Develop Reference

Android Fragment + AsyncTask

In a fragment I'm using an 'AsyncTask' to retrieve data from a URL. The main purpose of my code is to access the data (via AsyncTask) and to pass a 'JSONArray' to the fragment.
Problem is, on the fragment side, that when I check the variable that should have the result i get an error saying that the variable is null.
Here is the code:
public class MyFragment extends ListFragment {
//this is the variable that should have the result from the AsyncTask
JSONArray myResult = null;
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
(...)
//execute the asyncTask
new GetResult().execute(email, password);
(...)
}
//The AsyncTask
private class GetResult extends AsyncTask<String, Void, JSONArray> {
#Override
protected JSONArray doInBackground(String... params) {
(...)
JSONArray jsonArray = (JSONArray) json.get("customer");
return jsonArray;
}
protected void onPostExecute(JSONArray result){
(...)
//this is where I try to pass the data to the fragment
MyFragment.this.myResult = result;
}
}
}
Can someone can help me with that? Thank you in advance.

Are you using the value after postExecute method. You should know that after calling execute, your code on the main thread still runs and only after postExecute has completed, will you have the value.

Try to use this code, and make sure that your jsonstring is not null, because you won't post your complete code and I can't see if myResult variable was changed:
try {
JSONArray json = new JSONArray(jsonstring);
JSONObject json_customer = json.getJSONObject("customer");
return json_customer;
} catch (JSONException e) {
return null
}

For me it looks like problem with deserialization from String data to json's. We are lack of information what exactly data you are parsing. Consider using for this some more advanced json processing library lika Gson or Jackson. Usually its easier with them.

It sounds like you are trying to read from myResult variable before the AsyncTask finishes retrieving it. To make sure it populates properly, put this into your onPostExecute function, after this line MyFragment.this.myResult = result;:
Log.i("my result value is: ", MyFragment.this.myResult.toString())
It should print it out in the LogCat if you are retrieving it correctly, and you should be able to access that variable after that.
On a separate note, consider using Volley or Retrofit or another networking plugin to make your life easier.

Maybe you can use a clear way to transfer data back from the task to the fragment.
1) define an interface as the event listener to be called in the AsyncTask, like
public interface TaskEventListener(){
public void onTaskStarted(int messageId);
public void onTaskStopped(int messageId, int resultCode);
public void onDataReady(Object parameter);
}
2) let the fragment implement this EventListener, put the processing code in the onDataReady() function.
3) pass the fragment instance as the EventListener instance.
4) in the class extended from the AsyncTask, call the onTaskStarted() in the onPreExecute() function, call the onDataReady() and onTaskStopped() in the onPostExecute() function, and call the onTaskStopped() only in the onCancelled() function.
5) please use the weak references in the background task class to prevent crashing for referring the invalid UI component.

Black screen while AsyncTask is running

I'm new to asking questions here so any advice about that will be appreciated...but to my problem:
I'm trying to switch from one activity to another, where I'll show some data from the server. I'm pulling the data from server via AsyncTask but there is black screen showing between the activities, when the download is longer. I already tried to put the AsyncTask from onCreate() to onStart() but no change there.
New activity:
public class SalesMenu extends Activity {
#Override
protected void onCreate(Bundle savedInstanceState) {
requestWindowFeature(Window.FEATURE_INDETERMINATE_PROGRESS);
setContentView(R.layout.sales_menu);
setProgressBarIndeterminateVisibility(true);
super.onCreate(savedInstanceState);
}
#Override
protected void onStart() {
List<HashMap<String, Object>> result = null;
AsyncTask<Void, Boolean, List<HashMap<String, Object>>> aTask = new AsyncSearchRead(
"sale.order",
new Object[0],
new String[] {"name"},
this).execute();
try {
result = aTask.get();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
Log.d("test", (result!=null ? result.toString() : "nothing"));
super.onStart();
}
}
My AsyncTask:
public class AsyncSearchRead extends AsyncTask<Void, Boolean, List<HashMap<String, Object>>> {
String mModel;
boolean mCount;
Integer mOffset;
Integer mLimit;
String mOrder;
boolean mReverseOrder;
Object[] mConditions;
String[] mFields;
Activity parentActivity;
public AsyncSearchRead(String model, boolean count, Integer offset, Integer limit,
String order, boolean reverseOrder, Object[] conditions, String[] fields,
Activity activity) {
mModel = model;
mCount = count;
mOffset = offset;
mLimit = limit;
mOrder = order;
mReverseOrder = reverseOrder;
mConditions = conditions;
mFields = fields;
parentActivity = activity;
}
#Override
protected List<HashMap<String, Object>> doInBackground(Void... params) {
Long[] ids;
List<HashMap<String, Object>> results;
//Downloading the data
ids = SetServer.connection.search(mModel, mCount, mOffset, mLimit, mOrder, mReverseOrder, mConditions);
results = SetServer.connection.read(mModel, ids, mFields);
return results;
}
#Override
protected void onPostExecute(List<HashMap<String, Object>> result) {
parentActivity.setProgressBarIndeterminateVisibility(false);
}
}
When I did not turn visibility off onPostExecute it kept spinning after showing the activity but it's only shown after the loading.
How can I get it to show the activity and only after that start to load the data and show some progress bar/loader?
Thanks for any help.

It's because you're using .get(). .get() essentially makes an AsyncTask synchronous (makes the main thread wait until it's done), so it's basically like not running the AsyncTask at all.
My suggestion is as follows:
Move List<HashMap<String, Object>> result = null; to be a member variable.
Remove the whole try/catch statement (the one with .get() in it), and the call to Log.*.
Move your AsyncTask to be an inner class of SalesMenu.
Do this.result = result in onPostExecute(), as well as whatever other processing you need to do on it.

In addition to #Eric's answer, which is very good advice (+1), try putting super.onCreate() before setContentView(). As a general rule, the super call should happen before anything else in overridden Android lifecycle methods, and is especially important in the case of these two methods.

In addition to the other excellent asnwers.
In your doInBackground() method, to show progress you call publichProgress() at regular intervals, which will in turn call onProgressUpdate(). onPostExecute() is only called AFTER doInBackground() has returned.
There is a full usage example in the AsyncTask docs.

There some points to keep under consideration while doing any long running tasks:
1) Keep them out of main thread
2) Use Async task class for short lived tasks and make them Asynchronous, use handlers with threads
3) Keep out any long running/heavy tasks from oncreate method
4) Do any task after super.onStart(); call under onStart() or onCreate super calls.
Hope this will help you out.

How to pass variables in and out of AsyncTasks?

I haven't spent much time working with AsyncTasks in Android. I'm trying to understand how to pass variables to and from the class. The syntax:
class MyTask extends AsyncTask<String, Void, Bitmap>{
// Your Async code will be here
}
it's a little bit confusing with the < > syntax on the end of the class definition. Never seen that type of syntax before. It seems like I'm limited to only passing one value into the AsyncTask. Am I incorrect in assuming this? If I have more to pass, how do I do that?
Also, how do I return values from the AsyncTask?
It's a class and when you want to use it you call new MyTask().execute() but the actual method you use in the class is doInBackground(). So where do you actually return something?

Note: all of the information below is available on the Android Developers AsyncTask reference page. The Usage header has an example. Also take a look at the Painless Threading Android Developers Blog Entry.
Take a look at the source code for AsynTask.
The funny < > notation lets you customize your Async task. The brackets are used to help implement generics in Java.
There are 3 important parts of a task you can customize:
The type of the parameters passed in - any number you want
The type for what you use to update the progress bar / indicator
The type for what you return once done with the background task
And remember, that any of the above may be interfaces. This is how you can pass in multiple types on the same call!
You place the types of these 3 things in the angle brackets:
<Params, Progress, Result>
So if you are going to pass in URLs and use Integers to update progress and return a Boolean indicating success you would write:
public MyClass extends AsyncTask<URL, Integer, Boolean> {
In this case, if you are downloading Bitmaps for example, you would be handling what you do with the Bitmaps in the background. You could also just return a HashMap of Bitmaps if you wanted. Also remember the member variables you use are not restricted, so don't feel too tied down by params, progress, and result.
To launch an AsyncTask instantiate it, and then execute it either sequentially or in parallel. In the execution is where you pass in your variables. You can pass in more than one.
Note that you do not call doInBackground() directly. This is because doing so would break the magic of the AsyncTask, which is that doInBackground() is done in a background thread. Calling it directly as is, would make it run in the UI thread. So, instead you should use a form of execute(). The job of execute() is to kick off the doInBackground() in a background thread and not the UI thread.
Working with our example from above.
...
myBgTask = new MyClass();
myBgTask.execute(url1, url2, url3, url4);
...
onPostExecute will fire when all the tasks from execute are done.
myBgTask1 = new MyClass().execute(url1, url2);
myBgTask2 = new MyClass().execute(urlThis, urlThat);
Notice how you can pass multiple parameters to execute() which passes the multiple parameter on to doInBackground(). This is through the use of varargs (you know like String.format(...). Many examples only show the extraction of the first params by using params[0], but you should make sure you get all the params. If you are passing in URLs this would be (taken from the AsynTask example, there are multiple ways to do this):
// This method is not called directly.
// It is fired through the use of execute()
// It returns the third type in the brackets <...>
// and it is passed the first type in the brackets <...>
// and it can use the second type in the brackets <...> to track progress
protected Long doInBackground(URL... urls)
{
int count = urls.length;
long totalSize = 0;
// This will download stuff from each URL passed in
for (int i = 0; i < count; i++)
{
totalSize += Downloader.downloadFile(urls[i]);
publishProgress((int) ((i / (float) count) * 100));
}
// This will return once when all the URLs for this AsyncTask instance
// have been downloaded
return totalSize;
}
If you are going to be doing multiple bg tasks, then you want to consider that the above myBgTask1 and myBgTask2 calls will be made in sequence. This is great if one call depends on the other, but if the calls are independent - for example you are downloading multiple images, and you don't care which ones arrive first - then you can make the myBgTask1 and myBgTask2 calls in parallel with the THREAD_POOL_EXECUTOR:
myBgTask1 = new MyClass().executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, url1, url2);
myBgTask2 = new MyClass().executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, urlThis, urlThat);
Note:
Example
Here is an example AsyncTask that can take as many types as you want on the same execute() command. The restriction is that each type must implement the same interface:
public class BackgroundTask extends AsyncTask<BackgroundTodo, Void, Void>
{
public static interface BackgroundTodo
{
public void run();
}
#Override
protected Void doInBackground(BackgroundTodo... todos)
{
for (BackgroundTodo backgroundTodo : todos)
{
backgroundTodo.run();
// This logging is just for fun, to see that they really are different types
Log.d("BG_TASKS", "Bg task done on type: " + backgroundTodo.getClass().toString());
}
return null;
}
}
Now you can do:
new BackgroundTask().execute(this1, that1, other1);
Where each of those objects is a different type! (which implements the same interface)

I recognize that this is a late answer, but here's what I've been doing for the last while.
When I'm needing to pass in a bunch of data to an AsyncTask, I can either create my own class, pass that in and then access it's properties, like this:
public class MyAsyncTask extends AsyncTask<MyClass, Void, Boolean> {
#Override
protected Boolean doInBackground(MyClass... params) {
// Do blah blah with param1 and param2
MyClass myClass = params[0];
String param1 = myClass.getParam1();
String param2 = myClass.getParam2();
return null;
}
}
and then access it like this:
AsyncTask asyncTask = new MyAsyncTask().execute(new MyClass());
or I can add a constructor to my AsyncTask class, like this:
public class MyAsyncTask extends AsyncTask<Void, Void, Boolean> {
private String param1;
private String param2;
public MyAsyncTask(String param1, String param2) {
this.param1 = param1;
this.param2 = param2;
}
#Override
protected Boolean doInBackground(Void... params) {
// Do blah blah with param1 and param2
return null;
}
}
and then access it like this:
AsyncTask asyncTask = new MyAsyncTask("String1", "String2").execute();
Hope this helps!

Since you can pass array of objects in the square bracket, that is the best way to pass data based on which you want to do processing in the background.
You could pass the reference of your activity or the view in the Constructor and use that to pass data back into your activity
class DownloadFilesTask extends AsyncTask<URL, Integer, List> {
private static final String TAG = null;
private MainActivity mActivity;
public DownloadFilesTask(MainActivity activity) {
mActivity = activity;
mActivity.setProgressBarIndeterminateVisibility(true);
}
protected List doInBackground(URL... url) {
List output = Downloader.downloadFile(url[0]);
return output;
}
protected void onProgressUpdate(Integer... progress) {
setProgressPercent(progress[0]);
}
private void setProgressPercent(final Integer integer) {
mActivity.setProgress(100*integer);
}
protected void onPostExecute(List output) {
mActivity.mDetailsFragment.setDataList((ArrayList<Item>) output);
//you could do other processing here
}
}

Alternatively, you could just use a regular thread and usea handler to send data back to the ui thread by overriding the handlemessage function.

Passing a simple String:
public static void someMethod{
String [] variableString= {"hello"};
new MyTask().execute(variableString);
}
static class MyTask extends AsyncTask<String, Integer, String> {
// This is run in a background thread
#Override
protected String doInBackground(String... params) {
// get the string from params, which is an array
final String variableString = params[0];
Log.e("BACKGROUND", "authtoken: " + variableString);
return null;
}
}

AsyncTask Save Variables from doInBackground

I have two AsyncTasks as inner classes in my Activity. One returns an ArrayList in doInBackground and asigns a ListAdapter to it on postExecute. The other AsyncTask returns a StringArray and sets some TextViews.
On Rotation everything is gone, also the layout changes on Rotation.
I'd like to have access to the results of the doInBackground-Methods. If I had access I could just simply save the variables in onSaveInstanceState and reasign the values manually.

You can access the results of doInBackground in onPostExecute.
Simply change your class to:
public class YourTask extends AsyncTask<Void, Void, ObjectYouWantToReturn> {
#Override
protected ObjectYouWantToReturn doInBackground(Beneficiary... params) {
ObjectYouWantToReturn obj = new ObjectYouWantToReturn();
//... do your stuff
return obj;
}
#Override
protected void onPostExecute(ObjectYouWantToReturn result) {
//there you go, here you have the results from doInBackground
}
}

Shared preferences is solution of your problem i think.
revise the link given below.
http://developer.android.com/guide/topics/data/data-storage.html#pref
http://thedevelopersinfo.com/2009/11/25/getting-sharedpreferences-from-other-application-in-android/

Android: How can I pass parameters to AsyncTask's onPreExecute()?

I use an AsyncTask for loading operations that I implemented as an inner class.
In onPreExecute() I show a loading dialog which I then hide again in onPostExecute(). But for some of the loading operations I know in advance that they will finish very quickly so I don't want to display the loading dialog.
I wanted to indicate this by a boolean parameter that I could pass to onPreExecute() but apparently for some reason onPreExecute() doesn't take any parameters.
The obvious workaround would probably be to create a member field in my AsyncTask or in the outer class which I would have to set before every loading operation but that does not seem very elegant. Is there a better way to do this?

You can override the constructor. Something like:
private class MyAsyncTask extends AsyncTask<Void, Void, Void> {
public MyAsyncTask(boolean showLoading) {
super();
// do stuff
}
// doInBackground() et al.
}
Then, when calling the task, do something like:
new MyAsyncTask(true).execute(maybe_other_params);
Edit: this is more useful than creating member variables because it simplifies the task invocation. Compare the code above with:
MyAsyncTask task = new MyAsyncTask();
task.showLoading = false;
task.execute();

1) For me that's the most simple way passing parameters to async task
is like this
// To call the async task do it like this
Boolean[] myTaskParams = { true, true, true };
myAsyncTask = new myAsyncTask ().execute(myTaskParams);
Declare and use the async task like here
private class myAsyncTask extends AsyncTask<Boolean, Void, Void> {
#Override
protected Void doInBackground(Boolean...pParams)
{
Boolean param1, param2, param3;
//
param1=pParams[0];
param2=pParams[1];
param3=pParams[2];
....
}
2) Passing methods to async-task
In order to avoid coding the async-Task infrastructure (thread, messagenhandler, ...) multiple times you might consider to pass the methods which should be executed in your async-task as a parameter. Following example outlines this approach.
In addition you might have the need to subclass the async-task to pass initialization parameters in the constructor.
/* Generic Async Task */
interface MyGenericMethod {
int execute(String param);
}
protected class testtask extends AsyncTask<MyGenericMethod, Void, Void>
{
public String mParam; // member variable to parameterize the function
#Override
protected Void doInBackground(MyGenericMethod... params) {
// do something here
params[0].execute("Myparameter");
return null;
}
}
// to start the asynctask do something like that
public void startAsyncTask()
{
//
AsyncTask<MyGenericMethod, Void, Void> mytest = new testtask().execute(new MyGenericMethod() {
public int execute(String param) {
//body
return 1;
}
});
}

why, how and which parameters are passed to Asynctask<>, see detail here. I think it is the best explanation.
Google's Android Documentation Says that :
An asynchronous task is defined by 3 generic types, called Params, Progress and Result, and 4 steps, called onPreExecute, doInBackground, onProgressUpdate and onPostExecute.
AsyncTask's generic types :
The three types used by an asynchronous task are the following:
Params, the type of the parameters sent to the task upon execution.
Progress, the type of the progress units published during the background computation.
Result, the type of the result of the background computation.
Not all types are always used by an asynchronous task. To mark a type as unused, simply use the type Void:
private class MyTask extends AsyncTask<Void, Void, Void> { ... }
You Can further refer : http://developer.android.com/reference/android/os/AsyncTask.html
Or You Can clear whats the role of AsyncTask by refering Sankar-Ganesh's Blog
Well The structure of a typical AsyncTask class goes like :
private class MyTask extends AsyncTask<X, Y, Z>
protected void onPreExecute(){
}
This method is executed before starting the new Thread. There is no input/output values, so just initialize variables or whatever you think you need to do.
protected Z doInBackground(X...x){
}
The most important method in the AsyncTask class. You have to place here all the stuff you want to do in the background, in a different thread from the main one. Here we have as an input value an array of objects from the type “X” (Do you see in the header? We have “...extends AsyncTask” These are the TYPES of the input parameters) and returns an object from the type “Z”.
protected void onProgressUpdate(Y y){
}
This method is called using the method publishProgress(y) and it is usually used when you want to show any progress or information in the main screen, like a progress bar showing the progress of the operation you are doing in the background.
protected void onPostExecute(Z z){
}
This method is called after the operation in the background is done. As an input parameter you will receive the output parameter of the doInBackground method.
What about the X, Y and Z types?
As you can deduce from the above structure:
X – The type of the input variables value you want to set to the background process. This can be an array of objects.
Y – The type of the objects you are going to enter in the onProgressUpdate method.
Z – The type of the result from the operations you have done in the background process.
How do we call this task from an outside class? Just with the following two lines:
MyTask myTask = new MyTask();
myTask.execute(x);
Where x is the input parameter of the type X.
Once we have our task running, we can find out its status from “outside”. Using the “getStatus()” method.
myTask.getStatus();
and we can receive the following status:
RUNNING - Indicates that the task is running.
PENDING - Indicates that the task has not been executed yet.
FINISHED - Indicates that onPostExecute(Z) has finished.
Hints about using AsyncTask
Do not call the methods onPreExecute, doInBackground and onPostExecute manually. This is automatically done by the system.
You cannot call an AsyncTask inside another AsyncTask or Thread. The call of the method execute must be done in the UI Thread.
The method onPostExecute is executed in the UI Thread (here you can call another AsyncTask!).
The input parameters of the task can be an Object array, this way you can put whatever objects and types you want.

You can either pass the parameter in the task constructor or when you call execute:
AsyncTask<Object, Void, MyTaskResult>
The first parameter (Object) is passed in doInBackground.
The third parameter (MyTaskResult) is returned by doInBackground. You can change them to the types you want. The three dots mean that zero or more objects (or an array of them) may be passed as the argument(s).
public class MyActivity extends AppCompatActivity {
TextView textView1;
TextView textView2;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main2);
textView1 = (TextView) findViewById(R.id.textView1);
textView2 = (TextView) findViewById(R.id.textView2);
String input1 = "test";
boolean input2 = true;
int input3 = 100;
long input4 = 100000000;
new MyTask(input3, input4).execute(input1, input2);
}
private class MyTaskResult {
String text1;
String text2;
}
private class MyTask extends AsyncTask<Object, Void, MyTaskResult> {
private String val1;
private boolean val2;
private int val3;
private long val4;
public MyTask(int in3, long in4) {
this.val3 = in3;
this.val4 = in4;
// Do something ...
}
protected void onPreExecute() {
// Do something ...
}
#Override
protected MyTaskResult doInBackground(Object... params) {
MyTaskResult res = new MyTaskResult();
val1 = (String) params[0];
val2 = (boolean) params[1];
//Do some lengthy operation
res.text1 = RunProc1(val1);
res.text2 = RunProc2(val2);
return res;
}
#Override
protected void onPostExecute(MyTaskResult res) {
textView1.setText(res.text1);
textView2.setText(res.text2);
}
}
}