What is Worker Thread in Android ?

Worker threads are background threads. They are the threads that are created separately, other than the UI thread. Since blocking the UI thread is restricted according to the rule, user should run the child processes and tasks in worker threads. 

An example for creation and working of worker thread is given below:

public void onClick(View v) {

    new Thread(new Runnable() {

        public void run() {

            Bitmap b = loadImageFromNetwork("http://example.com/image.png");

            mImageView.setImageBitmap(b);

        }

    }).start();

}

In the above example code, the download operation is handled by a second thread other than the UI thread. But the program violates the second rule. The imageView from UI thread is manipulating from this worker thread.

According to the second rule, UI could not be accessed from outside the UI thread. Solution for such a restriction is runOnUiThread(Runnable) method. The main or UI thread can be accessed from other threads using runOnUiThread(Runnable) method.

As a result, the specified runnable action passed through this method will run on the UI thread. The action will execute immediately, if the current thread is in the UI itself. Else the action will be posted to the event queue.

An example code:

MainClassName.this.runOnUiThread(new Runnable() {

                    public void run() {

                        textViewObject.setText("Set this " + value from worker thread + "!");

                    }

                });

Only using the methods like runOnUiThread(Runnable), the program can touch the UI views.

Handlers and Runnable

A better solution to process the messages delivered from the UI thread is handlers. The runnable objects can also be processed using the handlers. Runnable is a command that can be executed. It is used to run the code in other thread.

An example that users handlers and runnable objects in worker thread is given here;

WorkerThreadActivity.class

package com.febi.worker;

import java.io.InputStream;

import java.net.HttpURLConnection;

import java.net.URL;

import android.app.Activity;

import android.app.AlertDialog;

import android.content.Context;

import android.graphics.Bitmap;

import android.graphics.BitmapFactory;

import android.os.Bundle;

import android.os.Handler;

import android.util.Log;

import android.view.View;

import android.widget.ImageView;

public class WorkerThreadActivity extends Activity{

          ImageView image;

          String url="http://icons.iconarchive.com/icons/icons-land/sport/256/Soccer-Ball-icon.png";

    /** Called when the activity is first created. */

    @Override

    public void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.main); 

    }

    public void onClick(View v){ 

        final Handler threadHandler; 

        final Context cbt; 

        cbt = this; 

        threadHandler = new Handler(); 

        new Thread(new Runnable() { 

            @Override 

            public void run() {

                  

                threadHandler.post(new Runnable() { 

                    @Override 

                    public void run() { 

                             InputStream is = null;

                             try{

                             URL ulrn = new URL(url);

                        HttpURLConnection con = (HttpURLConnection)ulrn.openConnection();

                        is = con.getInputStream();

                             }catch(Exception e)

                             {

                                      Log.i("LOGCAT Run", ""+e);

                             }

                        Bitmap bmp = BitmapFactory.decodeStream(is);

                             ImageView image=(ImageView)findViewById(R.id.imageView1);

                             image.setImageBitmap(bmp);

                    } 

                }); 

            } 

        }).start();

       AlertDialog.Builder ab = new AlertDialog.Builder(cbt); 

      ab.setMessage("A message sent from UI thread"); 

      ab.show();

    } 

         

}

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" >

    <TextView

        android:layout_width="fill_parent"

        android:layout_height="wrap_content"

        android:text="@string/hello" />

   

    <Button android:layout_width="100dp"

        android:layout_height="wrap_content"

        android:text="Click Me"

        android:onClick="onClick"

        />

    <ImageView

        android:id="@+id/imageView1"

        android:layout_width="wrap_content"

        android:layout_height="wrap_content"

        />

</LinearLayout>

Set permission for Internet in manifest file ;

<uses-permission android:name="android.permission.INTERNET"/>