AsyncTask Android example

android, android-asynctask

Solution

Ok, you are trying to access the GUI via another thread. This, in the main, is not good practice.

The AsyncTask executes everything in `doInBackground()` inside of another thread, which does not have access to the GUI where your views are.

`preExecute()` and `postExecute()` offer you access to the GUI before and after the heavy lifting occurs in this new thread, and you can even pass the result of the long operation to `postExecute()` to then show any results of processing.

See these lines where you are later updating your TextView:

TextView txt = findViewById(R.id.output);
txt.setText("Executed");

Put them in `onPostExecute()`.

You will then see your TextView text updated after the `doInBackground` completes.

I noticed that your onClick listener does not check to see which View has been selected. I find the easiest way to do this is via switch statements. I have a complete class edited below with all suggestions to save confusion.

import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.provider.Settings.System;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.view.View.OnClickListener;

public class AsyncTaskActivity extends Activity implements OnClickListener {

    Button btn;
    AsyncTask<?, ?, ?> runningTask;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        btn = findViewById(R.id.button1);

        // Because we implement OnClickListener, we only
        // have to pass "this" (much easier)
        btn.setOnClickListener(this);
    }

    @Override
    public void onClick(View view) {
        // Detect the view that was "clicked"
        switch (view.getId()) {
        case R.id.button1:
            if (runningTask != null)
                runningTask.cancel(true);
            runningTask = new LongOperation();
            runningTask.execute();
            break;
        }
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        // Cancel running task(s) to avoid memory leaks
        if (runningTask != null)
            runningTask.cancel(true);
    }

    private final class LongOperation extends AsyncTask<Void, Void, String> {

        @Override
        protected String doInBackground(Void... params) {
            for (int i = 0; i < 5; i++) {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    // We were cancelled; stop sleeping!
                }
            }
            return "Executed";
        }

        @Override
        protected void onPostExecute(String result) {
            TextView txt = (TextView) findViewById(R.id.output);
            txt.setText("Executed"); // txt.setText(result);
            // You might want to change "executed" for the returned string
            // passed into onPostExecute(), but that is up to you
        }
    }
}

Problem

I was reading about `AsyncTask`, and I tried the simple program below. But it does not seem to work. How can I make it work? ``` public class AsyncTaskActivity extends Activity { Button btn; /** Called when the activity is first created. */ @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); btn = (Button) findViewById(R.id.button1); btn.setOnClickListener((OnClickListener) this); } public void onClick(View view){ new LongOperation().execute(""); } private class LongOperation extends AsyncTask<String, Void, String> { @Override protected String doInBackground(String... params) { for(int i=0;i<5;i++) { try { Thread.sleep(1000); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } } TextView txt = (TextView) findViewById(R.id.output); txt.setText("Executed"); return null; } @Override protected void onPostExecute(String result) { } @Override protected void onPreExecute() { } @Override protected void onProgressUpdate(Void... values) { } } } ``` I am just trying to change the label after 5 seconds in the background process. This is my main.xml: ``` <?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="fill_parent" android:layout_height="fill_parent" android:orientation="vertical" > <ProgressBar android:id="@+id/progressBar" style="?android:attr/progressBarStyleHorizontal" android:layout_width="match_parent" android:layout_height="wrap_content" android:indeterminate="false" android:max="10" android:padding="10dip"> </ProgressBar> <Button android:id="@+id/button1" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="Start Progress" > </Button> <TextView android:id="@+id/output" android:layout_width="match_parent" android:layout_height="wrap_content" android:text="Replace"/> </LinearLayout> ```

Original source

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