Night Dreaming (by Sudar)

This week’s homework in the android class was to persist the task information in the sample TaskManager application across program runs, which was created during the class.

Preference API

Android provides a preference API, using which we can store task information. The preference API is very simple to use and all we have to do is to get the instance to the preference object from the Application object and then call the getString() and putString() method.

Serializing ArrayList to string

The only tricky part of the home work is that, since the preference API supports only storing and retrieving of strings, we have to serialize and de-serialize the ArrayList object which has the list of tasks into string.

Instead of writing my own code to do this conversion, I used the code from the Apache Pig project. You can check out the class from the pig’s github page.

Storing the task

In the addTask() method of the TaskManagerApplication class, we have to get the instance of the shared preference and then store the serialized ArrayList using the putString() method.

	public void addTask(Task t) {
		assert(null != t);
		if (null == currentTasks) {
			currentTasks = new ArrayList<task>();
		}
		currentTasks.add(t);

		//save the task list to preference
		SharedPreferences prefs = getSharedPreferences(SHARED_PREFS_FILE, Context.MODE_PRIVATE);
		Editor editor = prefs.edit();
		try {
			editor.putString(TASKS, ObjectSerializer.serialize(currentTasks));
		} catch (IOException e) {
			e.printStackTrace();
		}
		editor.commit();
	}

Retrieving the task

Similarly we have to retrieve the list of tasks from the preference in the onCreate() method

	public void onCreate() {
		super.onCreate();
		if (null == currentTasks) {
			currentTasks = new ArrayList<task>();
		}

		//		load tasks from preference
		SharedPreferences prefs = getSharedPreferences(SHARED_PREFS_FILE, Context.MODE_PRIVATE);

		try {
			currentTasks = (ArrayList<task>) ObjectSerializer.deserialize(prefs.getString(TASKS, ObjectSerializer.serialize(new ArrayList<task>())));
		} catch (IOException e) {
			e.printStackTrace();
		} catch (ClassNotFoundException e) {
			e.printStackTrace();
		}
	}

Finishing up

So that’s it, you are done. 🙂 All you have to do now is to save the project and run it in the emulator. If everything is done properly, the tasks that you enter will be saved even after you restart the app.

Source code

I have uploaded the entire project source code into github and you download it from there and verify it with your code.

Try to complete the homework, before the next session and do come back to view the notes and the homework for the next session too. 🙂

You can also subscribe to my blog’s RSS feed or follow me in Twitter to receive updates about my notes for the next sessions.

Like last week, I attended the session on Developing Android applications in Java by Creative Techs and O’Reilly and here are my notes which I took during the session.

Demo Application

In this week’s session, he created a sample task manager app which can be used for maintaining list of things to track. This app was created using Android 1.6 with Google API support. You can download the source code of this demo app below.

The sample app will have two activities. One will have a simple data entry form which can be used to add tasks and the other to view tasks which were entered. Check the screenshots below to see how the sample app looks like.

Layouts

Tony (the instructor) briefly explained about the different types of layouts and for the sample apps he used Relative Layout. You can read more about the relative layout from the android documentation.

The major pain point in using Relative layout is that, the controls should be specified in the order in which they are referenced and not in the order in which they will be displayed.

EditText control

EditText control is an editable control which can be used to get user input. It is similar to the HTML textbox or the Java Swing JTextField.

You can read more about EditText control from android documentation.

Sharing data between views

An application can have multiple activities (views) and to share data between these multiple activities, the android framework provides a class called Application. This Application class can be accessed from all the activities of the app by calling the getApplication() of the Activity class. Tony explained this about this class and also used it in the demo app to store and retrieve tasks from multiple activities.

You can find this class in the TaskManagerApplication.java in the sample app. You can download the source code of this demo app below.

Safe cancelling

Tony also explained about how to listen to text changes and make sure the user is not moving away from the activity when there are unsaved work.

You can check the cancel() method in AddTaskActivity class, where we will be showing an alert box (see screenshot) using the built in AlertDialog, whenever the user clicks the cancel button without saving the task that he has entered.

You can read about the AlertDialog class in the android documentation.

Source code

I have uploaded source code for yesterday’s session in github and you can download it from there. I am also working in this week’s homework and will be posting the explanation and source code once I am done.

You can also subscribe to my blog’s RSS feed or follow me in Twitter to receive updates about my notes for the next sessions.

As you know, I am following the free course on Developing Android applications in Java by CreativeTech and O’Reilly (even you should, if you are interested in developing apps for android) and this week’s homework in the course is to create an app which will show the phone dial screen when a button is clicked.

I just finished it and I thought of posting the source code and explanation so that it will be useful for others too.

Creating the project

The first step is to create the android project. You should follow the instructions given in the android documentation. You would need Eclipse and the android SDK to be installed to do this.

You will have an empty project to start with and with the default project structure, which I explained in my previous post.

Adding the button to the view

If you have followed the first session then you know that adding a button to the view is quite easy. All you have to do is to open your view file (/res/layout/main.xml) and add the following code.

<Button
    android:id="@+id/dialer_button"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="@string/dialer"
/>

You should also declare the value for the @string/dialer key in your values (/res/values/strings.xml) file by adding the following line

<string name="dialer">Phone Dialer</string>

Adding the button to the activity

After adding the button to the view, we have to declare the button in the activity. To do so, we have to add the following line in the Activity file.

Button dialerButton = (Button)findViewById(R.id.dialer_button);

Bind the Listener to the button

After creating the button instance in the activity, we have to bind a click listener to it. In the click listener we have to invoke the Phone dialer intent. Add the following code to the activity

dialerButton.setOnClickListener(new OnClickListener() {
	@Override
	public void onClick(View v) {
		//open the phone dialer on clicking the button
		Intent intent = new Intent(Intent.ACTION_DIAL);
		startActivity(intent);
	}
});

In the above snippet the following line does the trick

Intent intent = new Intent(Intent.ACTION_DIAL);

It creates the intent, which will open the phone dialer.

Finishing up

So that’s it, you are done 🙂 All you have to do now is to save the project and run it in the emulator. If everything is done properly, you will see the following screen in the emulator.

When you click the button, it should open the phone dialer.

Source code

I have uploaded the entire project source code into github and you download it from there and verify it with your code.

Try to complete the homework, before the next session and do come back to view the notes and the homework for the next session too. 🙂

You can also subscribe to my blog’s RSS feed or follow me in Twitter to receive updates about my notes for the next sessions.