I am trying to download a file at android using AsyncTask. Because I want to download many files I want to pass the url of the file by arguments. Also I want to change the UI when a user clicks the button that triggers the AsyncTask. As I figured out, apart from the URL I need to pass and the Activity itself so I can have access to it's views so I can change them. And my problem is this how can make the AsyncTask take multiple and different arguments.
Questions:
1.The only way to do this I think is to pass as Object the first argument so anything can pass, right?
public final Example extends AsyncTask<Objects, String, String>
2.When I call the AsyncTask how do I write the function?
Example a=new Example(??????); //I want to pass both a URL and something to give me access to the UI
a.execute(?????);
3. How can I use the "something" stated above argument to change visibility for example at a view?
Thx In advance for your time
Try this..
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB)
new Example("URL","something").executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, new String[]{null});
else
new Example("URL","something").execute(new String[]{null});
and Example AsyncTask Class
public class Example extends AsyncTask<String, Void, JSONObject> {
// variables passed in:
String url;
String something;
// constructor
public Example(String url, String something) {
this.url = url;
this.something = something;
}
#Override
protected void onPreExecute() {
}
#Override
protected JSONObject doInBackground(String... args) {
}
#Override
protected void onPostExecute(JSONObject jObjOut) {
}
}
AsyncTask takes 3 generic types:
1st is the argument which you send to execution;
2nd is for publishing updates on UI
3rd is for returning result
If you want to skip one or all of them use Void class:
example: new AsyncTask();
If you want to use your own constructor in AsyncTask you should define it in your own class that extends AsyncTask, something like this:
import android.app.Activity;
import android.os.AsyncTask;
public class Example extends AsyncTask<Object, String, String>{
private Object data;
private String data1;
private String data2;
private Activity activity;
public Example(Object data, String data1, String data2, Activity activity){
this.data = data;
this.data1 = data1;
this.data2 = data2;
this.activity = activity;
}
#Override
protected String doInBackground(Object... arg0) {
//Do here all your needs except UI updating
//transfer data you need for UI updating like this:
publishProgress("UI update data argument");
return null;
}
#Override
protected void onProgressUpdate(String... arg){
super.onProgressUpdate(arg);
//Update your UI using activity
}
}
To execute your Async task :
new Example(data, data1, data2, activity).execute(object);
You can use a parameterized constructor to achieve the same. But be aware that if you have multiple arguments you would end up creating multiple constructors. Since you need to modify multiple activities in response you need to pass a context as well.
query = UtilityClass.getQueries(UtilityClass.CHECKRIDE, paramsList);
new Example(HomeActivity.this, UtilityClass.CHECKRIDE).execute(query);
This is how you can create a query/url and pass it to the AsyncTask.
A Sample AsyncTask :
public class Example extends AsyncTask<String , Void, String> {
public Example(Context contextVar, int calloutTypeVar){
context = contextVar;
CALLOUTTYPE = calloutTypeVar;
}
#Override
protected void onPreExecute() {
progressDialog = ProgressDialog.show(context, "Please wait", "Loading...");
}
#Override
protected String doInBackground(String...sqlQueryList) {
String result = "";
String sqlQuery = sqlQueryList[0];
result = UtilityClass.doHttpCallout(sqlQuery);
return result;
}
// Handle the response from the callout
#Override
protected void onPostExecute(String result) {
// TODO Auto-generated method stub
super.onPostExecute(result);
progressDialog.dismiss();
switch (CALLOUTTYPE) {
case UtilityClass.SCHEDULERIDE:
ResponseHandlerClass.scheduleRide(result, context);
break;
}
}
I have implemented a response handler to handle the response from various callout results.
Hope this helps in handling your response from different urls.
The idea behind using AsyncTask is to avoid penalizing the responsiveness of the UI when performing blocking operations such as the file download of your scenario. When you use AsyncTask, the doInBackground method executes in another thread than the UI and the UI can not be modified from it so there is no point in passing any UI related data to it.
In order to modify your UI upon your background work completion, you can use the onPostExecute method. As #Tamilan suggested, you may pass the Activity object to the constructor of your AsyncTask if you want to use it there.
An example task could be as follows. Notice that you may replace String with URL, or even Object if you still want to pass different object types to the doInBackground method. Also, this example does not cover error handling beyond returning a simple boolean.
private class ExampleTask extends AsyncTask<String, Integer, Boolean>
{
private Activity mActivity;
public ExampleTask(Activity activity)
{
mActivity = activity;
}
#Override
protected Boolean doInBackground(String... params) {
// this is executed in a background thread
boolean success = true;
// do your background operations
publishProgress(50);
// more operations
return Boolean.valueOf(success);
}
#Override
protected void onProgressUpdate(Integer... progress) {
updateProgressbar(progress[0]); // update UI
}
#Override
protected void onPostExecute(Boolean result) {
// this is executed on your UI thread
if (!result)
// show error
else
// modify your UI
}
}
In order to execute this task:
ExampleTask task = new ExampleTask(anActivity);
task.execute(new String[] { "oneUrl", "anotherUrl" });
I retreive data from server by means of external class which contains AsyncTask:
public class GetTask extends AsyncTask<String, Void, JSONObject> {
private Context context;
ProgressDialog dialog;
public GetTask(Context cxt) {
context = cxt;
dialog = new ProgressDialog(context);
}
protected void onPreExecute() {
dialog.setTitle("Load...");
dialog.setMessage("Data...");
dialog.show();
super.onPreExecute();
}
#Override
protected JSONObject doInBackground(String... url) {
// code for retreive data
return jArray;
}
protected void onPostExecute(JSONObject object) {
dialog.dismiss();
super.onPostExecute(object);
}
}
I call this task from my activity:
Tasks task = new Tasks();
JSONObject json = task.new GetTask(this).execute(ServerURL).get();
My data retrieved successfully but ProgressDialog show after super.onPostExecute(object); method, why?
P.S. dialog show after:
// Make sure the identity of this thread is that of the local process,
// and keep track of what that identity token actually is.
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();
on internal Looper.class
Sorry my english is bad. ))
I found the solution, need use callback and not use .get() method. I call my task:
callTask(linkODeatails, obj);
callTask:
void callTask(String link, String object){
task.new GetTask(this).execute(link + object);
}
I create interface:
public interface AsyncTaskCompleteListener {
public void onTaskComplete(JSONObject result);
}
And added on my task:
private Activity activity;
private AsyncTaskCompleteListener callback;
public GetTask(Activity act){
this.activity = act;
this.callback = (AsyncTaskCompleteListener)act;
}
them called:
callback.onTaskComplete(result, object);
Hi I'm making Login page that access MySQL database. But my Activity always runs the code that check fail/success before it finishes the AsyncTask.
I tried using asynctask.get() method, but it just freeze my UI and doesn't work.
I tried this answer that said I should call the result-checker method inside onPostExecute().
But since I need to change the TextView to show success/failed, it results in NullPointerException because I instantiate the TextView inside onCreate().
I can't move the TextView instantiation into constructor because it will return NullPointerException unable to instantiate activity ComponentInfo.
Login.java
public class Login extends Activity{
//declare global Views here
protected void onCreate(Bundle bundle){
//Setup views
}
protected void onClick(View v){
//Setup necessary variables
AsyncClass async = new AsyncClass(this);
async.execute(username, password);
}
public void checkSuccess(boolean success){
if(success)
textView1.setText("Success");
else
textView1.setText("Failed");
}
}
AsyncClass.java
public class AsyncClass extends AsyncTask<String, String, JSONObject>{
protected JSONObject doInBackground(String... params){
//access database
}
protected void onPostExecute(JSONObject json){
//read the json result
Login login = new Login();
login.checkSuccess(true);
}
}
Any solution? Thanks
How about making AsyncTask as your inner class?
So your code should look something like below.
public class Login extends Activity {
//declare global Views here
protected void onCreate(Bundle bundle) {
//Setup views
}
protected void onClick(View v) {
new AsyncClass().execute(username, password);
}
public void checkSuccess(boolean success) {
if (success) textView1.setText("Success");
else textView1.setText("Failed");
}
class AsyncClass extends AsyncTask < String, String, JSONObject > {
protected JSONObject doInBackground(String...params) {
//access database
}
protected void onPostExecute(JSONObject json) {
checkSuccess(true / false);
}
}
}
try this
protected void onPostExecute(JSONObject json){
//read the json result
Login login = (Login)context; // object that you pass to task constructor
login.checkSuccess(true);
}
Also you can add progress dialog to your task to indicate some job execution
public class BaseTask<T> extends AsyncTask<Object, Void, T> {
public Context context;
public ProgressDialog dialog;
public BaseTask(Context context) {
this.context = context;
this.dialog = new ProgressDialog(context);
}
#Override
protected void onPreExecute() {
this.dialog.setMessage(context.getResources().getString(R.string.loading));
this.dialog.show();
}
#Override
protected T doInBackground(Object... objects) {
//....
return something;
}
#Override
protected void onPostExecute(T result) {
if (dialog != null && dialog.isShowing())
dialog.dismiss();
// do something
}
}
You cannot edit the UI from the async task thread. In order to make updates to the UI thread, use the onProgressUpdate() method. This method is part of your AsyncTask class, is actually executed in the main UI Thread (I hope you use the async task as a nested class btw, since it is declared public I guess your not. You should change that). The onProgressUpdate() Method is called by the OS itself if you call publishProgress(...) inside your Async task.
A small sample:
protected JSONObject doInBackground(String... params){
publishProgress("test");
}
/**
* This method is part of the Async Task
*/
protected void onProgressUpdate(String... progress) {
login.checkSuccess(true);
}
I would use it this way, just override your onPostExecute where you need it or create a own interface
//create a object f your asyncclass and
//override the onPostExecute where you need it
mInfo = new ASYNCCLASS({
#Override
public void onPostExecute(Object result){
//doSomething something with your views!
}
}).execute();
Waiting is not the answer, because you do not know how long your Asynctask will take to end.
Code above is not tested, just pseudoce, but it should show what i mean.
Do not have my IDE round here, so if anybody would correct the brackets if neccessary would be great!
Greetz
I have this two classes. My main Activity and the one that extends the AsyncTask, Now in my main Activity I need to get the result from the OnPostExecute() in the AsyncTask. How can I pass or get the result to my main Activity?
Here is the sample codes.
My main Activity.
public class MainActivity extends Activity{
AasyncTask asyncTask = new AasyncTask();
#Override
public void onCreate(Bundle aBundle) {
super.onCreate(aBundle);
//Calling the AsyncTask class to start to execute.
asyncTask.execute(a.targetServer);
//Creating a TextView.
TextView displayUI = asyncTask.dataDisplay;
displayUI = new TextView(this);
this.setContentView(tTextView);
}
}
This is the AsyncTask class
public class AasyncTask extends AsyncTask<String, Void, String> {
TextView dataDisplay; //store the data
String soapAction = "http://sample.com"; //SOAPAction header line.
String targetServer = "https://sampletargeturl.com"; //Target Server.
//SOAP Request.
String soapRequest = "<sample XML request>";
#Override
protected String doInBackground(String... string) {
String responseStorage = null; //storage of the response
try {
//Uses URL and HttpURLConnection for server connection.
URL targetURL = new URL(targetServer);
HttpURLConnection httpCon = (HttpURLConnection) targetURL.openConnection();
httpCon.setDoOutput(true);
httpCon.setDoInput(true);
httpCon.setUseCaches(false);
httpCon.setChunkedStreamingMode(0);
//properties of SOAPAction header
httpCon.addRequestProperty("SOAPAction", soapAction);
httpCon.addRequestProperty("Content-Type", "text/xml; charset=utf-8");
httpCon.addRequestProperty("Content-Length", "" + soapRequest.length());
httpCon.setRequestMethod(HttpPost.METHOD_NAME);
//sending request to the server.
OutputStream outputStream = httpCon.getOutputStream();
Writer writer = new OutputStreamWriter(outputStream);
writer.write(soapRequest);
writer.flush();
writer.close();
//getting the response from the server
InputStream inputStream = httpCon.getInputStream();
BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream));
ByteArrayBuffer byteArrayBuffer = new ByteArrayBuffer(50);
int intResponse = httpCon.getResponseCode();
while ((intResponse = bufferedReader.read()) != -1) {
byteArrayBuffer.append(intResponse);
}
responseStorage = new String(byteArrayBuffer.toByteArray());
} catch (Exception aException) {
responseStorage = aException.getMessage();
}
return responseStorage;
}
protected void onPostExecute(String result) {
aTextView.setText(result);
}
}
Easy:
Create interface class, where String output is optional, or can be whatever variables you want to return.
public interface AsyncResponse {
void processFinish(String output);
}
Go to your AsyncTask class, and declare interface AsyncResponse as a field :
public class MyAsyncTask extends AsyncTask<Void, Void, String> {
public AsyncResponse delegate = null;
#Override
protected void onPostExecute(String result) {
delegate.processFinish(result);
}
}
In your main Activity you need to implements interface AsyncResponse.
public class MainActivity implements AsyncResponse{
MyAsyncTask asyncTask =new MyAsyncTask();
#Override
public void onCreate(Bundle savedInstanceState) {
//this to set delegate/listener back to this class
asyncTask.delegate = this;
//execute the async task
asyncTask.execute();
}
//this override the implemented method from asyncTask
#Override
void processFinish(String output){
//Here you will receive the result fired from async class
//of onPostExecute(result) method.
}
}
UPDATE
I didn't know this is such a favourite to many of you. So here's the simple and convenience way to use interface.
still using same interface. FYI, you may combine this into AsyncTask class.
in AsyncTask class :
public class MyAsyncTask extends AsyncTask<Void, Void, String> {
// you may separate this or combined to caller class.
public interface AsyncResponse {
void processFinish(String output);
}
public AsyncResponse delegate = null;
public MyAsyncTask(AsyncResponse delegate){
this.delegate = delegate;
}
#Override
protected void onPostExecute(String result) {
delegate.processFinish(result);
}
}
do this in your Activity class
public class MainActivity extends Activity {
MyAsyncTask asyncTask = new MyAsyncTask(new AsyncResponse(){
#Override
void processFinish(String output){
//Here you will receive the result fired from async class
//of onPostExecute(result) method.
}
}).execute();
}
Or, implementing the interface on the Activity again
public class MainActivity extends Activity
implements AsyncResponse{
#Override
public void onCreate(Bundle savedInstanceState) {
//execute the async task
new MyAsyncTask(this).execute();
}
//this override the implemented method from AsyncResponse
#Override
void processFinish(String output){
//Here you will receive the result fired from async class
//of onPostExecute(result) method.
}
}
As you can see 2 solutions above, the first and third one, it needs to create method processFinish, the other one, the method is inside the caller parameter. The third is more neat because there is no nested anonymous class.
Tip: Change String output, String response, and String result to different matching types in order to get different objects.
There are a few options:
Nest the AsyncTask class within your Activity class. Assuming you don't use the same task in multiple activities, this is the easiest way. All your code stays the same, you just move the existing task class to be a nested class inside your activity's class.
public class MyActivity extends Activity {
// existing Activity code
...
private class MyAsyncTask extends AsyncTask<String, Void, String> {
// existing AsyncTask code
...
}
}
Create a custom constructor for your AsyncTask that takes a reference to your Activity. You would instantiate the task with something like new MyAsyncTask(this).execute(param1, param2).
public class MyAsyncTask extends AsyncTask<String, Void, String> {
private Activity activity;
public MyAsyncTask(Activity activity) {
this.activity = activity;
}
// existing AsyncTask code
...
}
You can try this code in your Main class.
That worked for me, but i have implemented methods in other way
try {
String receivedData = new AsyncTask().execute("http://yourdomain.com/yourscript.php").get();
}
catch (ExecutionException | InterruptedException ei) {
ei.printStackTrace();
}
I felt the below approach is very easy.
I have declared an interface for callback
public interface AsyncResponse {
void processFinish(Object output);
}
Then created asynchronous Task for responding all type of parallel requests
public class MyAsyncTask extends AsyncTask<Object, Object, Object> {
public AsyncResponse delegate = null;//Call back interface
public MyAsyncTask(AsyncResponse asyncResponse) {
delegate = asyncResponse;//Assigning call back interfacethrough constructor
}
#Override
protected Object doInBackground(Object... params) {
//My Background tasks are written here
return {resutl Object}
}
#Override
protected void onPostExecute(Object result) {
delegate.processFinish(result);
}
}
Then Called the asynchronous task when clicking a button in activity Class.
public class MainActivity extends Activity{
#Override
public void onCreate(Bundle savedInstanceState) {
Button mbtnPress = (Button) findViewById(R.id.btnPress);
mbtnPress.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
MyAsyncTask asyncTask =new MyAsyncTask(new AsyncResponse() {
#Override
public void processFinish(Object output) {
Log.d("Response From Asynchronous task:", (String) output);
mbtnPress.setText((String) output);
}
});
asyncTask.execute(new Object[] { "Your request to aynchronous task class is giving here.." });
}
});
}
}
Thanks
This answer might be late but I would like to mention few things when your Activity dependent on AsyncTask. That would help you in prevent crashes and memory management. As already mentioned in above answers go with interface, we also say them callbacks. They will work as an informer, but never ever send strong reference of Activity or interface always use weak reference in those cases.
Please refer to below screenshot to findout how that can cause issues.
As you can see if we started AsyncTask with a strong reference then there is no guarantee that our Activity/Fragment will be alive till we get data, so it would be better to use WeakReference in those cases and that will also help in memory management as we will never hold the strong reference of our Activity then it will be eligible for garbage collection after its distortion.
Check below code snippet to find out how to use awesome WeakReference -
MyTaskInformer.java Interface which will work as an informer.
public interface MyTaskInformer {
void onTaskDone(String output);
}
MySmallAsyncTask.java AsyncTask to do long running task, which will use WeakReference.
public class MySmallAsyncTask extends AsyncTask<String, Void, String> {
// ***** Hold weak reference *****
private WeakReference<MyTaskInformer> mCallBack;
public MySmallAsyncTask(MyTaskInformer callback) {
this.mCallBack = new WeakReference<>(callback);
}
#Override
protected String doInBackground(String... params) {
// Here do whatever your task is like reading/writing file
// or read data from your server or any other heavy task
// Let us suppose here you get response, just return it
final String output = "Any out, mine is just demo output";
// Return it from here to post execute
return output;
}
#Override
protected void onPostExecute(String s) {
super.onPostExecute(s);
// Here you can't guarantee that Activity/Fragment is alive who started this AsyncTask
// Make sure your caller is active
final MyTaskInformer callBack = mCallBack.get();
if(callBack != null) {
callBack.onTaskDone(s);
}
}
}
MainActivity.java This class is used to start my AsyncTask implement interface on this class and override this mandatory method.
public class MainActivity extends Activity implements MyTaskInformer {
private TextView mMyTextView;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mMyTextView = (TextView) findViewById(R.id.tv_text_view);
// Start your AsyncTask and pass reference of MyTaskInformer in constructor
new MySmallAsyncTask(this).execute();
}
#Override
public void onTaskDone(String output) {
// Here you will receive output only if your Activity is alive.
// no need to add checks like if(!isFinishing())
mMyTextView.setText(output);
}
}
You can do it in a few lines, just override onPostExecute when you call your AsyncTask. Here is an example for you:
new AasyncTask()
{
#Override public void onPostExecute(String result)
{
// do whatever you want with result
}
}.execute(a.targetServer);
I hope it helped you, happy codding :)
in your Oncreate():
`
myTask.execute("url");
String result = "";
try {
result = myTask.get().toString();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}catch (ExecutionException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}`
Why do people make it so hard.
This should be sufficient.
Do not implement the onPostExecute on the async task, rather implement it on the Activity:
public class MainActivity extends Activity
{
#Override
public void onCreate(Bundle savedInstanceState) {
//execute the async task
MyAsyncTask task = new MyAsyncTask(){
protected void onPostExecute(String result) {
//Do your thing
}
}
task.execute("Param");
}
}
You can call the get() method of AsyncTask (or the overloaded get(long, TimeUnit)). This method will block until the AsyncTask has completed its work, at which point it will return you the Result.
It would be wise to be doing other work between the creation/start of your async task and calling the get method, otherwise you aren't utilizing the async task very efficiently.
You can write your own listener. It's same as HelmiB's answer but looks more natural:
Create listener interface:
public interface myAsyncTaskCompletedListener {
void onMyAsynTaskCompleted(int responseCode, String result);
}
Then write your asynchronous task:
public class myAsyncTask extends AsyncTask<String, Void, String> {
private myAsyncTaskCompletedListener listener;
private int responseCode = 0;
public myAsyncTask() {
}
public myAsyncTask(myAsyncTaskCompletedListener listener, int responseCode) {
this.listener = listener;
this.responseCode = responseCode;
}
#Override
protected void onPreExecute() {
super.onPreExecute();
}
#Override
protected String doInBackground(String... params) {
String result;
String param = (params.length == 0) ? null : params[0];
if (param != null) {
// Do some background jobs, like httprequest...
return result;
}
return null;
}
#Override
protected void onPostExecute(String finalResult) {
super.onPostExecute(finalResult);
if (!isCancelled()) {
if (listener != null) {
listener.onMyAsynTaskCompleted(responseCode, finalResult);
}
}
}
}
Finally implement listener in activity:
public class MainActivity extends AppCompatActivity implements myAsyncTaskCompletedListener {
#Override
public void onMyAsynTaskCompleted(int responseCode, String result) {
switch (responseCode) {
case TASK_CODE_ONE:
// Do something for CODE_ONE
break;
case TASK_CODE_TWO:
// Do something for CODE_TWO
break;
default:
// Show some error code
}
}
And this is how you can call asyncTask:
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// Some other codes...
new myAsyncTask(this,TASK_CODE_ONE).execute("Data for background job");
// And some another codes...
}
Hi you can make something like this:
Create class which implements AsyncTask
// TASK
public class SomeClass extends AsyncTask<Void, Void, String>>
{
private OnTaskExecutionFinished _task_finished_event;
public interface OnTaskExecutionFinished
{
public void OnTaskFihishedEvent(String Reslut);
}
public void setOnTaskFinishedEvent(OnTaskExecutionFinished _event)
{
if(_event != null)
{
this._task_finished_event = _event;
}
}
#Override
protected void onPreExecute()
{
super.onPreExecute();
}
#Override
protected String doInBackground(Void... params)
{
// do your background task here ...
return "Done!";
}
#Override
protected void onPostExecute(String result)
{
super.onPostExecute(result);
if(this._task_finished_event != null)
{
this._task_finished_event.OnTaskFihishedEvent(result);
}
else
{
Log.d("SomeClass", "task_finished even is null");
}
}
}
Add in Main Activity
// MAIN ACTIVITY
public class MyActivity extends ListActivity
{
...
SomeClass _some_class = new SomeClass();
_someclass.setOnTaskFinishedEvent(new _some_class.OnTaskExecutionFinished()
{
#Override
public void OnTaskFihishedEvent(String result)
{
Toast.makeText(getApplicationContext(),
"Phony thread finished: " + result,
Toast.LENGTH_SHORT).show();
}
});
_some_class.execute();
...
}
Create a static member in your Activity class. Then assign the value during the onPostExecute
For example, if the result of your AsyncTask is a String, create a public static string in your Activity
public static String dataFromAsyncTask;
Then, in the onPostExecute of the AsyncTask, simply make a static call to your main class and set the value.
MainActivity.dataFromAsyncTask = "result blah";
I make it work by using threading and handler/message.
Steps as follow:
Declare a progress Dialog
ProgressDialog loadingdialog;
Create a function to close dialog when operation is finished.
private Handler handler = new Handler() {
#Override
public void handleMessage(Message msg) {
loadingdialog.dismiss();
}
};
Code your Execution details:
public void startUpload(String filepath) {
loadingdialog = ProgressDialog.show(MainActivity.this, "Uploading", "Uploading Please Wait", true);
final String _path = filepath;
new Thread() {
public void run() {
try {
UploadFile(_path, getHostName(), getPortNo());
handler.sendEmptyMessage(0);
} catch (Exception e) {
Log.e("threadmessage", e.getMessage());
}
}
}.start();
}
You need to use "protocols" to delegate or provide data to the AsynTask.
Delegates and Data Sources
A delegate is an object that acts on behalf of, or in coordination with, another object when that object encounters an event in a program. (Apple definition)
protocols are interfaces that define some methods to delegate some behaviors.
Here is a complete example!!!
try this:
public class SomAsyncTask extends AsyncTask<String, Integer, JSONObject> {
private CallBack callBack;
public interface CallBack {
void async( JSONObject jsonResult );
void sync( JSONObject jsonResult );
void progress( Integer... status );
void cancel();
}
public SomAsyncTask(CallBack callBack) {
this.callBack = callBack;
}
#Override
protected JSONObject doInBackground(String... strings) {
JSONObject dataJson = null;
//TODO query, get some dataJson
if(this.callBack != null)
this.callBack.async( dataJson );// asynchronize with MAIN LOOP THREAD
return dataJson;
}
#Override
protected void onProgressUpdate(Integer... values) {
super.onProgressUpdate(values);
if(this.callBack != null)
this.callBack.progress(values);// synchronize with MAIN LOOP THREAD
}
#Override
protected void onPostExecute(JSONObject jsonObject) {
super.onPostExecute(jsonObject);
if(this.callBack != null)
this.callBack.sync(jsonObject);// synchronize with MAIN LOOP THREAD
}
#Override
protected void onCancelled() {
super.onCancelled();
if(this.callBack != null)
this.callBack.cancel();
}
}
And usage example:
public void onCreate(#Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
final Context _localContext = getContext();
SomeAsyncTask.CallBack someCallBack = new SomeAsyncTask.CallBack() {
#Override
public void async(JSONObject jsonResult) {//async thread
//some async process, e.g. send data to server...
}
#Override
public void sync(JSONObject jsonResult) {//sync thread
//get result...
//get some resource of Activity variable...
Resources resources = _localContext.getResources();
}
#Override
public void progress(Integer... status) {//sync thread
//e.g. change status progress bar...
}
#Override
public void cancel() {
}
};
new SomeAsyncTask( someCallBack )
.execute("someParams0", "someParams1", "someParams2");
}
Probably going overboard a bit but i provided call backs for both the execution code and the results. obviously for thread safety you want to be careful what you access in your execution callback.
The AsyncTask implementation:
public class AsyncDbCall<ExecuteType,ResultType> extends AsyncTask<ExecuteType, Void,
ResultType>
{
public interface ExecuteCallback<E, R>
{
public R execute(E executeInput);
}
public interface PostExecuteCallback<R>
{
public void finish(R result);
}
private PostExecuteCallback<ResultType> _resultCallback = null;
private ExecuteCallback<ExecuteType,ResultType> _executeCallback = null;
AsyncDbCall(ExecuteCallback<ExecuteType,ResultType> executeCallback, PostExecuteCallback<ResultType> postExecuteCallback)
{
_resultCallback = postExecuteCallback;
_executeCallback = executeCallback;
}
AsyncDbCall(ExecuteCallback<ExecuteType,ResultType> executeCallback)
{
_executeCallback = executeCallback;
}
#Override
protected ResultType doInBackground(final ExecuteType... params)
{
return _executeCallback.execute(params[0]);
}
#Override
protected void onPostExecute(ResultType result)
{
if(_resultCallback != null)
_resultCallback.finish(result);
}
}
A callback:
AsyncDbCall.ExecuteCallback<Device, Device> updateDeviceCallback = new
AsyncDbCall.ExecuteCallback<Device, Device>()
{
#Override
public Device execute(Device device)
{
deviceDao.updateDevice(device);
return device;
}
};
And finally execution of the async task:
new AsyncDbCall<>(addDeviceCallback, resultCallback).execute(device);
Hope you been through this , if not please read.
https://developer.android.com/reference/android/os/AsyncTask
Depending on the nature of result data, you should choose best possible option you can think of.
It is a great choice to use an Interface
some other options would be..
If the AsyncTask class is defined inside the very class you want to
use the result in.Use a static global variable or get() , use it from
outer class (volatile variable if necessary). but should be aware of the AsyncTask progress or should at least make sure that it have finished the task and result is
available through global variable / get() method. you may use
polling, onProgressUpdate(Progress...), synchronization or interfaces (Which ever suits best for you)
If the Result is compatible to be a sharedPreference entry or it is okay to be saved as a file in the memory you could save it even from
the background task itself and could use the onPostExecute() method
to get notified when the result is available in the memory.
If the string is small enough, and is to be used with start of an
activity. it is possible to use intents (putExtra()) within
onPostExecute() , but remember that static contexts aren't that safe
to deal with.
If possible, you can call a static method from the
onPostExecute() method, with the result being your parameter
Is there anyway to use AsyncTask without passing in any parameters? I am currently passing in an empty string, but I'm not doing anything with it:
DownloadWebPageTask task = new DownloadWebPageTask();
task.execute(new String[] { "" });
class DownloadWebPageTask extends AsyncTask<String, Void, String> {
private final ProgressDialog dialog = new ProgressDialog(MainScreen.this);
#Override
protected void onPreExecute() {
dialog.setMessage("Gathering data for\n"+selectedSportName+".\nPlease wait...");
dialog.show();
}
#Override
protected String doInBackground(String... urls) {
//go do something
return "";
}
#Override
protected void onPostExecute(String result) {
dialog.dismiss();
startTabbedViewActivity();
}
}
private void startTabbedViewActivity(){
Intent intent = new Intent(MainScreen.this, TabbedView.class);
intent.putExtra(SPORT_NAME_EXTRA, selectedSportName);
intent.putExtra(HEADLINES_FOR_SPORT_EXTRA, existingSportHeadlines.get(selectedSportName));
intent.putExtra(SCORES_FOR_SPORT_EXTRA, existingSportScores.get(selectedSportName));
intent.putExtra(SCHEDULE_FOR_SPORT_EXTRA, existingSportSchedule.get(selectedSportName));
startActivity(intent);
}
For some reason, when I run the code as shown, with nothing happening in doInBackground(), the dialog dissapears, and the TabbedView activity starts.
But, when I use doInBackground() to run some code, the dialog dissapears, but, the TabbedView activity won't start. So I'm wondering if I could be doing anything differently?
Yes you can use AsyncTask without passing a String to the doInBackground() method.
Just change the first and third generic types from String to Void:
class DownloadWebPageTask extends AsyncTask<Void, Void, Void> {
}
From the doc:
The three types used by an asynchronous task are the following:
1)Params, the type of the parameters sent to the task upon execution.
2)Progress, the type of the progress units published during the
3) background computation. Result, the type of the result of the
background computation.
For the signature of onPostExecutetry this:
#Override
protected void onPostExecute(Void unused) {
}
EDIT:
For the parameters it's actually very simple.
Consider this:
class MyTask extends AsyncTask<Type1, Type2, Type3> {
#Override
protected void onPreExecute() {
}
#Override
protected Type3 doInBackground(Type1... param) {
}
#Override
protected void onProgressUpdate(Type2... progress) {
}
#Override
protected void onPostExecute(Type3 result) {
}
}
In order to determine what Type1, Type2 and Type3 should be, you'll have to ask yourself 3 questions:
What parameter should I pass to my task? => Type1
What is the type of the intermediate results that need to be shown during processing? =>
Type2
What is the type of the resulting operation? => Type3
In your case, I don't know whether the URL that you are downloading data from is defined in a globally or you want to pass it to the task. If it's defined globally, then you don't Type1 should be Void.
If you want to pass it to the task, then you can pass it to the doInBackground() method and Type1 would be String.
Obviously for your case you don't need to worry about Type2 since you aren't publishing any intermediate results and since you aren't doing anything with the result in onPostExecute() then Type3 is Void too.
To summarize you'll have:
class DownloadWebPageTask extends AsyncTask<String, Void, Void> {
#Override
protected Void doInBackground(String... urls) {
// do something with urls[0]
}
#Override
protected void onPostExecute(Void unused) {
// dismiss pd and start new intent
}
}
Or:
class DownloadWebPageTask extends AsyncTask<Void, Void, Void> {
#Override
protected Void doInBackground(Void... unused) {
}
#Override
protected void onPostExecute(Void unused) {
// dismiss pd and start new intent
}
}
Depending on your needs. Is that clear enough?
You simple specify Void for AsyncTask generics
class DownloadWebPageTask extends AsyncTask<Void, Void, Void> {
Look at the AsyncTask generics section