I have a layout with a TextView say mainTextView.
My activity file looks something like :
public class SecondActivity extends AppCompatActivity {
private static Integer i = 0;
private TextView tv = null;
#override
public void onCreate (Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main_layout);
tv = (TextView) findViewById(R.id.mainTextView);
new MyThread().execute();
}
private void notifyAChange () {
tv.setText(i.toString());
}
private class MyThread extends AsyncTask<String, Void, String> {
#Override
protected String doInBackground (String... params) {
while (true) {
try {
i++;
Thread.sleep(1000);
notifyAChange();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
}
This returns a FATAL EXCEPTION on runtime. I know I can't touch a view from another Thread except the original one but in that case I am touching the view from the main thread so what is wrong ?
Nope you are in the doInBackground part of the AsyncTask (worker thread).
You are calling notifyAChange() form worker thread, not main UI thread.
You should update the UI from onPostExecute. Or you can also use runOnUiThread for the part updating the view.
Main use of asyncktask to perform long running task so there are three methods preExecute to set progress bar befor starting executing task, doinbackground to perform task like download data (main thread),post execute to perform task after completion of task
you can only change UI component from Post execute while using asynctask.
For more info related to asynktask refer https://developer.android.com/reference/android/os/AsyncTask.html
and for Implementation and explanation refer AsynkTask ImplementationUpdate UI from Thread
you can also use RunonUI therad method and Handler
Related
Edit: It was suggested that it would be helpful to add my code so, my AsyncTask code is now pasted below...
I'm just learning Android and I have a UI with a few buttons. I want to animate the UI, changing the color of the buttons, in a sequence.
I shouldn't do that from the main thread of course, and it doesn't work anyway. The code manipulating the UI runs but the UI doesn't update until the end of the sequence.
So I created a thread and tried to run through the sequence from a background thread however, I would get an error trying to manipulate the UI components from the background thread. Only the main thread can touch the UI components.
Then I discovered AsyncTask. What I figured was, I could run through the sequence in doInBackground(). Every time I needed to update the UI I'd call publishProgress() which would cause onProgressUpdate() to be called from the main thread so I could access UI components without error.
Every time I call publishProgress() I would follow it with a SystemClock.sleep(500) to let time pass until the next animated UI update.
What I found though was that doInBackground() would run through the 4 UI state changes in about 2 seconds (500 ms each) but the UI would not update with each call to publishProgress(). Instead doInBackground() completes and then onProgressUpdate() is called 4 times in a row.
From the description, publishProgress & onProgressUpdate are designed to update a progress bar as doInBackground cranks through some longish running task so, obviously, onProgressUpdate must execute multiple times before doInBackground completes, right?
Am I missing something?
public class MainActivity extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
public void startGame(View view) {
MyTask task = new MyTask();
task.doInBackground();
}
class MyTask extends AsyncTask<Void,Void,Void> {
private int current_int;
#Override
protected Void doInBackground(Void... voids) {
this.current_int = 1;
Log.e("doInBackground","light up button "+this.current_int);
publishProgress();
SystemClock.sleep(500);
this.current_int = 2;
Log.e("doInBackground","light up button "+this.current_int);
publishProgress();
SystemClock.sleep(500);
this.current_int = 1;
Log.e("doInBackground","light up button "+this.current_int);
publishProgress();
SystemClock.sleep(500);
this.current_int = 2;
Log.e("doInBackground","light up button "+this.current_int);
publishProgress();
SystemClock.sleep(500);
return null;
}
#Override
protected void onProgressUpdate(Void... voids) {
super.onProgressUpdate(voids);
Log.e("onProgressUpdate","Updating button "+this.current_int);
Button btn1 = (Button) findViewById(R.id.button1);
Button btn2 = (Button) findViewById(R.id.button2);
if (this.current_int==1){
btn1.setBackgroundColor(getResources().getColor(android.R.color.holo_blue_light));
btn2.setBackgroundColor(getResources().getColor(android.R.color.holo_blue_dark));
} else {
btn2.setBackgroundColor(getResources().getColor(android.R.color.holo_blue_light));
btn1.setBackgroundColor(getResources().getColor(android.R.color.holo_blue_dark));
}
}
}
}
Just for reference : An Asynctask presents a systematic way to transition from main thread (calling thread) to the new thread (called thread). The onPreExecute() and onPostExecute() methods execute on the calling thread and the doInBackground() is the actual method executing on the new thread. Doing UI updates on main thread will hence lead to exception if done from doInBackground() method.
Your core background logic should hence be placed in the doInBackground() method.
If you want to update UI from background thread (Asynctask or otherwise), you can do it using this :
YourActivity.this.runOnUiThread(new Runnable(){
#Override
public void run()
{
//UI update operations here
}
});
You can use Handler for this,
public class TestClass extends AsyncTask<Void,Void,Void> {
boolean isRunning = true; //set false after executing UI logic.
Handler mHandler = new Handler();
#Override
protected Void doInBackground(Void... params) {
YourFunctionToUpdateUI();
return null;
}
public void YourFunctionToUpdateUI()
{
new Thread(new Runnable() {
#Override
public void run() {
while (isRunning) {
try {
mHandler.post(new Runnable() {
#Override
public void run() {
// your code to update the UI.
}
});
} catch (Exception e) {
//handle exception
}
}
}
}).start();
}
}
I'm an idiot. The problem is, I'm creating an AsyncTask and then calling doInBackground() directly, from my main thread instead of calling execute() which creates the background thread
I need to call task() function from doInBackground() of AsyncTask class. The task function contains 2 sub-async task. So the task() return immediately from doInBackground().
Is it possible to stop(or mark this task done) this task from anywhere else?
How to wrap two sub async task in one.
You don't need to call another AsyncTask from within doInBackground. As a matter of fact, once you get to a high enough API, you'll get an exception. You can call another long running method from within AsyncTask without needing to worry about a thread; you're already in a background thread. If you really feed the need, call another service, but there is no reason to do this.
To stop an AsyncTask, just override OnCancelled. Then you can just call:
task.cancel(true).
EDIT:
If you want to wait for another process to finish before you move on, you can wait for that process to finish by setting a global variable in your class or application and then doing a Thread sleep until done. You will not get an ANR because you are already in a background thread and not on the Main UI:
private boolean processIsDone = false.
//then in your method you are calling from AsyncTask:
private void myLongRunningMethod() {
//do your work here....
//at the end set
processIsDone = true;
}
//in your AsyncTask:
protected Void doInBackground(Void... params) {
//do first part of AsyncTask here
myLongRunningMethod();
do {
try {
Thread.sleep(1500);
} catch (InterruptedException e) {
e.printStackTrace();
}
} while (!processIsDone);
//finish the process here.
return null;
}
I do not understand the question exactly but maybe this can help. Use this class in your Activity like this:
myTask = new BackgroundAsyncTask().execute();
And cancel this way:
myTask.cancel(true);
This is the code of the class:
private class BackgroundAsyncTask extends AsyncTask<Object , Object ,String> {
protected void onPreExecute(){
// Do Before execute the main task
}
protected String doInBackground(Object... param) {
//Execute the main task and return for example and String
return res;
}
protected void onPostExecute(String result) {
// You can use the String returned by the method doInBackground and process it
}
}
Hope it's help
About your first question, you can catch event when your task is cancelled by onCancelled() method. try like this:
private CancelTask extends AsyncTask {
private boolean cancelled = false;
protected void onCancelled() {
cancelled = true;
}
protected Object doInBackground(Object... obj) {
do {
// do something...
}while(!cancelled)
}
}
and call AsyncTask.cancel(true); when you want to stop.
CancelTask task = new CancelTask();
task.execute();
...
task.cancel(true);
And about second question, I want to know how you handle two "Sub-AsyncTask"s.
I will try to find a solution after you update your code.
I'm having an issue with AsyncTask and onPostExecute. I am finding that onPostExecute is executing on a different thread than the main ui thread, which is causing a CalledFromWrongThreadException to happen when I modify any views.
I put in some logging to see what threads onPreExecute, doInBackground, and onPostExecute are running on. I would see a result like this...
onPreExecute ThreadId: 1
doInBackground ThreadId: 25
onPostExecute ThreadId: 18
I believe the main ui thread id is 1 and I would expect both onPre and onPost to both execute on thread 1. I am making sure to create and also call the execute method from the ui thread (for example in onCreate of an Activity).
Another thing to note that I have noticed is that later async tasks will run their onPostExecute method on the same thread as previous async task onPostExecute methods (in this case thread 18).
Right now in order to get around this I am wrapping the code in my onPostExecute methods in a call to runOnUiThread, but I think this is hacky and would like to get to the real issue.
I am out of ideas! Any one have any insight? I'm happy to answer any questions that could helper with further investigation!
EDIT:
There are two ways that async tasks are being run in the code. I am wondering if the latter in these examples is causing something weird to happen?
public class SomeActivity extends Activity {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main_layout);
new SomeAsyncTask().execute();
}
private class SomeAsyncTask extends AsyncTask<String, Void, Integer> {
#Override
public void onPreExecute() {
Thread.currentThread().getId() // 1
//Show a dialog
}
#Override
public Integer doInBackground(String... params) {
Thread.currentThread().getId() // 25
return 0;
}
#Override
public void onPostExecute(Integer result) {
Thread.currentThread().getId() // 18
//hide dialog
//update text view -> CalledFromWrongThreadException!!!
}
}
}
The above seems like a vanilla use of AsyncTask, but I still see this issue occurring even in simple cases like this. The next example uses an async task to run other async tasks. Maybe there is something I don't know about what happens when an async task gets constructed that is causing some weird behavior?
public class SomeActivity extends Activity implements TaskRunner.OnFinishListener {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main_layout);
TaskRunner taskRunner = new TaskRunner();
taskRunner.setOnFinishListener(this);
taskRunner.addTask(new SingleTask());
taskRunner.addTask(new SingleTask());
taskRunner.execute();
}
#Override
public void onTaskFinish(List<Integer> results) {
//Thread id is 18 when it should be 1
//do something to a view - CalledFromWrongThreadException!!
}
}
//In a different file
public class SingleTask extends AsyncTask<String, Void, Integer> {
//This is a an async task so we can run it separately as an asynctask
//Or run it on whatever thread runnerExecute is called on
#Override
public Integer doInBackground(String... params) {
return runnerExecute(params);
}
//Can be called outside of doInBackground
public Integer runnerExecute(String... params) {
//some long running task
return 0;
}
}
//In a different file
public class TaskRunner {
private List<SingleTask> tasks;
private OnFinishListener onFinishListener;
public interface OnFinishListener {
public void onTaskFinish(List<Integer> results);
}
public TaskRunner() {
this.tasks = new ArrayList<SingleTask>();
}
public void setOnFinishListener(OnFinishListener listener) {
this.onFinishListener = listener;
}
public void addTask(SingleTask task) {
tasks.add(task);
}
public void executeTasks() {
new RunnerTask().execute((SingleTask[]) tasks.toArray());
}
//Calls the runnerExecute method on each SingleTask
private class RunnerTask extends AsyncTask<SingleTask, Integer, List<Integer>> {
#Override
public void onPreExecute() {
//Runs on thread 1
}
#Override
public List<Integer> doInBackground(SingleTask... params) {
//Runs on arbitrary thread
List<Integer> results = new ArrayList<Integer>();
for(SingleTask task : params) {
int result =task.runnerExecute(task.getParams());
results.add(result);
}
return results;
}
#Override
public void onPostExecute(List<Integer> results) {
//Runs on thread 18
onFinishListener.onTaskFinish(results);
}
}
}
Maybe what is going on here is just super weird, and not at all how async tasks are meant to be used, either way it would be nice to get to the bottom of the issue.
Let me know if you need any more context.
I have been experiencing the same problem and it turned out the the issue was using Flurry 3.2.1. However, the issue is not limited to the Flurry library.
The issue behind the scenes is having the first ever (when the app is loaded for the first time) AsyncTask call from a looper thread which is not the Main UI thread. This call initializes a sHandler static variable in AsyncTask to the wrong thread id, and this id is then used in all subsequent AsyncTask$onPostExecute() calls.
To solve the problem, I call an empty (do-nothing) AsyncTask on first app load, just to initialize AsyncTask correctly.
try using:
getBaseContext().runOnUiThread(new Runnable()
{
#override
public void run()
{
}
});
and write your code inside the run function
The AsyncTask is designed to be used from the main thread. Your problem is the second case, and is that you call execute on the SingleTask from a background thread. You call it in the doInBackground method of RunnerTask. The onPostExecute is then run from the backgroundthread of RunnerTask
Two options for you.
1: Trash RunnerTask, and execute the SingleTasks from you main thread, they'll all run in parallell and you won't know which finishes first, but onPreExecute and onPostExecute is called on the main thread
2: Trash the SingleTask and define them as Runnables instead, then you can run them in sequence in the RunnerTask's doInBackground. They'll all run in the background thread of RunnerTask, in the order you call Run. When it is finished, the onPostExecute of RunnerTask is run on the main thread.
i just tried your code and onPreExecute and onPostExecute does run on the same thread, how do you output the thread id ? try:
Log.d("THREADTEST","PRE"+Long.toString(Thread.currentThread().getId()));
Log.d("THREADTEST","BACKGROUND"+Long.toString(Thread.currentThread().getId()));
Log.d("THREADTEST","POST"+Long.toString(Thread.currentThread().getId()));
P.S. it should be:
new SomeAsyncTask().execute();
and
private class SomeAsyncTask extends AsyncTask<String, Void, Integer> { ... }
you are actually executing the SingleTask from RunnerTask's doinbackground method which is incorrect as asynctask should be executed from a main thread only. You need to relook into the logic which runs the set of SingleTasks from RunnerTask.
I have an activity class as below.
public class LoginActivity extends Activity implements OnClickListener {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
button1 = (ImageView) findViewById(R.id.button1);
button1.setOnClickListener(this);
}
#Override
public void onClick(View v) {
loader = (ProgressBar) findViewById(R.id.loader);
Thread processThread = new Thread(loaderThread);
loader.setVisibility(View.VISIBLE);
processThread.start();
try {
Thread.currentThread().join();
Log.i("Activity","gone past join()");
loader.setVisibility(View.INVISIBLE);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
private Runnable loaderThread = new Runnable() {
public void run() {
ServiceCaller serviceCaller = new ServiceCaller();
boolean status = serviceCaller.checkProcess(url);
}
};
}
Further Question [EDITED]
Here is the scenario. The main activity class creates a thread on a click. The then created thread fetches some data from the server. It is a time consuming task. So a progress bar is displayed on the UI. Currently I am using AsyncTask (not shown here) to accomplish server data retrieval. But the real challenge is wait for the background task to complete and get the value from it. What I am looking for is:
wait until server calls are made and get the results. Meanwhile show the progress bar.
Any thoughts? Apologies in case I confuse you.
Thanks in advance!
You must have a look at AsyncTask
http://developer.android.com/reference/android/os/AsyncTask.html
http://www.vogella.com/articles/AndroidPerformance/article.html
and you can show the ProgressBar in onPreExecute()
do the task in doInBackground()
and hide the ProgressBar in onPostExecute()
Join method blocks the current thread. In your case Onclick method is called in UI thread, so all UI operations are blocked. It is never a good idea to block Ui thread.
So you probably should use either a Handler or Asynctask to keep updating Progressbar
I use AsyncTask to change text of TextView like this:
private class LongOperation extends AsyncTask<String, Void, String> {
#Override
protected String doInBackground(String... urls) {
String response = "";
for (String url : urls) {
response += url;
}
return response;
}
#Override
protected void onPostExecute(String result) {
textView.setText(result);
}
}
Everything will fine if I call it in OnClick event:
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
textView = (TextView) findViewById(R.id.txt);
Button button = (Button)this.findViewById(R.id.button);
button.setOnClickListener(new OnClickListener() {
#Override
public void onClick(View v) {
new LongOperation().execute(new String[]{"Hello"});
}
});
}
But the problem when I called it in my thread, the program forced close
this.closeButton.setOnClickListener(new OnClickListener() {
#Override
public void onClick(View v) {
Thread t= new Thread(){
#Override
public void run(){
try{
//Do something
//Then call AsyncTask
new LongOperation().execute(new String[]{"Hello"});
}catch(Exception e){}
}
};
t.start();
}
});
Where am I wrong? I dont' understand how difference call AsyncTask in thread or not.
I recommend you consult the AsyncTask documentation and Processes and Threads for a better understanding of how it works. Essentially, you should create your AsyncTask subclass on the main thread.
When you call AsyncTask.execute(), your provided, AsyncTask.onPreExecute is called on the main thread, so you can do UI setup.
Next AsyncTask.doInBackground method is called, and runs in its own thread.
Finally, when your AsyncTask.doInBackground method completes, a call is made to AsyncTask.onPostExecute on the main thread, and you can do any UI cleanup.
If you need to update the UI from within your AsyncTask.doInBackground method, call AsyncTask.publishProgress, which will invoke onProgressUpdate in the main thread.
When you call it from the UI thread, the associated Context is the running Activity. When you call it from a regular thread, there is no valid Context associated with that thread. AsyncTask executes in its own thread, you shouldn't be creating its own thread. If that is actual code, then you have missunderstood the point of AsyncTask. Search for tutorials on how to use it.
Adding to what the others have said: I think you can use AsyncTask to launch off a task in another thread, even if you start the AsyncTask from a different thread than the UI already.
But in that case, the only way you'll only be able to modify the UI indirectly, for example: pass the handler of the current Activity somehow to this AsyncTask instance, and send messages to it (handler messages get processed on the UI thread). Or use broadcast intents that the Activity catches and updates the UI accordingly, etc. These solutions seem to be overkills though.