Develop Reference

android autocomplete AsyncTask delay

I'm implementing a location suggestion activity which populates suggestions from an external server as the user types in a text view. I'm using an AsyncTask to fetch suggestions each time the text in the text view changes. When a new letter is typed, we cancel the task that already exists and execute a new one. Most of the time doInBackground starts immediately after execute is called, but other times it can take a few seconds. (Once doInBackground starts, performance is fine.)
Set listener:
private void init() {
// respond to any text change
textView.addTextChangedListener(new TextWatcher() {
#Override
public void onTextChanged(final CharSequence s, int start, int b, int c) {
showSuggestions(s.toString());
}
});
}
Here we start a new task and cancel the previous one:
private void showSuggestions(String query) {
// suggestionsTask is an instance variable of the activity
if (suggestionsTask != null) {
suggestionsTask.cancel(true);
}
suggestionsTask = new AsyncTask<String, Void, List<Suggestion>>() {
#Override
protected void onPostExecute(List<Suggestion> resultList) {
// set suggestions with adapter - CHANGES STATE
}
#Override
protected List<Suggestion> doInBackground(String... query) {
// one local db call for recent searches - DOES NOT CHANGE STATE
// one network call to external server - DOES NOT CHANGE STATE
// return results
}
};
suggestionsTask.execute(query);
}
Is there a better threading mechanism to use for this? Do you know why there is a delay between execute and doInBackground?

From the AsyncTask reference:
A task can be cancelled at any time by invoking cancel(boolean). Invoking this method will cause subsequent calls to isCancelled() to return true. After invoking this method, onCancelled(Object), instead of onPostExecute(Object) will be invoked after doInBackground(Object[]) returns. To ensure that a task is cancelled as quickly as possible, you should always check the return value of isCancelled() periodically from doInBackground(Object[]), if possible (inside a loop for instance.)
So you're actually not canceling the doInBackground() step unless you're manually checking whether isCancelled() is set periodically in doInBackground. In most versions of Android, all AsyncTasks share a single thread, so one has to finish before the next can start. So that's the reason for your delay, but I don't have enough information (i.e., you didn't post the code) from your doInBackground() code to come up with a suggestion on where to check for isCancelled().
If being able to cancel the previous task not possible for some reason, you can also try making your AsyncTasks execute in parallel, using executeOnExecutor(java.util.concurrent.Executor, Object[]) with THREAD_POOL_EXECUTOR as the same documentation suggests, but with what you're trying to do that seems like it could cause some frustrating threading issues which would probably be worse than the one you're having right now.

Getting results from an AsyncTask

I'm trying to use AsyncTask to download a string and return the string. I want to use AsyncTask because it might take a while.
One problem is that nowhere on the internet can I find an example of an AsyncTask returning any kind of value. So I took the example in the Commonsware book and modified it to return a value and I get the value as follows:
String mystr = new AddStringTask().execute().get();
While this works, it seem that this line of code is waiting for the return value and therefore synchronous. There must be some way to have an event trigger with the results of the AddStringTask.
How is that done?
Thanks, Gary

An AsyncTask cannot return a value, because to get the returned value you would have to wait before the task is finished. That would make the AsyncTask meaningless.
Instead, you should move your code in onPostExecute() (which runs on the UI thread, if this is what you worry about). This is where you handle the value returned by doInBackground() and typically update the UI or show an error message.

Also if you wanted to implement a more general AsyncTask you could implement something like the following to compartmentalize your code inside the activity.
#Override
protected void onPostExecute(Bitmap r){
if (r != null) {
processListeners(r);
}
}
protected void processListeners(Object data) {
for (final AsyncTaskDone l : listeners) l.finished(data);
}
public void addAsyncTaskListener (final AsyncTaskDone l){
listeners.add(l);
}
Where AsyncTaskListener is an interface with one function called finished implemented in the Activity the same way an onClickListener would be.

Android wait AsyncTask to finish

I have a function, AppHelper.isOnline(Context context), I call in various parts of my application to check that a session didn't timeout before making an HTTP request.
public void onClick(View v) {
Intent intent = null;
switch (v.getId()) {
case R.id.buttonPagamenti:
if (AppHelper.isOnline(this))
{
//here AppHelper.isOnline should have finished it's async task
intent = new Intent(this, OrdineCreaActivity.class);
this.startActivityForResult(intent, R.id.buttonPagamenti);
}
break;
...
Inside AppHelper.isOnline(), I am executing an AsyncTask that logs in, thus making a network request, which can't be run on UI because otherwise I get an exception. I need to wait for it to finish BEFORE resuming with the code inside the if. How can I do this ?
Problem is the activity starts firsts, then the AsyncTask executes, so when the activity expects a valid logged in session, it breaks.

You have two options:
Either use the AsyncTask's method get(long timeout, TimeUnit unit) like that:
task.get(1000, TimeUnit.MILLISECONDS);
This will make your main thread wait for the result of the AsyncTask at most 1000 milliseconds (as per #user1028741 comment: actually there is also infinetly waiting method - AsyncTask#get() which might also do the work for you in some cases).
Alternatively you can show a progress dialog in the async task until it finishes. See this thread (No need for me to copy past the code). Basically a progress dialog is shown while the async task runs and is hidden when it finishes.
You have even third option:" if Thread is sufficient for your needs you can just use its join method. However, if the task is taking a long while you will still need to show a progress dialog, otherwise you will get an exception because of the main thread being inactive for too long.

try using
if (AppHelper.isOnline(this))
{
while(!task.isCancelled()){
// waiting until finished protected String[] doInBackground(Void... params)
}
intent = new Intent(this, OrdineCreaActivity.class);
this.startActivityForResult(intent, R.id.buttonPagamenti);
}
For more information read http://developer.android.com/reference/android/os/AsyncTask.html

Rafiq's response did not work for me - the app hung. I think the reason has to do with the nature of isCancelled(): "Returns true if this task was cancelled before it completed normally." If the task completes normally (i.e. is not cancelled) then while(!task.isCancelled()) { } will loop forever.
To solve this create a Boolean flag that you instatiate to false and then flip to true in task.onPostExecute(). Then do while(!flag) { } before switching Activities. Additionally, if you'd like to give the main thread a 'break' to let the AsyncTask process a little faster, you can do try this:
while (!flag) {
try { Thread.sleep(100); }
catch (InterruptedException e) { e.printStackTrace(); }
}
It seems to be working well for me.

intent = new Intent(this, OrdineCreaActivity.class);
context.startActivityForResult(intent, R.id.buttonPagamenti);
Write the above lines in onPostExecute() of you AysncTask. Because if we are using AsyncTask it wont wait there until the task complete.

Check if all AsyncTasks have finished

I have 3 AsyncTasks and 1 ProgressBar. I want when any of task executes, the progress bar is visible and when all of them finish, the progress bar is invisible.
In Java, there is ExecutorService::isTerminated to check if all runnables finished but Android doesn't have it.
Update: 3 tasks execute at the same time.
Figure.

Nice graphic. But I am afraid there is no build in mechanism for this. You'll have to implement it by yourself. There are few solutions you could use -
Keep a reference to all 3 task. When task finishes check if the other two tasks are finished too, if yes than close the progress dialog if no wait for some other task to finish and check again. Make sure you free the references when you're done.
If you don't want to keep a reference store a counter. When the task finishes, increment the counter and check if it's equal to 3. If all tasks finished and you are done. If you implement this make sure to synchronized the access to the counter.

Try using AsyncTask.getStatus(). This works perfectly fine. Refer below sample code.
List<AsyncTask<String, String, String>> asyncTasks = new ArrayList<AsyncTask<String, String, String>>();
AsyncTask<String, String, String> asyncTask1 = new uploadTask().execute(string);
AsyncTask<String, String, String> asyncTask2 = new downloadTask().execute(string);
AsyncTask<String, String, String> asyncTask3 = new createTask().execute(string);
asyncTasks.add(asyncTask1);
asyncTasks.add(asyncTask2);
asyncTasks.add(asyncTask3);
You can later loop the AsyncTaskList and find each of the tasks' status as below.
for(int i=0;i<asyncTasks.size();i++){
AsyncTask<String, String, String> asyncTaskItem = (AsyncTask<String, String, String>)asyncTasks.get(i);
// getStatus() would return PENDING,RUNNING,FINISHED statuses
String status = asyncTaskItem.getStatus().toString();
//if status is FINISHED for all the 3 async tasks, hide the progressbar
}

A simple workaround would be to use three boolean variables one each for each AsyncTask and then check them accordingly.
A better approach would be to create a separate class that extends AsynTask and defines a callback interface which is fired in onPostExecute.

create a field to hold all tasks:
private ArrayList<HtmlDownloaderTask> mTasks;
Start your tasks this way:
HtmlDownloaderTask = new HtmlDownloaderTask(page.getHtml());
task.execute(page.getUrl());
//if you want parallel execution try this:
//task.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,page.getUrl());
mTasks.add(task);
on the onPostExecute of MyAsyncTask:
int unfinishedTasks = 0;
for (HtmlDownloaderTask myDT : mTasks){
if(!(myDT.getStatus() == AsyncTask.Status.FINISHED)){
unfinishedTasks++;
}
}
if (unfinishedTasks == 1){
//We are all done. 1 Because its the current one that hasnt finished post execute
callWhateverMethod();
}

Well as you do know when an AsyncTask ends (when onPostExecute gets called):
one solution could be to create a method setProgressBarVisible() that keeps a counter and when first called sets visible, and a method setProgressBarInvisible() that decreases the counter and when zero sets the progress bar invisible.

:-? i think it's just a trick. you will return some message at onPostExecute of each Asyntask and compare it. (this message can contain a time, for example)

A official support of CompletableFuture was introduced since API level 24.
It's also available in Java 8 here.
Can use simply use something like:
taskA.thenCombine(taskB).thenCombine(taskC)

I would simply notify it at onPostExecute(), refer to onPostExecute and 4 steps in the document for detail and you can use EventBus to do some subscribe things.

This is a common question when you want to run a bunch of AsynTasks on a THREAD_POOL_EXECUTOR. It's much more faster than if you just call .execute() and all your tasks are done one by one.
So if you have multiple jobs and objects are not depending on each other states - try to run on a thread pool.
But the question is: how do I know that all of my tasks are done?
There is no built in methods in AsyncTask so you should do a little workaround.
In my case I added a static Hashmap field to my Asynctask class to keep track of all started and finished tasks. As a bonus of a map I can always know which task is currently in progress.
private static HashMap<Uri, Boolean> mapOfAttachmentTasks = new HashMap<>();
and ad simple three methods to access this map.
Important: they should be synchronized
public static synchronized void addTask(Uri uri){
mapOfAttachmentTasks.put(uri, true);
}
public static synchronized void removeTask(Uri uri){
mapOfAttachmentTasks.remove(uri);
}
public static synchronized boolean isTasksEmpty(){
return mapOfAttachmentTasks.isEmpty();
}
You want to add a new item to the tracking Map in an AsyncTask constructor and remove it in onPostExecute():
public AttachmentTask(Uri uri) {
this.uri = uri;
addTask(uri);
}
#Override
protected void onPostExecute(Attachment attachment) {
removeTask(uri);
if(isTasksEmpty())
EventBus.getDefault().post(new AttachmentsTaskFinishedEvent(attachment));
}
Everytime a task is finished it calls onPostEexecute and you check if it was the last task. If there is no tasks left - send a signal that you're done.
Now, here I used EventBus to send event to my Fragment but you can use a callback. In this case you should create an interface with callbackMethod, your Fragment (any of your UI components which are waiting for the event) should implement this interface and have that method. Then in AsyncTask constructor you get your Fragment as an argument and keep a reference to it, so you can call it's callback method when everything is done.
But I dont like such approach. First you need to keep the reference of your Fragment (or any other UI) in a WeakReference wrapper becasue you will get a memory leak when your fragment is dead (but still kept in memory becasue your AsyncTask has it's reference).
Also you would need to make a lot of checks and it will look something like that:
private boolean isAlive() {
return mFragmentWeakReference != null
&& mFragmentWeakReference.get() != null
&& mFragmentWeakReference.get().isAdded()
&& mFragmentWeakReference.get().getActivity() != null
&& !mFragmentWeakReference.get().getActivity().isFinishing();
yep, in production you should be a little paranoic and do all these checks :)
That's why you can use EventBus and if your UI is dead - whatever.

try this, maybe can help you...
final ImageUploader _upload = new ImageUploader();
_upload.setValue(getApplicationContext(), _imagepath, _urlPHP);
_upload.execute();
Runnable _run;
Handler _h2;
_run = new Runnable() {
public void run() {
_h2 = new Handler();
_h2.postDelayed(this, 1000);
Toast.makeText(getApplicationContext(), "not finished", Toast.LENGTH_LONG).show();
if (_upload.getStatus() == AsyncTask.Status.FINISHED) {
Toast.makeText(getApplicationContext(), "finished", Toast.LENGTH_LONG).show();
_h2.removeCallbacks(_run);
}
}
};
_h2 = new Handler();
_h2.postDelayed(_run, 1);

Android AsyncTask - avoid multiple instances running

I have AsyncTask that processes some background HTTP stuff. AsyncTask runs on schedule (Alarms/service) and sometime user executes it manually.
I process records from SQLite and I noticed double-posts on server which tells me that sometime scheduled task runs and at the same time user runs it manually causing same record to be read and processed from DB twice. I remove records after they processed but still get this.
How should I handle it ? Maybe organize some kind of queing?

You can execute your AsyncTask's on an Executor using executeOnExecutor()
To make sure that the threads are running in a serial fashion please use: SERIAL_EXECUTOR.
Misc: How to use an Executor
If several activities are accessing your DB why don't create a sort of gateway database helper and use the synchronized block to ensure only one thread has access to it at an instant

Or, you can try this to see if the Task is currently running or not:
if (katitsAsyncTask.getStatus().equals(AsyncTask.Status.FINISHED))
katitsAsyncTask.execute();
else
// wait until it's done.

Initialize the AsyncTask to null. Only create a new one if it is null. In onPostExecute, set it to null again, at the end. Do the same in onCancelled, in case the user cancels this. Here's some untested code to illustrate the basic idea.
import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
public class FooActivity extends Activity {
private class MyAsyncTask extends AsyncTask<Foo, Foo, Foo> {
#Override
protected void onPostExecute(Foo foo) {
// do stuff
mMyAsyncTask = null;
}
#Override
protected void onCancelled() {
// TODO Auto-generated method stub
mMyAsyncTask = null;
}
#Override
protected Foo doInBackground(Foo... params) {
try {
// dangerous stuff
} catch (Exception e) {
// handle. Now we know we'll hit onPostExecute()
}
return null;
}
}
private MyAsyncTask mMyAsyncTask = null;
#Override
public void onCreate(Bundle bundle) {
Button button = (Button) findViewById(R.id.b2);
button.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
if (mMyAsyncTask == null) {
mMyAsyncTask = new MyAsyncTask();
mMyAsyncTask.execute(null);
}
}
});
}
}

I know this was a while ago now, and you have solved your problem. but I just had a similar problem. Reno's suggestion put me on the right track, but for those who have been finding it difficult to fill in the gaps. Here is how I overcame a similar issue to that of katit's.
I wanted a particular AsyncTask to only run if it was not currently running. And as a forward from Reno's suggestion, the AsyncTask interface has been created to handle all the nitty gritty processes in properly dealing with threads for Android. Which means, the Executor is built in. As this blog suggests:
"When execute(Object.. params) is invoked on an AsyncTask the task is executed in a background thread. Depending on the platform AsyncTasks may be executed serially (pre 1.6 and potentially again in 4+), or concurrently (1.6-3.2).
To be sure of running serially or concurrently as you require, from API Level 11 onwards you can use the executeOnExecutor(Executor executor, Object.. params) method instead, and supply an executor. The platform provides two executors for convenience, accessable as AsyncTask.SERIAL_EXECUTOR and AsyncTask.THREAD_POOL_EXECUTOR respectively. "
So with this in mind, you can do thread blocking via the AsyncTask interface, it also implies you can simply use the AsyncTasks.getStatus() to handle thread blocking, as DeeV suggests on this post.
In my code, I managed this by:
Creating a global variable defined as:
private static AsyncTask<String, Integer, String> mTask = null;
And in onCreate, initialising it as an instance of my AsyncTask called CalculateSpecAndDraw:
mTask = new CalculateAndDrawSpec();
Now when ever I wish to call this AsyncTask I surround the execute with the following:
if(mTask.getStatus() == AsyncTask.Status.FINISHED){
// My AsyncTask is done and onPostExecute was called
mTask = new CalculateAndDrawSpec().execute(Integer.toString(progress));
}else if(mTask.getStatus() == AsyncTask.Status.PENDING){
mTask.execute(Integer.toString(progress));
}else{
Toast.makeText(PlaySpecActivity.this, "Please Wait..", 1000).show();
}
This spawns a new thread if it is finished, or if the thread state is PENDING, the thread is defined but has not been started we start it. But otherwise if the thread is running we don't re-run it, we simply inform the user that it is not finished, or perform what ever action we wish. Then if you wanted to schedule the next event rather than just block it from re running, take a look at this documentation on using executors.

How about just wrapping your check-what-to-send and send-it logic in a synchronized method? This approach seems to work for us.

try having some instance boolean value that gets set to "true" on the asynctask's preexecute then "false" on postexecute. Then maybe in doinbackground check if that boolean is true. if so, then call cancel on that particular duplicate task.

You could keep the state of the task in shared preferences. Check the value (Boolean perhaps) before starting the task. Set the state to finished(true?) in onPostExecute and false in onPreExecute or in the constructor