I am trying to retrieve JPGs from an MJPG Stream using an async task. I first thought, i could let the task continuously be running and just letting the buffered JPGs pop up in the onProgressUpdate() method. But this doesn't seem to work, because the method only displays integers, no drawables...what I tried:
private class StreamTask extends AsyncTask<String, Void, String> {
#Override
protected String doInBackground(String... params) {
image = readMJPGStream();
return null;
}
#Override
protected void onPostExecute(String result) {
}
#Override
protected void onPreExecute() {
}
#Override
protected void onProgressUpdate(Void... values) {
imView.setImageDrawable(image); // << doesn't work
}
}
Do you have any ideas? What approach should I try?
Thank you very much in advance!
Huck
When you start a AsyncTask, the doInBackground is called, but it is only run once. I'm not sure what readMJPGStream does, but assuming it blocks until an image is retrieved, its certainly possible that onProgressUpdate gets called several times while your image is still null, then once image get set to something valid and doInbackground quickly exits, there are no further calls to OnProgressUpdate.
You should probably put the call to imView.setImageDrawable in onPostExecute to make sure it gets called when the image has been downloaded. You may also need to call invalidate() on your image view to ensure it gets redrawn. Additionally, if you intend to loop and continue downloading images, you'll need to devise a mechanism of triggering updates. In reality, the issue you're having is because you want 'on demand' get on the UI Thread, but you're dependent on calls to onProgressUpdate. You'd have much better luck using a Handler and your own Runnable/Thread because then when an image download completes, you can post to the UI thread (using the Handler) without having to wait for the onProgressUpdate.
I now got it running this way:
public class StreamActivity extends Activity implements Runnable{
// ...
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
imView = (ImageView) findViewById(R.id.imView);
Thread thread = new Thread(this);
thread.start();
}
#Override
public void run() {
while (connected){
image = readMJPGStream();
handler.sendEmptyMessage(0);
}
}
private Handler handler = new Handler() {
#Override
public void handleMessage(Message msg) {
imView.setImageDrawable(image);
}
};
public Drawable readMJPGStream() {
//...
return image;
}
}
The only problem is that my while loop is not fast enough. Any ideas are very much appreciated!
Huck
Related
I am writing a Android application which reads data from a SQLite Database and then displays the data on a next screen. Whenever I was doing a query on the database I would get an error message that too much work is being done on the main thread.
I then put my query in a new Thread:
(new Thread()
{
public void run()
{
Looper.prepare();
try
{
FPJobCardWizard data = dbHelperInstance.loadFPJobCardWizardFull(fitmentHash);
wState.fitmentItemSet(data.fitmentItemGet());
} catch (Exception e) {e.printStackTrace();}
Looper.loop();
}
}).start();
Now the gui/main thread is completing it's operation prior to the Query being complete and as a result the data variable is still empty. I read a few posts and the API documentation and it seems that I need to use a Looper (this seems to be the correct fix) but I have never used a Looper and cannot seem to get it to work.
Please can you check the code above and guide me in the right direction.
Thank you all in advance.
the best choice here will be using an AsyncTask, as it will enables you to perform all the background work in a background thread, then when the result is generated it will apply it using the UI thread:
So, as explained in the life cycle of AsyncTask, you can do all of your background work in the method doInBackground() and then do all of your UI work on the method onPostExecute() which will be executed after taking the result from doInBackground() method according to the life cycle, and to put your hands more on the AsyncTask, have a look at this example which provides the following example code:
public class AsyncTaskTestActivity extends Activity {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
// This starts the AsyncTask
// Doesn't need to be in onCreate()
new MyTask().execute("my string paramater");
}
// Here is the AsyncTask class:
//
// AsyncTask<Params, Progress, Result>.
// Params – the type (Object/primitive) you pass to the AsyncTask from .execute()
// Progress – the type that gets passed to onProgressUpdate()
// Result – the type returns from doInBackground()
// Any of them can be String, Integer, Void, etc.
private class MyTask extends AsyncTask<String, Integer, String> {
// Runs in UI before background thread is called
#Override
protected void onPreExecute() {
super.onPreExecute();
// Do something like display a progress bar
}
// This is run in a background thread
#Override
protected String doInBackground(String... params) {
// get the string from params, which is an array
String myString = params[0];
// Do something that takes a long time, for example:
for (int i = 0; i <= 100; i++) {
// Do things
// Call this to update your progress
publishProgress(i);
}
return "this string is passed to onPostExecute";
}
// This is called from background thread but runs in UI
#Override
protected void onProgressUpdate(Integer... values) {
super.onProgressUpdate(values);
// Do things like update the progress bar
}
// This runs in UI when background thread finishes
#Override
protected void onPostExecute(String result) {
super.onPostExecute(result);
// Do things like hide the progress bar or change a TextView
}
}
}
I'm having an issue with AsyncTask and onPostExecute. I am finding that onPostExecute is executing on a different thread than the main ui thread, which is causing a CalledFromWrongThreadException to happen when I modify any views.
I put in some logging to see what threads onPreExecute, doInBackground, and onPostExecute are running on. I would see a result like this...
onPreExecute ThreadId: 1
doInBackground ThreadId: 25
onPostExecute ThreadId: 18
I believe the main ui thread id is 1 and I would expect both onPre and onPost to both execute on thread 1. I am making sure to create and also call the execute method from the ui thread (for example in onCreate of an Activity).
Another thing to note that I have noticed is that later async tasks will run their onPostExecute method on the same thread as previous async task onPostExecute methods (in this case thread 18).
Right now in order to get around this I am wrapping the code in my onPostExecute methods in a call to runOnUiThread, but I think this is hacky and would like to get to the real issue.
I am out of ideas! Any one have any insight? I'm happy to answer any questions that could helper with further investigation!
EDIT:
There are two ways that async tasks are being run in the code. I am wondering if the latter in these examples is causing something weird to happen?
public class SomeActivity extends Activity {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main_layout);
new SomeAsyncTask().execute();
}
private class SomeAsyncTask extends AsyncTask<String, Void, Integer> {
#Override
public void onPreExecute() {
Thread.currentThread().getId() // 1
//Show a dialog
}
#Override
public Integer doInBackground(String... params) {
Thread.currentThread().getId() // 25
return 0;
}
#Override
public void onPostExecute(Integer result) {
Thread.currentThread().getId() // 18
//hide dialog
//update text view -> CalledFromWrongThreadException!!!
}
}
}
The above seems like a vanilla use of AsyncTask, but I still see this issue occurring even in simple cases like this. The next example uses an async task to run other async tasks. Maybe there is something I don't know about what happens when an async task gets constructed that is causing some weird behavior?
public class SomeActivity extends Activity implements TaskRunner.OnFinishListener {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main_layout);
TaskRunner taskRunner = new TaskRunner();
taskRunner.setOnFinishListener(this);
taskRunner.addTask(new SingleTask());
taskRunner.addTask(new SingleTask());
taskRunner.execute();
}
#Override
public void onTaskFinish(List<Integer> results) {
//Thread id is 18 when it should be 1
//do something to a view - CalledFromWrongThreadException!!
}
}
//In a different file
public class SingleTask extends AsyncTask<String, Void, Integer> {
//This is a an async task so we can run it separately as an asynctask
//Or run it on whatever thread runnerExecute is called on
#Override
public Integer doInBackground(String... params) {
return runnerExecute(params);
}
//Can be called outside of doInBackground
public Integer runnerExecute(String... params) {
//some long running task
return 0;
}
}
//In a different file
public class TaskRunner {
private List<SingleTask> tasks;
private OnFinishListener onFinishListener;
public interface OnFinishListener {
public void onTaskFinish(List<Integer> results);
}
public TaskRunner() {
this.tasks = new ArrayList<SingleTask>();
}
public void setOnFinishListener(OnFinishListener listener) {
this.onFinishListener = listener;
}
public void addTask(SingleTask task) {
tasks.add(task);
}
public void executeTasks() {
new RunnerTask().execute((SingleTask[]) tasks.toArray());
}
//Calls the runnerExecute method on each SingleTask
private class RunnerTask extends AsyncTask<SingleTask, Integer, List<Integer>> {
#Override
public void onPreExecute() {
//Runs on thread 1
}
#Override
public List<Integer> doInBackground(SingleTask... params) {
//Runs on arbitrary thread
List<Integer> results = new ArrayList<Integer>();
for(SingleTask task : params) {
int result =task.runnerExecute(task.getParams());
results.add(result);
}
return results;
}
#Override
public void onPostExecute(List<Integer> results) {
//Runs on thread 18
onFinishListener.onTaskFinish(results);
}
}
}
Maybe what is going on here is just super weird, and not at all how async tasks are meant to be used, either way it would be nice to get to the bottom of the issue.
Let me know if you need any more context.
I have been experiencing the same problem and it turned out the the issue was using Flurry 3.2.1. However, the issue is not limited to the Flurry library.
The issue behind the scenes is having the first ever (when the app is loaded for the first time) AsyncTask call from a looper thread which is not the Main UI thread. This call initializes a sHandler static variable in AsyncTask to the wrong thread id, and this id is then used in all subsequent AsyncTask$onPostExecute() calls.
To solve the problem, I call an empty (do-nothing) AsyncTask on first app load, just to initialize AsyncTask correctly.
try using:
getBaseContext().runOnUiThread(new Runnable()
{
#override
public void run()
{
}
});
and write your code inside the run function
The AsyncTask is designed to be used from the main thread. Your problem is the second case, and is that you call execute on the SingleTask from a background thread. You call it in the doInBackground method of RunnerTask. The onPostExecute is then run from the backgroundthread of RunnerTask
Two options for you.
1: Trash RunnerTask, and execute the SingleTasks from you main thread, they'll all run in parallell and you won't know which finishes first, but onPreExecute and onPostExecute is called on the main thread
2: Trash the SingleTask and define them as Runnables instead, then you can run them in sequence in the RunnerTask's doInBackground. They'll all run in the background thread of RunnerTask, in the order you call Run. When it is finished, the onPostExecute of RunnerTask is run on the main thread.
i just tried your code and onPreExecute and onPostExecute does run on the same thread, how do you output the thread id ? try:
Log.d("THREADTEST","PRE"+Long.toString(Thread.currentThread().getId()));
Log.d("THREADTEST","BACKGROUND"+Long.toString(Thread.currentThread().getId()));
Log.d("THREADTEST","POST"+Long.toString(Thread.currentThread().getId()));
P.S. it should be:
new SomeAsyncTask().execute();
and
private class SomeAsyncTask extends AsyncTask<String, Void, Integer> { ... }
you are actually executing the SingleTask from RunnerTask's doinbackground method which is incorrect as asynctask should be executed from a main thread only. You need to relook into the logic which runs the set of SingleTasks from RunnerTask.
I'm searching some pages from facebook and then downloading their pictures.
i want to view them immediately in my list and update the adapter everytime i download a new picture. i'm using the same method in one other activity and it works, but here i'm using a dialog and it freeze my UI till everything is finished, even if i'm doing it in background. why?
private synchronized void getUserLikesImages() {AsyncTask<Void, Void, Bitmap> task = new AsyncTask<Void, Void, Bitmap>() {
#Override
public Bitmap doInBackground(Void... params) {
for(...){
//download one picture...
FacebookeventsActivity.this.runOnUiThread(new Runnable() {
public void run() {
pagesILikeArrayAdapter.notifyDataSetChanged();
}
});
}
};
task.execute();
}
it works, but the UI is frozen till the end of the operations... i think it might be because my list is in a dialog and i'm running it on UIThread, but i don't know how to fix it.
#Override
public Bitmap doInBackground(Void... params) {
for(...){
//download one picture...
}
};
#Override
onPostExecute()
{
pagesILikeArrayAdapter.notifyDataSetChanged();
}
Just add onPostExecute() and notify adapter from that.. Or just Add onProgreesUpdate() method and use publishProgress() from doInBackGround()
notify adapter periodically from that..
The goal:
Using Google App Engine server and Android client, I'm trying to put on the Google map at the Android client Users overlays. Every 30 seconds I'm polling the server and getting Vector that contains users and adding it to the map.
Current status:
I'm dong all that using in one new thread, So after running the app I got:
weird behaviors(delayed overlays, multiple overlays) and after that crushed with ConcurrentModificationException.
After reading a bit i figured out that I need to work with AsyncTask.
Correct me if I'm wrong,But I understand that everything done in the Activity at at onCreate is "running" in UIhread so I need to put the "Logic" (All the Network handling) in doInBackground and all the UI Handling like putting overlays on the map in onPostExecute.
My Question are:
1) In the current status I'm doing:
new Thread()
{
#Override
public void run()
{
super.run();
while(true)
{
SystemClock.sleep(30000);
Vector responseFromServer = getUsersVectorFromServer();
putNewOnlineUserOnTheMap();
}
}
}.start();
What is the right way to convert this To AsyncTask?
Do I poll the server still using new thread in the doInBackground or there is right way to do this?
2) Is there a specific list of what counts as UI to put in onPostExecute or any concepts list?
In my case I guess that in need to put putNewOnlineUserOnTheMap() in onPostExecute.
Thanks.
Something similar to the following:
class UpdateTask extends AsyncTask<Void, Vector, Void>{
#Override
protected Void doInBackground(Void... params) {
// this is running in a background thread.
while (!isCancelled()) {
SystemClock.sleep(30000);
Vector responseFromServer = getUsersVectorFromServer();
// send the result back to the UI thread
// onProgressUpdate will be called then
publishProgress(responseFromServer);
}
return null;
}
#Override
protected void onProgressUpdate(Vector... values) {
// this is executed on the UI thread where we can safely touch UI stuff
putNewOnlineUserOnTheMap(values[0]);
}
}
You can't use the result of the task since the task is finished then. But you can use the progress publishing mechanism to get periodic results. If you use it like that and do the modification on the UI thread you should not get ConcurrentModificationException because you do the modifications on the one thread that can safely modify the UI.
One thing to note here: create new instances of your Vector in the background thread and then use it to update the UI. But don't touch the same object afterwards in the backgroundthread. That way you don't need any synchronization since after the background thread sends it away it is only the UI thread that touches it. (and you could use a simple ArrayList instead of a Vector)
AsyncTask uses generics and varargs.The parameters that are passed to the asyntask are . TypeOfVariableArgumentsParameters is passed into the doInBackground(), ProgressParam is used for progress information and ResultParam must be returned from doInBackground() and is passed to onPostExecute() as parameter.
example:--
protected class ParsingTask extends AsyncTask> {
private ProgressDialog loadingDialog = new ProgressDialog(JsonParserActivity.this);
protected void onPreExecute() {
loadingDialog.setMessage("loading app store..");
loadingDialog.show();
}
#Override
protected ArrayList<Items> doInBackground( Context... params ) {
// do ur process here.
return result;
}
if (!this.isCancelled()) {
}
return result;
}
#Override
protected void onProgressUpdate(String... s) {
super.onProgressUpdate(s);
Toast.makeText(getApplicationContext(), s[0], Toast.LENGTH_SHORT).show();
}
#Override
protected void onPostExecute( ArrayList<Items> response ) {
//if u r dealing with list view and adapters set the adapter here at the onPostExecute()
loadingDialog.dismiss();
}
#Override
protected void onCancelled() {
super.onCancelled();
Toast.makeText(getApplicationContext(), "The operation was cancelled", 1).show();
}
}
You can use AsyncTask like below. Hope this will help you..
Class YourClass{
void YourClass(){
NetworkTask nT = new NetworkTasK();
nT.execute();
}
}
protected class NetworkTask extends AsyncTask<Void, String, Boolean>
{
#Override
protected Boolean doInBackground(Void... params)
{
try
{
String response;
while(keepreceiving)
{
response = in.readLine();//Prog Counter stops here until getting i/p.
if(response != null)
yourFunctionForResponse(response);
}
}
catch (Exception ex)
{
}
return null;
}
private void yourFunctionForResponse(String response){
//things to do....
}
}
You may also try runOnUiThread(Runnable action) along with this to implement your work.
I wanted to use AsyncTask to load images to the ListView.
private class LoadImageTask extends AsyncTask<HashMap<String,Bitmap>,Void,Bitmap>{
#SuppressWarnings("unchecked")
#Override
protected void onPostExecute(Bitmap result) {
if(model.getIconCache().get(cellIcon)!=null){
icon.setImageBitmap(model.getIconCache().get(cellIcon));
}else{
new LoadImageTask().execute(model.getIconCache());
}
}
#Override
protected Bitmap doInBackground(HashMap<String, Bitmap>... params) {
//return model.getIconCache().get(cellIcon);
return null;
}
}
Ok, I know that this not an affective code. However it works well but with a lot of memory allocation. When reading the documentation about AsyncTask it said that Asynctask can be called only from UI thread, how could it let to use inside itself? And of course I want to make my code work inside a single AsyncTask. "model" in the code is an object that is updated at runtime through another thread. So I need to find a way to use a single Asynctask with periodically control the state of an object. How do I do that? Thanks
only do in backGround runs on backGround thread and postExecute and preExecute run on UI thread itself.. For the same reason u can show and dismiss dialogs in it..
if u want to use single Asynctask for multiple purpose u can play around by passing Different constants.. in .execute() method..
I mean something like this.
Integer Constant1 = 1;
int Constant2 = 2;
and while calling,,
new Background.execute(Constan1 or 2)
and in AsyncTask
doInBackground(Object.. arg0)
{
if(arg0.equals())
{
}
else if(arg0.equals())
{
}
}
Take a look at the asynctask documentation: http://developer.android.com/reference/android/os/AsyncTask.html
private class MyTask extends AsyncTask<Void, Integer, Boolean> {
protected Boolean doInBackground(Void...) {
int i = 0;
while(true){
publishProgress(i++);
}
}
protected void onProgressUpdate(Integer... progress) {
myObject.setState(progress[0]);
}
}
You do your background stuff in the doInBackground method (which runs in the background thread).
You control the state of your object in the onProgressUpdate (which runs on the ui thread)
You send messages between the background thread and the ui thread using publishProgress. These messages could contain the state of your object.