Nomi Nomi - 1 month ago 13
Android Question

How to Display toast in AsyncTask in android

I am Trying to display Taost in my initial_bakcground class extended with AsyncTask i am receiving this error in logcat.

public class InitialBackgroundTask extends AsyncTask<URL, Integer, Long> {

@Override
protected Long doInBackground(URL... params) {
// TODO Auto-generated method stub
show a = new show();
a.loop();
return null;
}

public class show {

void loop()
{
for(int i=0; i<10; i++)
{
Toast.makeText(MainActivity.me, "test", Toast.LENGTH_LONG).show();
}
}
}


05-30 12:08:12.641: E/AndroidRuntime(30840): FATAL EXCEPTION: AsyncTask #1
05-30 12:08:12.641: E/AndroidRuntime(30840): java.lang.RuntimeException: An error occured while executing doInBackground()
05-30 12:08:12.641: E/AndroidRuntime(30840): at android.os.AsyncTask$3.done(AsyncTask.java:278)
05-30 12:08:12.641: E/AndroidRuntime(30840): at java.util.concurrent.FutureTask$Sync.innerSetException(FutureTask.java:273)
05-30 12:08:12.641: E/AndroidRuntime(30840): at java.util.concurrent.FutureTask.setException(FutureTask.java:124)
05-30 12:08:12.641: E/AndroidRuntime(30840): at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:307)
05-30 12:08:12.641: E/AndroidRuntime(30840): at java.util.concurrent.FutureTask.run(FutureTask.java:137)
05-30 12:08:12.641: E/AndroidRuntime(30840): at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:208)
05-30 12:08:12.641: E/AndroidRuntime(30840): at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1076)
05-30 12:08:12.641: E/AndroidRuntime(30840): at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:569)
05-30 12:08:12.641: E/AndroidRuntime(30840): at java.lang.Thread.run(Thread.java:856)
05-30 12:08:12.641: E/AndroidRuntime(30840): Caused by: java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
05-30 12:08:12.641: E/AndroidRuntime(30840): at android.os.Handler.<init>(Handler.java:121)
05-30 12:08:12.641: E/AndroidRuntime(30840): at android.widget.Toast$TN.<init>(Toast.java:317)
05-30 12:08:12.641: E/AndroidRuntime(30840): at android.widget.Toast.<init>(Toast.java:91)
05-30 12:08:12.641: E/AndroidRuntime(30840): at android.widget.Toast.makeText(Toast.java:233)
05-30 12:08:12.641: E/AndroidRuntime(30840): at com.example.toast.show.loop(show.java:11)
05-30 12:08:12.641: E/AndroidRuntime(30840): at com.example.toast.InitialBackgroundTask.doInBackground(InitialBackgroundTask.java:13)
05-30 12:08:12.641: E/AndroidRuntime(30840): at com.example.toast.InitialBackgroundTask.doInBackground(InitialBackgroundTask.java:1)
05-30 12:08:12.641: E/AndroidRuntime(30840): at android.os.AsyncTask$2.call(AsyncTask.java:264)
05-30 12:08:12.641: E/AndroidRuntime(30840): at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:305)
05-30 12:08:12.641: E/AndroidRuntime(30840): ... 5 more


The above code is showing the whole story. Actually, I want to show toast in doInBackground method

Answer

You cannot update ui on the background thread. doinBAckground is invoked on the background thread. You should update ui on the ui thread.

      runOnUiThread(new Runnable(){

          @Override
          public void run(){
            //update ui here
            // display toast here 
          }
       });

onPreExecute(),onPostExecute(Result), are invoked on the ui thread. So you can display toast here.

onProgressUpdate(Progress...), invoked on the UI thread after a call to publishProgress(Progress...). can be used to animate a progress bar or show logs in a text field.

The result of doInbackground() computation is a parameter to onPostExecute(Result) so return the result in doinBackground() and show your toast in onPostExecute(Result)

You can also use a handler as suggested by @Stine Pike

For clarity check the link below under the topic The 4 steps.

http://developer.android.com/reference/android/os/AsyncTask.html

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