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How to show AlertDialog in AsyncTask without leaking Context?

I have the below code working fine to find and read a record in a Room database via an id. Android Studio required adding a try/catch block which I've included below.
Two questions:
Can I leave the if{} section blank in onPostExecute() if there is no Exception?
How do I show an AlertDialog here without leaking contect and without using a hack via WeakReference?
// AsyncTask for reading an existing CardView from the database.
private static class ReadAsyncTask extends AsyncTask<Integer, Void, Quickcard> {
private QuickcardDao asyncTaskDao;
Exception e;
ReadAsyncTask(QuickcardDao dao) {
asyncTaskDao = dao;
}
#Override
public Quickcard doInBackground(final Integer... params) {
Quickcard result;
try {
result = asyncTaskDao.readCardForUpdate(params[0]);
} catch (Exception e) {
this.e = e;
result = null;
}
return result;
}
#Override
protected void onPostExecute(Quickcard quickcard) {
if (e == null) {
// *** Okay to leave this blank? If not, what should go here?
}
else {
// *** How do I show an AlertDialog here with leaking context?
}
}
}

You cannot use a view object while using a thread that operates in background.. U must implement the dialog in the UI thread. When you are implementing the the asynchronous class, in that method you should show the alert dialog. Hope this helps..

This is what I will do to prevent leaks from happening.
private static class showDialog extends AsyncTask<String, String, String> {
private WeakReference<MainActivity> mainActivityWeakReference;
showDialog(MainActivity mainActivity){
this.mainActivityWeakReference = new WeakReference<>(mainActivity);
}
#Override
protected String doInBackground(String... params) {
//do your long long time consuming tasks here.
return "Done";
}
#Override
protected void onPostExecute(String result) {
// execution of result of Long time consuming operation
//Just building an alert dialog to show.
if (mainActivityWeakReference.get() != null){
final MainActivity activity = mainActivityWeakReference.get();
new AlertDialog.Builder(activity).setPositiveButton(android.R.string.yes, new DialogInterface.OnClickListener() {
#Override
public void onClick(DialogInterface dialogInterface, int i) {
Toast.makeText(activity, "Yes was clicked", Toast.LENGTH_SHORT).show();
}
}).show();
}
}
#Override
protected void onPreExecute() {
}
#Override
protected void onProgressUpdate(String... text) {
}
}

How do I refactor my code to use AsyncTask?

I made an application for Android that originally targeted a lower version (2.3). After I got my proof-of-concept working, I tried to get it to work on Android 4. That's when I got the NetworkOnMainThread exception.
After doing some research, I quickly found the AsyncTask, which sounded awesome. The problem is, I'm having a hard time wrapping my mind around it. For instance, here's my original code:
public void Refresh(Context c)
{
SummaryModel model = MobileController.FetchSummary(c);
TextView txtCurrentWeight = (TextView)findViewById(R.id.txtCurrentWeight);
TextView txtWeightChange = (TextView)findViewById(R.id.txtWeightChange);
TextView txtAvgPerWeek = (TextView)findViewById(R.id.txtAvgPerWeek);
if(model.ErrorMessage == "")
{
txtCurrentWeight.setText(model.CurrentWeight);
txtWeightChange.setText(model.WeightChange);
txtAvgPerWeek.setText(model.Average);
}
else
{
Toast.makeText(c, model.ErrorMessage, Toast.LENGTH_LONG).show();
txtCurrentWeight.setText("");
txtWeightChange.setText("");
txtAvgPerWeek.setText("");
}
}
I created an AsychTask like this:
public class WebMethodTask extends AsyncTask<Object, Integer, Object> {
#Override
protected void onPreExecute() {
super.onPreExecute();
}
#Override
protected void onProgressUpdate(Integer... values) {
super.onProgressUpdate(values);
}
#Override
protected void onPostExecute(Object result) {
super.onPostExecute(result);
SummaryModel model = (SummaryModel)result;
// Can't seem to access UI items here??
}
#Override
protected Object doInBackground(Object... params) {
Context c = (Context)params[0];
return MobileController.FetchSummary(c);
}
}
How do I access the UI items from the onPostExecute method? Or, do I have the wrong idea on how to use AsyncTask?
Thanks!

You should be able to accessUI where you put your comments (in the postExecute method)
Additionally, I would suggest to use more specialized class with for AsyncTask, so that your code looks better :
public class WebMethodTask extends AsyncTask<Object, Integer, SummaryModel> {
private Activity source;
public WebMethodTask(Activity activity) {
this.source=activity;
}
#Override
protected void onPreExecute() {
super.onPreExecute();
}
#Override
protected void onProgressUpdate(Integer... values) {
super.onProgressUpdate(values);
}
#Override
protected void onPostExecute(SummaryModel model) {
super.onPostExecute(model );
TextView txtCurrentWeight = (TextView)source.findViewById(R.id.txtCurrentWeight);
TextView txtWeightChange = (TextView)source.findViewById(R.id.txtWeightChange);
TextView txtAvgPerWeek = (TextView)source.findViewById(R.id.txtAvgPerWeek);
if(model.ErrorMessage.length()==0)
{
txtCurrentWeight.setText(model.CurrentWeight);
txtWeightChange.setText(model.WeightChange);
txtAvgPerWeek.setText(model.Average);
}
else
{
Toast.makeText(c, model.ErrorMessage, Toast.LENGTH_LONG).show();
txtCurrentWeight.setText("");
txtWeightChange.setText("");
txtAvgPerWeek.setText("");
}
}
#Override
protected SummaryModel doInBackground(Context ... params) {
Context c = params[0];
return MobileController.FetchSummary(c);
}
}
Edit : Added a reference to your activity, to take your last comment into account.
However, if you acynctask can be long, it's maybe not a very good idea to keep a reference on an activity.
It would be a better design to create a listenerclass that will accept some displayModel(CummaryModel) method, and whose responsability is to cal the setText methods if the activity has not been paused / stopped in the meanwhile...

Fill the ui items with the loaded model data in the WebMethodTask#onPostExecute method.

You need a reference to your UI controls. When passing references to your UI controls to the ASyncTask you will create problems.
Assume the following scenario:
show activity (activity instance 1)
call async task with te activity as reference.
rotate your device (by default a device rotation will create a new activity) -> (activity instance 2)
when the sync task is finished, activity instance 1 is used to display the results. However the activity no longer exists causing exceptions.
The conclusion is that the ASyncTask should not be used for network activity related background tasks.
Fortunately there is a solution: RoboSpice.
RoboSpice uses another approach. Look at https://github.com/octo-online/robospice/wiki/Understand-the-basics-of-RoboSpice-in-30-seconds for a good explanation.
More information: https://github.com/octo-online/robospice

create an inner class in refresh method as
enter code herepublic void Refresh(Context c)
{
SummaryModel model = MobileController.FetchSummary(c);
TextView txtCurrentWeight = (TextView)findViewById(R.id.txtCurrentWeight);
TextView txtWeightChange = (TextView)findViewById(R.id.txtWeightChange);
TextView txtAvgPerWeek = (TextView)findViewById(R.id.txtAvgPerWeek);
if(model.ErrorMessage == "")
{
txtCurrentWeight.setText(model.CurrentWeight);
txtWeightChange.setText(model.WeightChange);
txtAvgPerWeek.setText(model.Average);
}
else
{
Toast.makeText(c, model.ErrorMessage, Toast.LENGTH_LONG).show();
txtCurrentWeight.setText("");
txtWeightChange.setText("");
txtAvgPerWeek.setText("");
}
class WebMethodTask extends AsyncTask<Object, Integer, Object> {
#Override
protected void onPreExecute() {
super.onPreExecute();
}
#Override
protected void onProgressUpdate(Integer... values) {
super.onProgressUpdate(values);
}
#Override
protected void onPostExecute(Object result) {
super.onPostExecute(result);
SummaryModel model = (SummaryModel)result;
// Can't seem to access UI items here??
}
#Override
protected Object doInBackground(Object... params) {
Context c = (Context)params[0];
return MobileController.FetchSummary(c);
}
}
}

How to set adapter of spinner by using Async Task Class

In my code I load a spinner adapter by using Async Task
In My case The ProgressDialog is Not dismissing
This is My code.
I want to show the item after adapter load and the progressDialog is to dismiss
Please Help me, Thanks
private class LoadMoreVehicals extends AsyncTask<Object, Integer, Object> {
#Override
protected void onPreExecute() {
progressBar = ProgressDialog.show(RegistrationScreen.this, "",
"Loading...");
progressBar.setIndeterminate(true);
progressBar.setIndeterminateDrawable(getResources().getDrawable(
R.anim.progressbar_handler));
super.onPreExecute();
}
#Override
protected Object doInBackground(Object... params) {
String countryUrl = ConstantURL.COUNTRY_URL;
getCounty(countryUrl);
countrySpinner
.setAdapter(new MyCustomSpinnerAdapter(
RegistrationScreen.this,
R.layout.spinner_dropdown,
countyList));
return null;
}
#Override
protected void onProgressUpdate(Integer... values) {
progressBar.getProgress();
}
#Override
protected void onPostExecute(Object result) {
progressBar.dismiss();
Log.e("Im in onPostExecute", "");
super.onPostExecute(result);
}
}

While programming in Android you should remember one thing that any task which draws something on the screen should be executed on the main thread. When you set the adapter then android calls the getView() method of the adapter and draws views on the screen. So you should set the adapter in the postExecute() method instead in doInBackground() method.
Here is a small sample to clear my point:
class MyTask extends AsyncTask<Void, Void, Void> {
ProgressDialog pd = new ProgressDialog(MainActivity.this);
#Override
protected void onPreExecute ( )
{
//starting the progress dialogue
pd.show();
}
#Override
protected Void doInBackground (Void... params)
{
//fetch data here
...
...
return null;
}
#Override
protected void onPostExecute (Void result)
{
//set adapter here
...
...
//dismissing the progress dialogue
pd.dismiss();
}
}

In my experience i have so many problems with async runs and UI so now always separate the stuff trying to place the "responsibilities" in each place. So i do something like this:
Create my Async class with the process i want to do and nothing that transform the UI in it
Create a function in UI thread that modify the UI when async task finish, something like OnAsyncTaskComplete(Object response)
Keep communicated the threads
public class MyActivity extends Activity {
private static MyAsyncClass backgroundTask;
private static ProgressDialog pleaseWaitDialog;
//......activity stuff.......
#Override
public void onPause()
{
super.onPause();
//Get rid of progress dialog in the event of a screen rotation or other state change. Prevents a crash.
if (pleaseWaitDialog != null)
pleaseWaitDialog.dismiss();
}
//Function to avoid lose the async thread if the app interrupts (phone rotation, incoming call, etc) RECOMENDED TO HANDLE THIS!!
//Sets the current state after app resume
#Override
public void onResume()
{
super.onResume();
//If there is a background task set it to the new activity
if ((backgroundTask != null) && (backgroundTask.getStatus() == Status.RUNNING))
{
if (pleaseWaitDialog != null)
pleaseWaitDialog.show();
backgroundTask.setActivity(this);
}
}
}
//Logic business after the web service complete here
//Do the thing that modify the UI in a function like this
private void onTaskCompleted(Object _response)
{
//For example _response can be a new adapter
MyList.setAdapter((BaseAdapter)_response);
//or can be a list to create the new adapter
MyList.setAdapter(new MyAdapter(this, (ArrayList<String>)_response));
//or can be anything you want, just try to make here the things that you need to change the UI
}
/**
* Class that handle the async task
*/
public class MyAsyncClass extends AsyncTask<Void, Void, Object>
{
//Maintain attached activity for states change propose
private MyActivity activity;
//Keep the response of the async task
private Object _response;
//Flag that keep async task completed status
private boolean completed;
//Constructor
private MyAsyncClass(MyActivity activity) {
this.activity = activity;
}
//Pre execution actions
#Override
protected void onPreExecute() {
//Start the splash screen dialog
if (pleaseWaitDialog == null)
pleaseWaitDialog= ProgressDialog.show(activity.this,
"PLEASE WAIT",
"Getting results...",
false);
}
//Execution of the async task
protected Object doInBackground(Object...params)
{
//return the thing you want or do want you want
return new ArrayList();
}
//Post execution actions
#Override
protected void onPostExecute(Object response)
{
//Set task completed and notify the activity
completed = true;
_response = response;
notifyActivityTaskCompleted();
//Close the splash screen
if (pleaseWaitDialog != null)
{
pleaseWaitDialog.dismiss();
pleaseWaitDialog = null;
}
}
//Notify activity of async task completion
private void notifyActivityTaskCompleted()
{
if ( null != activity ) {
activity.onTaskCompleted(_response);
}
}
//for maintain attached the async task to the activity in phone states changes
//Sets the current activity to the async task
public void setActivity(MyActivity activity)
{
this.activity = activity;
if ( completed ) {
notifyActivityTaskCompleted();
}
}
}
}
Hope its help you

First of all you cannot set the adapter in the doInBackground
follow this design:
private class LoadMoreVehicals extends AsyncTask<Object, Integer, Object>
{
private ArrayList<Country> countries;
#Override
protected void onPreExecute() {
progressBar = ProgressDialog.show(RegistrationScreen.this, "","Loading...");
progressBar.setIndeterminate(true);
progressBar.setIndeterminateDrawable(getResources().getDrawable(R.anim.progressbar_handler));
super.onPreExecute();
}
#Override
protected Object doInBackground(Object... params) {
String countryUrl = ConstantURL.COUNTRY_URL;
countries = getCounty(countryUrl);
return null;
}
#Override
protected void onProgressUpdate(Integer... values) {
progressBar.getProgress();
}
#Override
protected void onPostExecute(Object result) {
countrySpinner.setAdapter(new MyCustomSpinnerAdapter(RegistrationScreen.this,R.layout.spinner_dropdown,countries));
progressBar.dismiss();
Log.e("Im in onPostExecute", "");
super.onPostExecute(result);
}
}

ProgressDialog not shown in AsyncTask

I have a huge database (40MB) on an SDCard. I need fetch data, with LIKE in query, which is very slow.
DB request takes about 5 seconds. Therefore, I need to do it asynchronously and with ProgressDialog.
I tried it with AsyncTask, but problem is with ProgressDialog. It was implemented this way:
private class GetDataFromLangDB extends AsyncTask<String, String, String> {
private final ProgressDialog dialog = new ProgressDialog(TranslAndActivity.this);
#Override
protected void onPreExecute() {
super.onPreExecute();
urDBCursor.close();
curDBCursor = null;
scaAdapter = null;
this.dialog.setMessage("Loading data...");
this.dialog.show();
}
#Override
protected String doInBackground(String... whatSearch) {
String result = "";
if (myDatabaseAdapter != null) {
curDBCursor = myDatabaseAdapter.fetchAll(whatSearch[0]);
}
return result;
}
#Override
protected void onPostExecute(String result) {
super.onPostExecute(result);
if (this.dialog.isShowing()) {
this.dialog.dismiss();
}
prepareListView();
}
}
The problem is that ProgressDialog is not shown during the DB request.
After finished database query, it flash on screen for a short time. When user tries
to tap on screen during database request, UI is freezed, and after DB request
message about 'not responding' is shown.
I tried it with a thread this way:
public void startProgress(View view, final String aWhatSearch) {
final ProgressDialog dialog = new ProgressDialog(MyActivity.this);
if (curDBCursor != null){
curDBCursor.close();
curDBCursor = null;
}
dialog.setMessage("Loading data...");
dialog.show();
Runnable runnable = new Runnable() {
public void run() {
curDBCursor = myDatabaseAdapter.fetchAll(aWhatSearch);
// dirty trick
try {
Thread.sleep(250); // it must be here to show progress
} catch (InterruptedException e) {
e.printStackTrace();
}
handler.post(new Runnable() {
public void run() {
if (dialog.isShowing()) {
dialog.dismiss();
}
prepareListView();
}
});
}
};
new Thread(runnable).start();
}
The result was the same, but when I used the trick with Thread.sleep(250);
ProgressDialog was shown during the database request. But it is not spinning,
it looks freezed during the DB request.
DB stuff is called this way (after tap on search button):
btnSearchAll.setOnClickListener(new OnClickListener() {
public void onClick(View v) {
// AsyncTask
new GetDataFromLangDB().execute(edtTextToSearch.getText().toString());
// or Thread
//startProgress(null, edtTextToSearch.getText().toString());
}
});
I found a lot of problems like this in SO, but nothing was useful for me.
Could it be that DB is on SD Card?

I put the definition of the dialog into the AsyncTask Class and it works fine for me.
Take a look at this exampel (You have to change NAMEOFCLASS in the name of your CLASS:
private class doInBackground extends AsyncTask<Integer, Integer, Void> {
final ProgressDialog dialog = new ProgressDialog(NAMEOFCLASS.this) {
#Override
protected void onPreExecute() {
dialog.setCancelable(false);
dialog.setTitle(getString(R.string.daten_wait_titel));
dialog.setIcon(R.drawable.icon);
dialog.setMessage(getString(R.string.dse_dialog_speichern));
dialog.show();
}
#Override
protected void onCancelled() {
dialog.cancel();
}
....
#Override
protected void onProgressUpdate(Integer... values) {
// DO YOUR UPDATE HERE
}
#Override
protected void onPostExecute(Void result) {
dialog.dismiss();
}
}

Maybe this SO answer could help you. It looks like similar problem. Try to use AsyncQueryHandler for querying your database

declare you Dialog box on Class (Activity) level like this
private ProgressDialog dialog = null;
show the progress dialog and call the AsyncTask class when you want to start you Busy work..like onButton click or any
dialog = ProgressDialog.show(this,"Sending Email to your account please! wait...", true);
SendingEmailTask task = new SendingEmailTask();
String s = "";
task.execute(s);
create your inner class like
private class SendingEmailTask extends AsyncTask<String, Void, String> {
protected String doInBackground(String... urls) {
//do your work here..
// like fetching the Data from DB or any
return null;
}
#Override
protected void onPostExecute(String str) {
//hide progress dialog here
dialog.dismiss();
}
}
let me know if this help!!

More than one level of function call in AsyncTask doInBackground() causes "Activity has leaked window" crash! Why?

I have a simple AsyncTask whose doInBackground() calls a function fn1 which in turn simply calls another fn2. The result is a "Activity has leaked window" crash! When I call fn2 directly from doInBackground() everything works fine. Does that mean in a thread one cannot have more than 1 level of fn calling?( Thread-> fn1->fn2)
I am new to Java and android, so please bear with me if that is a basic question! Thnx.
My code algo is something ike this :
MyActivity:
MyLibrary myLib;
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.myXML);
myLib = new MyLibrary(MyActivity.this);
new myTask().execute();
}
private class myTask extends AsyncTask<Void, Void, Void>
{
#Override
protected void onPreExecute() {
progressDialog = ProgressDialog.show(MyActivity.this, "", "Fetching Data...");
}
#Override
protected Void doInBackground(Void... params) {
myLib.fn1();
return null;
}
#Override
protected void onPostExecute(Void result) {
progressDialog.dismiss();
}
}
MyLibrary :
Context ctx;
String iVar;
public MyLibrary(Context context){
this.ctx = context;
}
public void fn1()
{
fn2();
Log.d("TAG", "Function 1";
}
public void fn2()
{
iVar = "100";
Log.d("TAG", "Function 2";
}

Add the following in your Activity's code:
#Override
protected void onPause() {
super.onPause();
if(progressDialog != null)
progressDialog.dismiss();
}
It will dismiss the dialog window in case you move away from activity.

Usually this error happens when the Activity that you're referencing already got removed from the top of the ActivityStack. You should really check if the AsyncTask got embedded correctly into your Activity's Lifecycle.
Best wishes,
Tim

Somehow ProgressDialog may have got crashed during nested function call.