Develop Reference

UI thread and AsyncTask synchronization

How to keep UI thread waiting until i get data from AsyncTask.
i trying to load some information and image from server and display it into textview and imageview respectively. But UI thread access the field before background thread complete its task. So null value is retrieved and application get crashed.

You can update the UI in onPostExecute of AsyncTask after doing your background tasks.
For the crash, by accessing the field on UI thread => You need to shift this code also to the onPostExecute method.
So the required data will be available, the field will have a value.
So the crash issue can be resolved.

If you are building an application that basically scrapes the html source and then you manipulate this "string", what you can do is
private class Scrapper extends AsyncTask<String, Void, Void> {
#Override
protected Void doInBackground(String... params) {
//Get the source here
}
Then in the onCreate Method
Scrapper scrapper = new Scrapper();
String URLSource = scrapper.execute("url here").get();
//Additional code to manipulate the string
However it is generally not recommended to do it this way, since apparently it will put the entire UI Thread on hold and there will be a long delay if the source is large.

Multiple tasks inside an Async class

I need to perform two different functions inside the AsyncTask class:- 1) Fetching String data from server. 2) Download Images from server . Please suggest me how can I make them run both . I tried to do it as follow but that does not work .
class SignIn extends AsyncTask<String, String, String[]> {
#Override
protected String[] doInBackground(final String... params)
{
}
protected Bitmap[] doInBackground(Bitmap...urls)
{
}

Firstly, since your doInBackground() code will always stay the same, I recommend you move it into a general utility class.
Beyond that, you can go one of two ways:
1) Create a new AsyncTask for each type of request that can call your utility class, and have its own onPostExecute()
2) Create one AsyncTask that has a flag in it, which can be checked in the onPostExecute() method to see what code needs to be executed there. You will have to pass the flag in in the constructor or as a parameter in execute.

Why does this code take a long time?

I am using this following code for getting all songs stored in the sdcard.
https://stackoverflow.com/a/12227047/2714061
Well why does this code take so long to return this list of songs.
I have included this code in a function which is called from the oncreate method in my player's playlist.
This is what happens.
1: When the application runs is executed for the first time on my android ph, the playlist has nothing to show, and hence is seen empty.
2: Well after for instance-> 30sec when I again call for the playlist it returns instantly all the songs.
Hence, giving the feel as though this thing takes time to execute?
Why does this happen?

How about using an asynchronous task, reading a file or downloading something, takes time that requires the user to wait, you must think of using an Asynchronous task for this purpose,
1: From the developer reference we have :
AsyncTask enables proper and easy use of the UI thread. This class allows to perform background operations and publish results on the UI thread without having to manipulate threads and/or handlers. http://developer.android.com/reference/android/os/AsyncTask.html
An asynchronous task is defined by 3 generic types, called Params, Progress and Result, and 4 steps, called onPreExecute, doInBackground, onProgressUpdate and onPostExecute.
2: So you may include an Async task class as:
class DoBackgroundTask extends AsyncTask<URL, Void, ArrayList> {
/*
URL is the file directory or URL to be fetched, remember we can pass an array of URLs,
Void is simple void for the progress parameter, you may change it to Integer or Double if you also want to do something on progress,
Arraylist is the type of object returned by doInBackground() method.
*/
#Override
protected ArrayList doInBackground(URL... url) {
//Do your background work here
//i.e. fetch your file list here
return fileList; // return your fileList as an ArrayList
}
protected void onPostExecute(ArrayList result) {
//Do updates on GUI here
//i.e. fetch your file list from result and show on GUI
}
#Override
protected void onProgressUpdate(Integer... values) {
// Do something on progress update
}
}
//Meanwhile, you may show a progressbar while the files load, or are fetched.
This AsyncTask can be called from you onCreate method by calling its execute method and passing the arguments to it:
new DoBackgroundTask().execute(URL);
3: And at last, there is also a very nice tutorial about AsyncTasks here, http://www.vogella.com/articles/AndroidBackgroundProcessing/article.html

What is the purpose of adding parameters to an AsyncTask?

Sorry for being a newbie, I've looked everywhere and I just don't get it.
Asynctask needs 3 parameters; e.g.
but what is the point of these parameters?
I am trying to run a geocoder in a separate thread and I have this
private class GetCurrentCity extends AsyncTask<String, Void, Void>{
but I literally made those parameters up. I have no idea what I'm supposed to put there. I don't need a progress bar or anything to be transferred to the other thread except for the line of code that is already in doInBackground() . Then I need a string to be returned from that, and I am using onPostExecute(String returnedAddress) for that.
I am confused. Help please!

From the doc of 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> { ... }

Rather than an API reference, here's a little more description. First, many people will use an AsyncTask inline as an anonymous class; keep that in mind. It looks like you've extended it with your own class (which is totally fine), but it likely means you're passing the necessary data in the constructor and referencing it as class variables. In that case, some of the arguments won't make as much sense.
So picture an anonymous inline class of AsyncTask. The first thing it's going to do is run some processing in the background. To do that, you need a way to pass data to the doInBackground method (because you don't have a constructor to call and pass it data). So the first argument is the type of data you're going to pass to it. If you don't have to pass anything, use Void, or use Object, or anything at all, really, because it has to be part of the method signature but you'd ignore it anyway.
For many situations, one will want to provide progress updates. For example, you might use the Float type to represent percent complete, or Long to represent bytes read from a file, or String for general messages to a user. You use the progress type to pass out interim progress information (think of uploading a file to facebook or downloading a file, where it updates the notification status with the progress - this is where/how you'd do that). You've said you don't care about it in your case, so use Void and don't bother implementing any progress methods.
Finally, when the task completes, you need to get to the result in the onPostExecute. So your doInBackground will return a value (of this type) and the AsyncTask framework will pass it to onPostExecute. Again, this makes more sense for an anonymous class with no further body. If you'd hold any results in a class member, that's fine also (but unnecessary). If you don't need to do anything on complete, or don't need to pass any data, use Void (and return null from doInBackground). I find it's useful at the least to return a Boolean for "completed successfully or failed," so you have that information (which might influence whether you post a success or failure notification, as notification of task complete is a common onPostExecute operation).
Hope some more explanation with examples helps.

Those are for when you want to pass something to it at time of execution or passing between runInBackground and onPostExecute. You can simply make all three Void in class declaration.
AsyncTask | Android Developers
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> { ... }

What arguments are passed into AsyncTask<arg1, arg2, arg3>?

I don't understand what I am supposed to put in here and where these arguments end up? What exactly should I put, and where exactly will it go? Do I need to include all 3 or can I include 1,2,20?

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.

I'm too late to the party but thought this might help someone.

Keep it simple!
An AsyncTask is background task which runs in the background thread. It takes an Input, performs Progress and gives Output.
ie AsyncTask<Input,Progress,Output>.
In my opinion the main source of confusion comes when we try to memorize the parameters in the AsyncTask.
The key is Don't memorize.
If you can visualize what your task really needs to do then writing the AsyncTask with the correct signature would be a piece of cake.
Just figure out what your Input, Progress and Output are and you will be good to go.
For example:
Heart of the AsyncTask!
doInBackgound() method is the most important method in an AsyncTask because
Only this method runs in the background thread and publish data to UI thread.
Its signature changes with the AsyncTask parameters.
So lets see the relationship
doInBackground() and onPostExecute(),onProgressUpdate() are also related
Show me the code
So how will I write the code for DownloadTask?
DownloadTask extends AsyncTask<String,Integer,String>{
#Override
public void onPreExecute()
{}
#Override
public String doInbackGround(String... params)
{
// Download code
int downloadPerc = // calculate that
publish(downloadPerc);
return "Download Success";
}
#Override
public void onPostExecute(String result)
{
super.onPostExecute(result);
}
#Override
public void onProgressUpdate(Integer... params)
{
// show in spinner, access UI elements
}
}
How will you run this Task
new DownLoadTask().execute("Paradise.mp3");

Refer to following links:
http://developer.android.com/reference/android/os/AsyncTask.html
http://labs.makemachine.net/2010/05/android-asynctask-example/
You cannot pass more than three arguments, if you want to pass only 1 argument then use void for the other two arguments.
1. private class DownloadFilesTask extends AsyncTask<URL, Integer, Long>
2. protected class InitTask extends AsyncTask<Context, Integer, Integer>
An asynchronous task is defined by a computation that runs on a background thread and whose result is published on the UI thread. An asynchronous task is defined by 3 generic types, called Params, Progress and Result, and 4 steps, called onPreExecute, doInBackground, onProgressUpdate and onPostExecute.

in Short, There are 3 parameters in AsyncTask
parameters for Input use in DoInBackground(String... params)
parameters for show status of progress use in OnProgressUpdate(String... status)
parameters for result use in OnPostExcute(String... result)
Note : - [Type of parameters can vary depending on your requirement]