# AsyncTask in Android.

What is AsyncTask?
AsyncTask is an abstract Android class which helps the Android applications to handle the Main UI thread in efficient way. AsyncTask class allows us to perform long lasting tasks/background operations and show the result on the UI thread without affecting the main thread.

When to use AsyncTask?
Assume you have created a simple Android application which downloads Images file from Internet on launching the application.

How to implement AsyncTask in Android applications?

Create a new class inside Activity class and subclass it by extending AsyncTask as shown below

private classMyTask extends AsyncTask<URL, Integer, Long> {

 protected Long doInBackground(URL... urls) {

      //Yet to code

     }

 protected void onProgressUpdate(Integer... progress) {

     //Yet to code

     }

 protected void onPostExecute(Long result) {

     //Yet to code

     }

}

Execute the task simply by invoking execute method as shown below:

onPreExecute:

 Invoked before the task is executed ideally before doInBackground method is called on the UI thread. This method is normally used to setup the task like showing progress bar in the UI. 


doInBackground:  

Code running for long lasting time should be put in doInBackground method. When execute method is called in UI main thread, this method is called with the parameters passed. 


onProgressUpdate: 

 Invoked by calling publishProgress at anytime from doInBackground. This method can be used to display any form of progress in the user interface. 


onPostExecute:

 Invoked after background computation in doInBackground method completes processing. Result of the doInBackground is passed to this method. 


Task Cancellation

 The task can be cancelled by invoking cancel(boolean) method. This will cause subsequent calls to isCancelled() to return true. After invoking this method, onCancelled(Object) method is called instead of onPostExecute() after doInBackground() returns. 


AsyncTask – Rules to be followed 

The AsyncTask class must be loaded on the UI thread.
 The task instance must be created on the UI thread
 Method execute(Params…) must be invoked on the UI thread. 
 Should not call onPreExecute(), onPostExecute(Result), doInBackground(Params…), onProgressUpdate(Progress…) manually. 
 The task can be executed only once (an exception will be thrown if a second execution is attempted.)








Create New Projects Name AsyncTask

AsyncTask\app\src\main\res\layout



1.activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">


    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/download_img"
        android:layout_gravity="center_vertical" />

    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/download_next"
        android:layout_gravity="right" />

    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/download_nextimg"
        android:layout_gravity="center_horizontal" />
    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/download_nexttimeimg"
        android:layout_gravity="center_horizontal" />


    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Download"
        android:id="@+id/button"
        android:onClick="downloadImage" />

</LinearLayout>



2.AsyncTask\app\src\main\java\com\problemhunt\tanuj\asynctask:

MainActivity.java



package com.problemhunt.tanuj.asynctask;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.ImageView;

public class MainActivity extends AppCompatActivity {

    private ImageView mDownloadImgV;
    private ImageView mDownloadNext;
    private ImageView mDownloadImgNext;
    private ImageView mDownloadNextTimeImg;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mDownloadImgV=(ImageView) findViewById(R.id.download_img);
        mDownloadNext=(ImageView) findViewById(R.id.download_next);
        mDownloadImgNext=(ImageView)  findViewById(R.id.download_nextimg);
        mDownloadNextTimeImg=(ImageView) findViewById(R.id.download_nexttimeimg);
    }

    public void downloadImage(View view) {
        MyTask myTask=new MyTask(mDownloadImgV,mDownloadNext,mDownloadImgNext,mDownloadNextTimeImg,MainActivity.this);
        myTask.execute("http://images.indianexpress.com/2015/07/kalam_pranab_759.jpg","http://images.indianexpress.com/2015/07/kalam_dd_759.jpg","http://cdn.narendramodi.in/cmsuploads/0.34592100-1450848819-indian-pm-narendra-modi-russia-remains-our-principal-partner.jpg","http://astrotarot.net/wp-content/uploads/2015/09/army.jpg");
           }


    }




Create Class MyTask.java



 package com.smartpropertyhunt.tanuj.asynctask;

import android.app.ProgressDialog;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.util.Log;
import android.widget.ImageView;

import java.io.BufferedInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.URL;
import java.net.URLConnection;
import java.util.ArrayList;

/**
 * Created by Tanuj on 06/09/2016.
 */
public class MyTask extends AsyncTask<String,String,ArrayList<Bitmap>> {
    ImageView mImageView;
    ImageView  vImageView;
    ImageView nImageView;
    ImageView iImageView;
    private Context mContext;
    private ProgressDialog mprogressDialog;
    ArrayList<Bitmap> marry= new ArrayList<Bitmap>();


    public MyTask(ImageView mImageView,ImageView  vImageView,  ImageView nImageView,ImageView iImageView,Context mContext) {
        this.mImageView = mImageView;
        this.vImageView=vImageView;
        this.nImageView=nImageView;
        this.iImageView=iImageView;
        this.mContext = mContext;
        mprogressDialog = new ProgressDialog(mContext);

    }

    @Override
    protected ArrayList<Bitmap> doInBackground(String... params) {

        Bitmap bm = null;
        for (int i=0;i<=3;i++)
        {
            try {
                URL aURL = new URL(params[i]);
                URLConnection conn = aURL.openConnection();
                conn.connect();
                InputStream is = conn.getInputStream();
                BufferedInputStream bis = new BufferedInputStream(is);
                bm = BitmapFactory.decodeStream(bis);
                marry.add(bm);
                bis.close();
                is.close();
            } catch (IOException e) {
                Log.e("Hub", "Error getting the image from server : " + e.getMessage().toString());
            }
        }

        return marry;
    }

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        mprogressDialog.setMessage("Loading Image...");
        mprogressDialog.show();
    }

    @Override
    protected void onProgressUpdate(String... values) {
        super.onProgressUpdate(values);
    }

    @Override
    protected void onPostExecute(ArrayList<Bitmap> bitmaps){
        super.onPostExecute(bitmaps);
        mImageView.setImageBitmap(marry.get(0));
        vImageView.setImageBitmap(marry.get(1));
        nImageView.setImageBitmap(marry.get(2));
        iImageView.setImageBitmap(marry.get(3));
        mprogressDialog.dismiss();

    }
}



 