I have a class called "Class A"
a) I have a button called verify
b) onClick of verify i call auth.authenticate() which returns boolean
c) upon true - i have to call an intent
the authenticate function is in another class (auth is a ref)
authenticate function is as follow ()
boolean authenticate() {
new AsyncTask<String, String, String>() {
preExecute()
{
--starting a progress bar--
}
doInBackground(String )
{
---- huge authentication code-----
------ returns true or false -------isVerified
}
onPostExecute(String)
{
--- dismiss progress bar-----------
}
}.execute();
}
--------end of AsynTask block----
synchronized (this) {
try {
this.wait();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return isVerified; ---- this should return to MyClass - after async finishes
-- but this is returned before async completes.. actually it is correct - because these are thread-- runs parallel, so to make it work - i have waited till async to complete using that synchronized block -- but that will blocking my main thread - (also no progress bar)
I could call my next activity in post execute success - as i m making a library class..i think should not... please help
}
You can override onPostExecute() and return your value from there, if the task was cancelled it will not be called.
AsyncTasks are also threads. So you can make a new AuthenticateAsyncTask<Void,Void,Void> class for authentication and with overriding the onPostExecute method you can call a function with the parameter isVerified
For Ex:
boolean isVerified;
// Async Task
class AuthenticateAsyncTask extends AsyncTask<Void, Void, Void>{
#Override
protected Void doInBackground(Void... params) {
// Authentication
isVerified = returningValue // set your isVerified variable
return null;
}
#Override
protected void onPostExecute(Void result) {
// you can here call a function that indicates the authenticaion value is returned
}
#Override
protected void onPreExecute() {}
}
Hope this helps
Related
I need to call task() function from doInBackground() of AsyncTask class. The task function contains 2 sub-async task. So the task() return immediately from doInBackground().
Is it possible to stop(or mark this task done) this task from anywhere else?
How to wrap two sub async task in one.
You don't need to call another AsyncTask from within doInBackground. As a matter of fact, once you get to a high enough API, you'll get an exception. You can call another long running method from within AsyncTask without needing to worry about a thread; you're already in a background thread. If you really feed the need, call another service, but there is no reason to do this.
To stop an AsyncTask, just override OnCancelled. Then you can just call:
task.cancel(true).
EDIT:
If you want to wait for another process to finish before you move on, you can wait for that process to finish by setting a global variable in your class or application and then doing a Thread sleep until done. You will not get an ANR because you are already in a background thread and not on the Main UI:
private boolean processIsDone = false.
//then in your method you are calling from AsyncTask:
private void myLongRunningMethod() {
//do your work here....
//at the end set
processIsDone = true;
}
//in your AsyncTask:
protected Void doInBackground(Void... params) {
//do first part of AsyncTask here
myLongRunningMethod();
do {
try {
Thread.sleep(1500);
} catch (InterruptedException e) {
e.printStackTrace();
}
} while (!processIsDone);
//finish the process here.
return null;
}
I do not understand the question exactly but maybe this can help. Use this class in your Activity like this:
myTask = new BackgroundAsyncTask().execute();
And cancel this way:
myTask.cancel(true);
This is the code of the class:
private class BackgroundAsyncTask extends AsyncTask<Object , Object ,String> {
protected void onPreExecute(){
// Do Before execute the main task
}
protected String doInBackground(Object... param) {
//Execute the main task and return for example and String
return res;
}
protected void onPostExecute(String result) {
// You can use the String returned by the method doInBackground and process it
}
}
Hope it's help
About your first question, you can catch event when your task is cancelled by onCancelled() method. try like this:
private CancelTask extends AsyncTask {
private boolean cancelled = false;
protected void onCancelled() {
cancelled = true;
}
protected Object doInBackground(Object... obj) {
do {
// do something...
}while(!cancelled)
}
}
and call AsyncTask.cancel(true); when you want to stop.
CancelTask task = new CancelTask();
task.execute();
...
task.cancel(true);
And about second question, I want to know how you handle two "Sub-AsyncTask"s.
I will try to find a solution after you update your code.
I am calling another class's method from doInBackground of async task.
Now i need to stop the download when cancel is called. I am not sure where to check the value of isCancelled().
class myasync extends Asynctask{
protected String doInBackground(String... sURL) {
abc = new abc();
abc.getURLResult(sURL[0])
}
}
class abc()
{
getURLResult(String URL)
{
for(int i=0; i<fp.size(); i++){
//some text to download
}
}
}
class myclass
{
myclass()
{
myasync = new myasync();
myasync.execute("http:\\");
}
stopDownload()
{
myasync.cancel(true);
}
}
EDIT:
Have used the below solution by combining the two answers below:
1. myclass.cancel1(true);
class myclass
{
myclass()
{
myasync = new myasync();
myasync.execute("http:\\");
}
stopDownload()
{
myasync.cancel1(true);
}
}
2.
class myasync extends Asynctask{
protected String doInBackground(String... sURL) {
abc = new abc();
abc.getURLResult(sURL[0])
}
cancel1()
{
abc.cancel();
}
}
3.
class abc()
{
private boolean cancel = false;
getURLResult(String URL)
{
for(int i=0; i<fp.size(); i++){
//some text to download
if(cancel)
break;
}
}
cancel()
{
cancel = true;
}
}
The above method is working. However the methods myclass.stopDownload() is running in UI thread , and hence myasync.cancel1() and abc.cancel() are running the UI thread. And myAsync.doInBackground() and hence abc.getURLResult() are running in seperate thread. I dont know much about inter process communication. I hope this is right thing to do.
Not very nice, but you can do something like this by adding a static variable isDownloading:
protected String doInBackground(String... sURL) {
abc = new abc();
abc.getURLResult(sURL[0])
}
}
class abc()
{
getURLResult(String URL)
{
for(int i=0; i<fp.size(); i++){
if(!myclass.isDownloading){ //ADDED
break; // or Return or handle Cancel
}
//some text to dopwnload
}
}
class myclass
{
public static boolean isDownloading; // ADDED
myclass()
{
myasync = new myasync();
isDownloading = true; // ADDED
myasync.execute("http:\\");
}
stopDownload()
{
isDownloading = false; // ADDED
myclass.cancel(true);
}
}
Update:
From the AsyncTask Cancel doc. we have to check if the async task got cancelled as you say.
Calling this method will result in onCancelled(Object) being invoked
on the UI thread after doInBackground(Object[]) returns. Calling this
method guarantees that onPostExecute(Object) is never invoked. After
invoking this method, you should check the value returned by
isCancelled() periodically from doInBackground(Object[]) to finish the
task as early as possible.
To do that send the asyncTask itself to the getURLResult as parameter along with the URL:
protected String doInBackground(String... sURL) {
new abc().getURLResult("http://...", this); // this here is the asyncTask itself.
}
getURLResult(String URL, myasync myAsyncTask)
{
for(int i=0; i<fp.size(); i++){
if(myAsyncTask.isCancelled()){
break;
}
}
}
Don't use a boolean as other suggested. it's not safe at all since another AsyncTask could be started. and it is a background threads. you can't guarantee which will check the boolean first. could cancel all AsyncTasks.
Old post:
The only place you need to check for cancellation to guarantee the cancellation! is on the onPostExecute. You can't guarantee that the async task got cancelled on calling cancel method. Therefore, you need to check whether the client application asked to cancel it and the returned data is not wanted anymore.
private boolean askedForCancellation = true;
#Override
protected void onPostExecute(Object response) {
if (!askedForCancellation)
// parse response
else
// ignore. or send message to activity to stop loading if you didn't already did that when called cancel method.
}
To achieve that add the following cancel method to the AsyncTask:
public final boolean cancel(boolean mayInterruptIfRunning) {
askedForCancellation = true;
return mFuture.cancel(mayInterruptIfRunning);
}
In your class:
myasync.cancel(true);
myasync = null;
Set myasync to null is ok. because, you can't use it anymore for execution again. you will get a runtime error. you need to re-initialise it.
To check if AsyncTask asked for cancellation. check if the value of
myasync is equal to null. remember the AsyncTask asked to get
cancelled and not cancelled because there is no guarantee that it is
going to be cancelled on calling cancel. What you do is to ignore the
response on onPostExecute
I used this approach in more than 15 applications till now. No bugs and no unexpected behaviours.
I am currently working on an android application that has to handle a network connection using several AsyncTasks.
This is the first task that is establishing the connection and calling a new task which is handling the microphone input.
private class establishConnectionTask extends
AsyncTask<String, Integer, String> {
#Override
protected String doInBackground(String... params) {
try {
// initialize connection
initConnection();
MicrophoneTask micTask = new MicrophoneTask();
micTask.execute("");
}
} catch (InterruptedException e) {
e.printStackTrace();
}
return "Executed";
}
#Override
protected void onPostExecute(String result) {
mReadInputTask = new readInputTask();
mReadInputTask.execute("");
super.onPostExecute(result);
}
Everything works fine, the connection is working and I can transfer data. Also the MicrophoneTask is doing it's job.
Here comes the problem:
In the onPostExecute method I am creating a new AsyncTask which should handle all the network input.
This is how the readInputTask looks like:
private class readInputTask extends AsyncTask<String, Integer, String>
{
#Override
protected void onPreExecute()
{
Log.d("DEBUG", "pre");
super.onPreExecute();
}
#Override
protected String doInBackground(String... params) {
// blocking readInput method
Log.d("DEBUG", "doInBackground");
}
#Override
protected void onPostExecute(String result) {
Log.d("DEBUG", "post");
super.onPostExecute(result);
}
}
The readInputTask somehow gets stuck in the onPreExecute method of the readInputTask. The only output I get is "pre", eventhough I also expect "doInBackground" and "post".
Does anyone see an error or knows a solution for this?
Any help is appreciated!
mReadInputTask.execute("");
When you use AsyncTask#execute(params), the AsyncTasks are executed serially: one after the other. To execute AsyncTasks in parallel, use AsyncTask#executeOnExecutor(...).
From the docs on executeOnExecutor (Executor exec, Params... params):
This method is typically used with THREAD_POOL_EXECUTOR to allow
multiple tasks to run in parallel on a pool of threads managed by
AsyncTask, however you can also use your own Executor for custom
behavior.
I have two asyc task both perform separate network operation.I want one async task to wait for other task to finish for a single variable..I thought of doing it like perform other asyc operation onPostexecute of first one but for a single variable i have to make other task to wait first one to finish...is there any to achieve efficently
Referring to this, you can not use .execute() so;
First you have to start your tasks like this:
// Start first task
new Task1().executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, "");
// Start second task
new Task2().executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, "");
and then you can make a static variable so the both of tasks can access to it:
public static boolean task1Finished = false;
Then simple example of the tasks:
First task
private class Task1 extends AsyncTask<String, Void, String> {
#Override
protected String doInBackground(String... params) {
Log.d("myApp", "Task1 started");
for(int x = 0; x < 10; ++x)
{
try
{
Thread.sleep(1000);
//Log.d("myApp", "sleeped 1000 ms");
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
return "";
}
#Override
protected void onPreExecute() {
}
#Override
protected void onPostExecute(String result) {
// Lets the second task to know that first has finished
task1Finished = true;
}
}
Second task
private class Task2 extends AsyncTask<String, Void, String> {
#Override
protected String doInBackground(String... params) {
Log.d("myApp", "Task2 started");
while( task1Finished == false )
{
try
{
Log.d("myApp", "Waiting for Task1");
Thread.sleep(1000);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
Log.d("myApp", "Task1 finished");
// Do what ever you like
// ...
return "";
}
#Override
protected void onPreExecute() {
}
#Override
protected void onPostExecute(String result) {
Log.d("myApp", "All done here (Task2)");
}
}
Maybe asynctask is not the best tool? There are some interesting classes in the android api that can help doing specifically the synchronizing job :
Quote from android developper : "Four classes aid common special-purpose synchronization idioms.
Semaphore is a classic concurrency tool.
CountDownLatch is a very simple yet very common utility for blocking until a given number of signals, events, or conditions hold.
A CyclicBarrier is a resettable multiway synchronization point useful in some styles of parallel programming.
An Exchanger allows two threads to exchange objects at a rendezvous point, and is useful in several pipeline designs."
So I suggest looking into :
Cyclic Barrier
http://developer.android.com/reference/java/util/concurrent/CyclicBarrier.html
Exchanger
http://developer.android.com/reference/java/util/concurrent/Exchanger.html
You need to create another async in OnpostExecute of first one if you need to call the other synchronously.
I am using AsyncTask to perform some background calculations but I am unable to find a correct way to handle exceptions. Currently I am using the following code:
private class MyTask extends AsyncTask<String, Void, String>
{
private int e = 0;
#Override
protected String doInBackground(String... params)
{
try
{
URL url = new URL("http://www.example.com/");
}
catch (MalformedURLException e)
{
e = 1;
}
// Other code here...
return null;
}
#Override
protected void onPostExecute(String result)
{
if (e == 1)
Log.i("Some Tag", "An error occurred.");
// Perform post processing here...
}
}
I believe that the variable e maye be written/accessed by both the main and worker thread. As I know that onPostExecute() will only be run after doInBackround() has finished, can I omit any synchronization?
Is this bad code? Is there an agreed or correct way to handle exceptions in an AsyncTask?
I've been doing that in my apps, I guess there is not a better way.
You can also read Mark Murphy answer about it.
I think your code would to the job but there is already some kind of error handling built into the AsyncTask class.
You can avoid to use an extra variable by using the cancel() method and its handler method onCancelled(). When you call cancel within the doInBackground() method the onCancelled method in the UI thread. Whether you call cancel(true) or cancel(false) depends on your needs.
private class MyTask extends AsyncTask<String, Void, String>
{
#Override
protected NewsItem doInBackground(String... params)
{
try
{
URL url = new URL("http://www.example.com/");
}
catch (MalformedURLException e)
{
cancel(false/true);
}
// Other code here...
return null;
}
#Override
protected void onPostExecute(String result)
{
// Perform successful post processing here...
}
#Override
protected void onCancelled() {
super.onCancelled();
// Perform error post processing here...
}
}
This is guaranteed to work, even on SMP architecture. All the synchronization is done for you. It would however be better to use the return value to do this.