Develop Reference

Callbacks in Android

I am new to Android and programming as a whole and I need a little help with callbacks. I understand the gist of callbacks but I am unsure of how to go about implementing.
Context: I am writing a simple notetaking app that allows the user to write text and saving it to the app. The user can then request to read the file with a button. The text is then displayed on a textview in the main activity. There is an option to wipe this file and this is done with a confirmation pop up, which is another activity. This pop up contains 2 buttons, one to cancel and one to wipe. If the file is not empty it will wipe and does nothing if empty. I am not sure if this is the best way to implement it but I want to use the wipe button to callback to the main activity to clear the textview. The way I was thinking of was by using the callback to send a boolean value back. The main activity will check if the boolean is true and clear the textview if it is. I am unsure of how to implement the callback in my popup display to send this boolean value back to the main activity.
Code for main activity
public class MainActivity extends AppCompatActivity implements Popout.ClearTextView {
Button bnRead,bnWrite,bnClear;
TextView tvFileOP;
EditText etInput;
// private static final String INPUT_CONTENT = "inputContent";
public static final String TV_CONTENT = "textViewContent";
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
bnRead = (Button) findViewById(R.id.bnRead);
bnWrite = (Button) findViewById(R.id.bnWrite);
bnClear = (Button) findViewById(R.id.bnClear);
tvFileOP = (TextView) findViewById(R.id.tvFileOP);
etInput = (EditText) findViewById(R.id.etInput);
tvFileOP.setMovementMethod(new ScrollingMovementMethod());
final String fileName = "test_file";
String data;
bnRead.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
try {
FileInputStream fIn = openFileInput(fileName);
int c;
String temp = "";
while ( (c=fIn.read()) != -1){
temp = temp + Character.toString((char) c);
}
tvFileOP.setText(temp);
Toast.makeText(getBaseContext(),"file successfully read", Toast.LENGTH_LONG).show();
} catch (Exception e) {
e.printStackTrace();
}
}
});
bnWrite.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
String data = etInput.getText().toString();
try {
FileOutputStream fOut = openFileOutput(fileName,MODE_APPEND);
fOut.write(data.getBytes());
fOut.close();
etInput.setText("");
Toast.makeText(getBaseContext(),"file successfully written", Toast.LENGTH_LONG).show();
} catch (Exception e) {
e.printStackTrace();
}
}
});
bnClear.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
startActivity(new Intent(MainActivity.this,Popout.class));
}
});
}
#Override
protected void onSaveInstanceState(#NonNull Bundle outState) {
outState.putString(TV_CONTENT,tvFileOP.getText().toString());
super.onSaveInstanceState(outState);
}
#Override
protected void onRestoreInstanceState(#NonNull Bundle savedInstanceState) {
super.onRestoreInstanceState(savedInstanceState);
tvFileOP.setText(savedInstanceState.getString(TV_CONTENT));
}
#Override
public void clearTextView(Boolean clear) {
if (clear){
tvFileOP.setText("");
}
}
}
Code for popup confirmation menu
public class Popout extends AppCompatActivity {
Button bnClosepopup,bnWipe;
TextView tvConfirmation;
String fileName = "test_file";
TextView tvFileOP;
public interface ClearTextView {
public void clearTextView(Boolean clear);
}
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.popupwindow);
bnClosepopup = (Button) findViewById(R.id.bnClosepopup);
bnWipe = (Button) findViewById(R.id.bnWipe);
tvConfirmation = (TextView) findViewById(R.id.tvConfirmation);
//HIDING THE TOOL BAR AT THE TOP OF THE SCREEN
this.getSupportActionBar().hide();
//GETTING THE SIZE OF THE SCREEN
DisplayMetrics displayMetrics = new DisplayMetrics();
getWindowManager().getDefaultDisplay().getMetrics(displayMetrics);
int height = displayMetrics.heightPixels;
int width = displayMetrics.widthPixels;
getWindow().setLayout((int) (width*0.8) , (int) (0.8*height));
bnClosepopup.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
finish();
}
});
bnWipe.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
try {
File dir = getFilesDir();
File file = new File(dir, fileName);
boolean deleted = file.delete();
Toast.makeText(getBaseContext(),"file has been deleted",Toast.LENGTH_SHORT).show();
} catch (Exception e) {
Toast.makeText(getBaseContext(), e.getMessage(), Toast.LENGTH_LONG).show();
}
finish();
}
});
}
}
I am very new to android development and any tips on how to improve my code would be greatly appreciated :)

In this case there is no way to pass the interface to the other activity, because this is an activity to activity communication.
You have to use some other method, there is multiple ways to approach, the best way I can think of is to use startActivityForResult() to start the activity and then wait for a response to come back, and then query this response in the MainActivity by overriding the onActivityResult() method:
Example
In the MainActivity:
//on click of this button
bnClear.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
Intent intent = new Intent(MainActivity.this,Popout.class);
int requestCode = 12; //it could be whatever you want
startActivityForResult(intent , requestCode);
}
});
//override this method
#Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
super.onActivityResult(requestCode, resultCode, data);
//this is triggered when you finish the Popout Activity
if(requestCode == 12 && resultCode == Activity.RESULT_OK){
// get the boolean data returned from the Popout Activity
boolean deleted = data.getBooleanExtra("deleted_state" , false); //false is default if no value exists
}
}
In the Popout activity:
bnWipe.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
try {
File dir = getFilesDir();
File file = new File(dir, fileName);
boolean deleted = file.delete();
//send the result to onActivtyResult() in MainActivity
Intent result = new Intent();
result.putExtra("deleted_state", deleted );
setResult(Activity.RESULT_OK, result);
Toast.makeText(getBaseContext(),"file has been deleted",Toast.LENGTH_SHORT).show();
} catch (Exception e) {
Toast.makeText(getBaseContext(), e.getMessage(), Toast.LENGTH_LONG).show();
}
finish();
}
});
UPDATE:
It will be like this:
// get the boolean data returned from the Popout Activity
boolean deleted = data.getBooleanExtra("deleted_state" , false);
if (deleted){
tvFileOP.setText("");
}
..........

As far as what if Understood your problem correctly: You want to control your 'Wipe' button click event from your activity. Here is the solution which may help you.
1: Make an overridden constructor of your dialog class.
2: Create one abstract method in the dialog class. (say - onWipeButtonClick)
You need to make your dialog class abstract as well.
3: Inside on Click Listener of 'Wipe' button, call onWipeButtonClick abstract method.
4: Create the instance of dialog in the main activity where ever you want. The compiler will give you an error because you haven't implemented the call back method.
do implement your onWipeButtonClick method and do needful for wipe data inside the method.
public abstract class WipeDialog extends Dialog{
private Context context;
public WipeDialog(Context context){
this.context = context;
}
public abstract void onWipeButtonClick(boolean isTextEmpty);
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.<XML_FILE>);
<initialization>
btnWipe.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
onWipeButtonClick(<YOUR_BOOLEAN_CHECK>);
}
});
}
}
And now in Activity:
WipeDialog dialog = new WipeDialog(MainActivity.this) {
#Override
public void onWipeButtonClick(boolean isTextEmpty) {
//Do Need full with respected to your requirement on click of button 'WIPE'
}
};
Hope this will help.
Thanks!

putExtra doesn't seem to be working at all. Using ListView to get a string and using listView to display a string

I am programming a messaging app and I want to add users in a group. However, when a list of users pops up and I select one from the list, it doesn't pass the string (the username) to the other activity. All I get is an empty list.
Here is my code:
First Activity = Sending data (usernames from list) through putExtra()
public class ListUsersActivity extends Activity {
private String currentUserId;
private ArrayAdapter<String> namesArrayAdapter;
private ArrayList<String> names;
private ListView usersListView;
private Button logoutButton;
private ProgressDialog progressDialog;
private BroadcastReceiver receiver = null;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_list_users);
Parse.initialize(this, "embpZ0spRUv5XwDgI23innll1sgHg0KZNiKzg6kl", "LPsU4UffPeqFXkQB1GfLCIJ4kvg20llPgbOnLise");
currentUserId = ParseUser.getCurrentUser().getObjectId();
names = new ArrayList<>();
ParseQuery<ParseUser> query = ParseUser.getQuery();
query.whereNotEqualTo("objectId", currentUserId);
query.findInBackground(new FindCallback<ParseUser>() {
public void done(List<ParseUser> userList, com.parse.ParseException e) {
if (e == null) {
for (int i=0; i<userList.size(); i++) {
names.add(userList.get(i).getUsername().toString());
}
usersListView = (ListView)findViewById(R.id.usersListView);
namesArrayAdapter =
new ArrayAdapter<String>(getApplicationContext(),
R.layout.user_list_item, names);
usersListView.setAdapter(namesArrayAdapter);
usersListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
#Override
public void onItemClick(AdapterView<?> a, View v, int i, long l) {
Intent goBackToAddPoolIntent = new Intent(ListUsersActivity.this, addNewPoolActivity.class);
addNewPoolActivity checker = new addNewPoolActivity();
checker.checkIfUserIsSelected(usersListView.getItemAtPosition(i).toString());
goBackToAddPoolIntent.putExtra("username", usersListView.getItemAtPosition(i).toString());
startActivity(goBackToAddPoolIntent);
}
});
} else {
Toast.makeText(getApplicationContext(),
"Error loading user list",
Toast.LENGTH_LONG).show();
}
}
});
}
Second Activity = Receiving data from putExtra()
public class addNewPoolActivity extends Activity {
private static ArrayList<String> addedUsers;
private ArrayAdapter <String> addedUserAdapter;
private boolean userIsSelected;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_add_new_pool);
Button addMembers = (Button) findViewById(R.id.bAddMembers);
addedUsers = new ArrayList<>();
//addedUsers.add("Group Members");
addMembers.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
Intent showUsersToSelect = new Intent(addNewPoolActivity.this, ListUsersActivity.class);
startActivity(showUsersToSelect);
}
});
ListView addedUsersList = (ListView) findViewById(R.id.addedUsersListView);
addedUserAdapter = new ArrayAdapter<>(this, R.layout.user_list_item, addedUsers);
addedUsersList.setAdapter(addedUserAdapter);
if(userIsSelected){
Bundle extras = getIntent().getExtras();
addedUsers.add(extras.getString("username"));
}
}
public void checkIfUserIsSelected(String user){
if (user!=null){
userIsSelected = true;
}else{
userIsSelected = false;
}
}

Since the default value for a boolean is false, the code is never called because
if(userIsSelected){
will always evaluate to false since you have declared the varaible as
private boolean userIsSelected;
and the first snippet here is in onCreate() so it will only run the first time the Activity is created.
Maybe you are wanting to call checkIfUserIsSelected(someUser) before that code but without more context of what you hope to accomplish, it's hard to say.
Possibly, you want to use startActivityForResult() in some way?

In addition to #codeMagic 's answer (Since your boolean value is false, it won't call the statement that you are adding the new data). It's also because of you parse the Data "username" after you setAdapter of your ListView. So basically you are setting the data, and then trying to add the new data you parsed to the list. Either you need to do it before setting your data set to your adapter, or call addedUsersAdapter.notifyDataSetChanged() to refresh your listView's data set.
addedUserAdapter = new ArrayAdapter<>(this, R.layout.user_list_item, addedUsers);
addedUsersList.setAdapter(addedUserAdapter);
Bundle extras = getIntent().getExtras();
// Check if the username has been sent to this Activity.
if(extras != null && extras.containsKey("username")){
addedUsers.add(extras.getString("username"));
// Refresh Your Data Set
addedUserAdapter.notifyDataSetChanged();
}

How to get updated value of EditText in Android

I'm new to android and working on a basic screen to use a web-service with android application.
I am posting values using AsyncTask and fetching the result from the webservice. It works fine until displaying the returned value. While displaying the Toast Message on click, I get old value of TextView resultReturned
public class TestPost extends Activity{
private TextView result = null;
#Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.my_screen);
result = (TextView)findViewById(R.id.resultReturned);
Button submit = (Button)findViewById(R.id.btnSubmit);
submit.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
String[] strPost = new String[]{"value1", "value2"};
SendAsyncRequest asyncSend = new SendAsyncRequest();
asyncSend.execute(strPost);
// ResultView retains old value and gets correct value on second click
String returned = result.getText().toString();
Toast.makeText(getApplicationContext(), returned, Toast.LENGTH_LONG).show();
}
});
}
public class SendAsyncRequest extends AsyncTask<String, Void, String>{
private String fetchedData = "";
#Override
protected String doInBackground(String... params ) {
// perform async task
return fetchedData;
}
#Override
protected void onPostExecute(String result) {
setReturedValue(result);
}
}
private void setReturedValue(String data){
result.setText(data);
}
So, how do I get the updated text value of the TextView?

AsyncTask takes time to get response from request, Show toast message in postExecute() method, like this, and remove from onclick.
#Override
protected void onPostExecute(String result) {
Toast.makeText(getApplicationContext(), result, Toast.LENGTH_LONG).show();
}

Try this
submit.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
String[] strPost = new String[]{"value1", "value2"};
SendAsyncRequest asyncSend = new SendAsyncRequest();
asyncSend.execute(strPost);
// ResultView retains old value and gets correct value on second click
String jsonResult;
try {
jsonResult=asyncSend.get();
} catch (InterruptedException | ExecutionException e) {
e.printStackTrace();
}
Toast.makeText(getApplicationContext(), jsonResult, Toast.LENGTH_LONG).show();
}
});
And return your Json String in doInBackground().

Android save data from nested AsyncTask onPostExecute after screen rotation

I have spent many hours looking for a solution to this and need help.
I have a nested AsyncTask in my Android app Activity and I would like to allow the user to rotate his phone during it's processing without starting a new AsyncTask. I tried to use onRetainNonConfigurationInstance() and getLastNonConfigurationInstance().
I am able to retain the task; however after rotation it does not save the result from onPostExecute() to the outer class variable. Of course, I tried getters and setters. When I dump the variable in onPostExecute, that it is OK. But when I try to access to the variable from onClick listener then it is null.
Maybe the code will make the problem clear for you.
public class MainActivity extends BaseActivity {
private String possibleResults = null;
private Object task = null;
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
this.task = getLastNonConfigurationInstance();
setContentView(R.layout.menu);
if ((savedInstanceState != null)
&& (savedInstanceState.containsKey("possibleResults"))) {
this.possibleResults = savedInstanceState
.getString("possibleResults");
}
if (this.possibleResults == null) {
if (this.task != null) {
if (this.task instanceof PossibleResultWebService) {
((PossibleResultWebService) this.task).attach();
}
} else {
this.task = new PossibleResultWebService();
((PossibleResultWebService) this.task).execute(this.matchToken);
}
}
Button button;
button = (Button) findViewById(R.id.menu_resultButton);
button.setOnClickListener(resultListener);
}
#Override
protected void onResume() {
super.onResume();
}
OnClickListener resultListener = new OnClickListener() {
#Override
public void onClick(View v) {
Spinner s = (Spinner) findViewById(R.id.menu_heatSpinner);
int heatNo = s.getSelectedItemPosition() + 1;
Intent myIntent = new Intent(MainActivity.this,
ResultActivity.class);
myIntent.putExtra("matchToken", MainActivity.this.matchToken);
myIntent.putExtra("heatNo", String.valueOf(heatNo));
myIntent.putExtra("possibleResults",
MainActivity.this.possibleResults);
MainActivity.this.startActivityForResult(myIntent, ADD_RESULT);
}
};
private class PossibleResultWebService extends AsyncTask<String, Integer, Integer> {
private ProgressDialog pd;
private InputStream is;
private boolean finished = false;
private String possibleResults = null;
public boolean isFinished() {
return finished;
}
public String getPossibleResults() {
return possibleResults;
}
#Override
protected Integer doInBackground(String... params) {
// quite long code
}
public void attach() {
if (this.finished == false) {
pd = ProgressDialog.show(MainActivity.this, "Please wait...",
"Loading data...", true, false);
}
}
public void detach() {
pd.dismiss();
}
#Override
protected void onPreExecute() {
pd = ProgressDialog.show(MainActivity.this, "Please wait...",
"Loading data...", true, false);
}
#Override
protected void onPostExecute(Integer result) {
possibleResults = convertStreamToString(is);
MainActivity.this.possibleResults = possibleResults;
pd.dismiss();
this.finished = true;
}
}
#Override
protected void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
if (this.possibleResults != null) {
outState.putString("possibleResults", this.possibleResults);
}
}
#Override
public Object onRetainNonConfigurationInstance() {
if (this.task instanceof PossibleResultWebService) {
((PossibleResultWebService) this.task).detach();
}
return (this.task);
}
}

It is because you are creating the OnClickListener each time you instantiate the Activity (so each time you are getting a fresh, new, OuterClass.this reference), however you are saving the AsyncTask between Activity instantiations and keeping a reference to the first instantiated Activity in it by referencing OuterClass.this.
For an example of how to do this right, please see https://github.com/commonsguy/cw-android/tree/master/Rotation/RotationAsync/
You will see he has an attach() and detach() method in his RotationAwareTask to solve this problem.
To confirm that the OuterClass.this reference inside the AsyncTask will always point to the first instantiated Activity if you keep it between screen orientation changes (using onRetainNonConfigurationInstance) then you can use a static counter that gets incremented each time by the default constructor and keep an instance level variable that gets set to the count on each creation, then print that.

AsyncTask : passing value to an Activity (onCreate method )

Update1
activity:
public Integer _number = 0;
#Override
public void onCreate(Bundle savedInstanceState) {
if (_number >0)
{
Log.d("onSuccessfulExecute", ""+_number);
}
else
{
Log.d("onSuccessfulExecute", "nope empty songs lists");
}
}
public int onSuccessfulExecute(int numberOfSongList) {
_number = numberOfSongList;
if (numberOfSongList >0)
{
Log.d("onSuccessfulExecute", ""+numberOfSongList);
}
else
{
Log.d("onSuccessfulExecute", "nope empty songs lists");
}
return numberOfSongList;
}
end Update1
UPDATE: AsynchTask has its own external class.
How to pass an value from AsyncTask onPostExecute()... to activity
my code does returning value from onPostExecute() and updating on UI but i am looking for a way to set the activity variable (NumberOfSongList) coming from AsynchTask.
AsyncTask class:
#Override
public void onPostExecute(asynctask.Payload payload)
{
AsyncTemplateActivity app = (AsyncTemplateActivity) payload.data[0];
//the below code DOES UPDATE the UI textView control
int answer = ((Integer) payload.result).intValue();
app.taskStatus.setText("Success: answer = "+answer);
//PROBLEM:
//i am trying to populate the value to an variable but does not seems like the way i am doing:
app.NumberOfSongList = payload.answer;
..............
..............
}
Activity:
public Integer NumberOfSongList;
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
//Several UI Code
new ConnectingTask().execute();
Log.d("onCreate", ""+NumberOfSongList);
}

What about using a setter method? e.g.
private int _number;
public int setNumber(int number) {
_number = number;
}
UPDATE:
Please look at this code. This will do what you're trying to accomplish.
Activity class
public class TestActivity extends Activity {
public int Number;
#Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.test);
Button btnDisplay = (Button) findViewById(R.id.btnDisplay);
btnDisplay.setOnClickListener(new OnClickListener() {
public void onClick(View v) {
Toast toast = Toast.makeText(v.getContext(), "Generated number: " + String.valueOf(Number), Toast.LENGTH_LONG);
toast.show();
}
});
new TestTask(this).execute();
}
}
AsyncTask class
public class TestTask extends AsyncTask<Void, Void, Integer> {
private final Context _context;
private final String TAG = "TestTask";
private final Random _rnd;
public TestTask(Context context){
_context = context;
_rnd = new Random();
}
#Override
protected void onPreExecute() {
//TODO: Do task init.
super.onPreExecute();
}
#Override
protected Integer doInBackground(Void... params) {
//Simulate a long-running procedure.
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
Log.e(TAG, e.getMessage());
}
return _rnd.nextInt();
}
#Override
protected void onPostExecute(Integer result) {
TestActivity test = (TestActivity) _context;
test.Number = result;
super.onPostExecute(result);
}
}

Just a word of caution: Be very careful when attempting to hold a reference to an Activity instance in an AsyncTask - I found this out the hard way :). If the user happens to rotate the device while your background task is still running, your activity will be destroyed and recreated thus invalidating the reference being to the Activity.

Create a listener.
Make a new class file. Called it something like MyAsyncListener and make it look like this:
public interface MyAsyncListener() {
onSuccessfulExecute(int numberOfSongList);
}
Make your activity implement MyAsyncListener, ie,
public class myActivity extends Activity implements MyAsyncListener {
Add the listener to the constructor for your AsyncTask and set it to a global var in the Async class. Then call the listener's method in onPostExecute and pass the data.
public class MyCustomAsync extends AsyncTask<Void,Void,Void> {
MyAsyncListener mal;
public MyCustomAsync(MyAsyncListener listener) {
this.mal = listener;
}
#Override
public void onPostExecute(asynctask.Payload payload) {
\\update UI
mal.onSuccessfulExecute(int numberOfSongList);
}
}
Now, whenever your AsyncTask is done, it will call the method onSuccessfulExecute in your Activity class which should look like:
#Override
public void onSuccessfulExecute(int numberOfSongList) {
\\do whatever
}
Good luck.