Question

Exercise 10-4   Move a task to an asychronous thread

In this exercise, you’ll modify an app that downloads an image file from the Internet, writes that image to a file, and reads that image from the file.

Review and test the app

1. Start Android Studio and open the project named ch10_ex4_TestAsync.
2. Review the code for this app. Note that it uses an asynchronous task to download an image file from the Internet.
1. Run the app. When it starts, it should briefly display the standard Android image. Then, it should download another image from the Internet, write it to a file, read that file, and display the image in an ImageView widget.

Move a task from the UI thread to another thread

2. Create an asynchronous class named ReadFile that you can use to read and display the image file that’s stored on the device.
3. Move all code within the readFile method to the doInBackground method of the ReadFile class. Then, delete the readFile class and clean up the code so there are no syntax errors.
4. Run the app. You should get an error that indicates that you can’t update the UI thread from the background thread.
5. Modify the declaration of the ReadFile class so it returns a Drawable object.
6. Modify the doInBackground method so it returns a Drawable object.
7. Modify the onPostExecute method so it uses the Drawable object to display the image.
8. Run the app. It should work as before. However, the app now uses a separate thread to read the image file.

Answer 1

/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" >

    <ImageView

        android:id="@+id/imageView1"

        android:layout_width="wrap_content"

        android:layout_height="wrap_content"

        android:src="@drawable/android" />        

</LinearLayout>

--------------------------------------------------------------------------------------------------------------

MyAndroidAppActivity.java

import android.graphics.Bitmap;

import android.graphics.BitmapFactory;

import android.support.v7.app.AppCompatActivity;

import android.os.Bundle;

import android.widget.ImageView;

public class ReadFile extends AppCompatActivity, AsyncTask<URL, Integer, Long>

{

    @Override

    protected void readFile(Bundle savedInstanceState) {

      File imgFile = new File("/storage/emulated/0/DCIM/Camera/IMG_20151102_193132.jpg");

      if(imgFile.exists()){

        Bitmap myBitmap = BitmapFactory.decodeFile(imgFile.getAbsolutePath());

        ImageView myImage = (ImageView) findViewById(R.id.imageviewTest);

        myImage.setImageBitmap(myBitmap);

      }

   Drawable d = getResources().getDrawable(android.R.drawable.ic_dialog);

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

   image.setImageDrawable(d);

onPostExecute(d);

    }

protected Long doInBackground(URL urls) {
int count = urls.length;
long totalSize = 0;
for (int i = 0; i < count; i++) {
totalSize += Downloader.downloadFile(urls[i]);
publishProgress((int) ((i / (float) count) * 100));

onProgressUpdate(i);

if (isCancelled()) break;
}
return totalSize;
}

protected void onProgressUpdate(Integer  progress) {
setProgressPercent(progress[0]);
}

protected void onPostExecute(Drawable result) {
showDialog("Downloaded " + result + " bytes");
}

}