printStackTrace();
}
Reader reader = new InputStreamReader(source);
Type karaokeCollection = new TypeToken<Collection<KaraokeModel>>() {}.getType();
_response = new Gson().fromJson(reader, karaokeCollection);
Log.i("Response - KaraokeCategory" , _karaoke_category_response.toString());
return null;
}
protected void onPostExecute(Void Void){
super.onPostExecute(Void);
setSupportProgressBarIndeterminateVisibility(false);
_karaoke_adapter = new KaraokeAdapter(KaraokeActivity.this, _bundle.getString("_id"), _response);
if(_response.size() == 0){
Toast.makeText(getApplicationContext(), "Categoria sin karaoke", Toast.LENGTH_SHORT).show();
}
_list_view.setAdapter(_karaoke_adapter);
_karaoke_adapter.notifyDataSetChanged();
}
}
How should I do to call 2 times to "AsyncTask" method and prevent the activity is engaged by a few seconds?

The primary rule of AsyncTask is that it must always be create and run on the main thread. You will get an exception if you start another AsyncTask inside the doInBackground() method. Your options are to start the next AsyncTask in one of the callbacks. Generally, some people will chain AsyncTask in the onPostExecute() method, but you can also start them in onPreExecute() and onProgressUpdate().
EDIT:
Additionally, you can run AsyncTask in sequence of each other using AsyncTask#executeOnExecutor(). From HoneyComb on, you don't need to do this. All AsyncTask run in a serial thread pool in the order they are executed. Though it may be easier to understand that the code is running serially if you use it. You do need to chain if using Android Android 1.6 - 2.3.x though.

You should build the URL in the main activity, then run an AsyncTask to download the content and finally process the result back in your activity.
The syntax to run an AsyncTask is:
String category = "...";
new AsyncKaraoke().execute(category);
You can also remove the onPostExecute() method from your AsyncKaraoke class and put it in the activity:
String category = "...";
new AsyncKaraoke() {
#Override
protected void onPostExecute(Void Void){
// do stuff (and moving the third type of the AsyncKaraoke to something else
// than Void will allow you to get the result here.
}.execute(category);

Generally, we use AsyncTask to perform an action in another thread than the UI thread to prevent the user from being halt while performing some actions. SO, it does not make any sense to create an additional AsyncTask inside the outer one. Try to manage your code to do it all the those method soInBackground(), onPreExecution() and onPostExecution() and make use of their order of execution

int count = 0;
protected void onPostExecute(Void Void){
super.onPostExecute(Void);
// call same asynctask
if (count == 0)
{
execute asynctask
count++;
}
}