Develop Reference

Android AsyncTask Parameters

I'm having trouble understanding the use of the parameters for Asynctask in android.
The Android Developers documentation explains it as follows:
AsyncTask must be subclassed to be used.
The subclass will override at least one method (doInBackground(Params...)),
and most often will override a second one (onPostExecute(Result).)
Here is an example of subclassing:
private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> {
protected Long doInBackground(URL... urls) {
int count = urls.length;
long totalSize = 0;
for (int i = 0; i < count; i++) {
totalSize += Downloader.downloadFile(urls[i]);
publishProgress((int) ((i / (float) count) * 100));
// Escape early if cancel() is called
if (isCancelled()) break;
}
return totalSize;
}
protected void onProgressUpdate(Integer... progress) {
setProgressPercent(progress[0]);
}
protected void onPostExecute(Long result) {
showDialog("Downloaded " + result + " bytes");
}
}
Once created, a task is executed very simply:
new DownloadFilesTask().execute(url1, url2, url3);
For my extension of AsyncTask, I don't need to pass in any parameters, but I need to override the doInBackground(), onProgressUpdate(), and onPostExecute(). Why do I have to insert Void,Void,Void into AsyncTask<>?
What do the parameters do?

From the documentation it says using Void simply marks the type as unused. You don't have to have a type in AsyncTask.
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> { ... }

Well Async task parameters are simple,
First param : Array of object or single object which needed for background process.
Second param : Object type you are going to pass into to the onProgressUpdate
Third Param : The return type of doInBackground
example :-
private class ImageDownloader extends AsyncTask<Void, Void, Void> {//todo}
ImageDownloader downloader = new ImageDownloader();
downloader.execute();

You insert void void void if you have no need for the parameters. For example when the async task is done and in onPostExecute you just want to update the view and it isn't reliant on the result doInBackground you can put Void as the 3rd parameter.
I think it is preferred practice to use parameters however instead of holding variables in either the async task or instance variables if the async task is a private inner class.

How to get multiple implementations of a single AsyncTask class in Android?

I am developing an Android application, where most of my activities are fetching content from a MySQL database separately, through an http request.
For this, I am required to add the AsyncTask class separately to each such activity.
Is there a possibility that I may use a common AsyncTask class for all such data fetching and create their objects separately in the activities and fetch data based on the parameters?

Of course, you can create separate class extending AsyncTask for making HTTP requests, which will take appropriate parameters and re-use it in many activities. You don't have to create nested private classes in each activity. You can use input parameters of AsyncTask in doInBackground(parameters) method. See documentation of AsyncTask at: http://developer.android.com/reference/android/os/AsyncTask.html
Below, you can see very simple example of using AsyncTask with some pseudo-code:
public class DownloadDataTask extends AsyncTask<String, Integer, String> {
protected String doInBackground(String address) {
// you should have method for performing HTTP request
// and return result - in this case as String
String result = performHttpRequest(address);
return result;
}
protected void onPostExecute(String result) {
// this method is executed after downloading result
// now, you can perform some action - e.g. display data in a TextView
myTextView.setText(result);
}
}
In this definition: AsyncTask<String, Integer, String> first parameter is type of input parameter (in this case address of the end-point), second parameter is type of progress value (we are not using it in this example), last parameter is type of output value (in our case, HTTP response as String).
When DownloadDataTask class is placed in a separate file, you can re-use it in many activities with the following call:
new DownloadDataTask().execute(address);
Of course, it's very basic example with some pseudo-code. I haven't initialized TextView and I haven't provided implementation of performHttpRequest() method. You should adjust it to your needs. You can also upgrade this solution and pass reference to a TextView or another widget in which your result will be displayed.
Nevertheless, consider using RxJava and RxAndroid instead of AsyncTask. It will make you app simpler and less error-prone. I do not recommend using AsyncTask, because we have many better solutions for Android right now. Regardless of this fact, you decide which solution will be used in your project.

You can achieve this in much the same way as you would with any other Class - create a separate class file that extends AsyncTask and create an object where it's required.
For more flexibility, you could declare the class abstract and use an unknown type parameter, so that you can adjust it as your requirements change.
public abstract class MyTask<T> extends AsyncTask<T, Void, Void>{
#Override
protected void onPreExecute() {
Log.i("Tag", "onPreExecute");
}
#Override
protected void onPostExecute(Void aVoid) {
Log.i("Tag", "onPostExecute");
}
}
And then, whenever you want to create an instance:
MyTask<String> mt = null; //Or URL etc.
//...
if (mt == null) {
mt = new MyTask() {
#Override
protected Void doInBackground(String... params) {
return null;
}
};
}
mt.execute(params);

Should I give params to the constructor or to AsyncTask.execute(params)?

I'm trying to figure out why Android's AsyncTask gives parameters through execute and why passing then to the constructor doesn't seem to be done (in the docs, at least).
This is the way that seems to me to make the most sense (naturally, the actual task I'm working with is more than just a sum calculator):
public class FooTask extends AsyncTask<Void, Integer, Long> {
private ProgressBar progressBar;
private int[] data;
public FooTask(ProgressBar progressBar, int... data) {
this.progressBar = progressBar;
this.data = data;
}
protected void onPreExecute() {
progressBar.setMax(data.length);
progressBar.setProgress(0);
}
protected Long doInBackground(Void... _) {
long sum = 0;
for (int i = 0; i < data.length; i++) {
sum += data[i];
publishProgress(i);
}
return sum;
}
protected void onProgressUpdate(Integer... progress) {
progressBar.setProgress(progress[0]);
}
protected void onPostExecute(Long result) {
Log.i(TAG, "Sum: " + result);
}
}
This would be used thus:
new FooTask(progressBar, 1, 2, 3).execute();
However, this isn't the way the documentation talks about doing it; it uses arguments to execute(), like this (taken to the extreme of not using a constructor at all, but still using one field because otherwise it'd be too horrible):
public class FooTask extends AsyncTask<Object, Integer, Long> {
private ProgressBar progressBar;
private boolean isMaxSettingUpdate = true;
protected Long doInBackground(Object... params) {
progressBar = params[0];
long sum = 0;
for (int i = 1; i < data.length; i++) {
sum += (int) data[i];
publishProgress(i - 1, data.length);
}
return sum;
}
protected void onProgressUpdate(Integer... progress) {
progressBar.setMax(progress[1]);
progressBar.setProgress(progress[0]);
}
protected void onPostExecute(Long result) {
Log.i(TAG, "Sum: " + result);
}
}
Execution of this task would look more like this:
new FooTask().execute(progressBar, 1, 2, 3);
One other option I considered was giving the progress bar to the constructor and the data to the execute call, but then I still can't use onPreExecute as I don't know the max value. (I would prefer to use a true max rather than setting the max value arbitrarily and calculating the percentage... just seems nicer.)
Where's the balance? What should I do? Is there anything wrong with using the constructor?

As to why the docs do everything in the method can be as a result of their example choice. Usually, you're more likely to extend your AsyncTask and only use doInBackground(), not the constructor.
Anyways, the documentation states:
Memory observability
AsyncTask guarantees that all callback calls are synchronized in such
a way that the following operations are safe without explicit
synchronizations.
•Set member fields in the constructor or onPreExecute(), and refer to them in > doInBackground(Params...).
•Set member fields in doInBackground(Params...), and refer to them in
onProgressUpdate(Progress...) and onPostExecute(Result).
This means that you should be good with both approaches.
And as a side note, I have used an AsyncTask with a param-ed constructor without issues, so I can back up what the documentation states.
Also, for your specific case, unless the ProgressBar has had a different max value set beforehand, it should default to 100. This means that you can have your constructor accept in just the ProgressBar and have doInBackground() accept in the data (which should also be a member variable). Then when updating the progress, do
(progress[0]/data.length) * 100
It won't be perfect, and you can convert to double if you want to increase accuracy, but this should make the code easiest to understand.

In my opinion, if I want to pass some values to initial the UI, just like ProgressBar you mentioned, I need do it in OnPreExecute(). And because the OnPreExecute() don't accept any parameters, I prefer to put values in constructor as a parameters. If some values are not relative to UI, just need in doInBackground, for example, the download urls of files, I will pass them as parameters of execute.
So, in a summary, if the values are about UI, pass them in constructor,
if values are just used in doInBackground, pass them as parameters of execute.

If you want to do work (that requires parameters) on the main-thread, you have to pass those parameters to the constructor.
Everything you pass via .execute() will be run on the async-thread.
As buptcoder mentioned, mainly that will concern UI stuff, but also disk/database operations can be thread-critical

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;
}
}

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);
}
}
}