The data persistence techniques in Android provide options for balancing speed, efficiency, and robustness:

❑Shared Preferences When storing the UI state, user preferences, or application settings, you want a lightweight mechanism to store a known set of values. Shared Preferences let you save groups of key/value pairs of primitive data as named preferences.

❑Files It’s not pretty, but sometimes writing to, and reading from, files directly is the only way to go. Android lets you create and load files on the device’s internal or external media.

❑SQLite Databases When managed, structured data is the best approach, Android offers the SQLite relational database library. Every application can create its own databases over which it has total control.

❑Content Providers Rather than a storage mechanism in their own right, Content Providers let you expose a well-defined interface for using and sharing private data. You can control access to Content Providers using the standard permission system.

Saving Simple Application Data

There are two lightweight techniques for saving simple application data for Android applications — Shared Preferences and a pair of event handlers used for saving Activity instance details. Both mecha-nisms use a name/value pair (NVP) mechanism to store simple primitive values.

Using SharedPreferences, you can create named maps of key/value pairs within your application that can be shared between application components running in the same Context.

Shared Preferences support the primitive types Boolean, string, float, long, and integer, making them an ideal way to quickly store default values, class instance variables, the current UI state, and user preferences. They are most commonly used to persist data across user sessions and to share settings between application components.

Alternatively, Activities offer the onSaveInstanceState handler. It’s designed specifically to persist the UI state when the Activity becomes eligible for termination by a resource-hungry run time.

The handler works like the Shared Preference mechanism. It offers a Bundle parameter that represents a key/value map of primitive types that can be used to save the Activity’s instance values. This Bundle

is then made available as a parameter passed in to the onCreate and onRestoreInstanceState method handlers.

This UI state Bundle is used to record the values needed for an Activity to provide an identical UI fol-lowing unexpected restarts.

Creating and Saving Preferences

To create or modify a Shared Preference, call getSharedPreferences on the application Context, pass-ing in the name of the Shared Preferences to change. Shared Preferences are shared across an applica-tion’s components but aren’t available to other applications.

To modify a Shared Preference, use the SharedPreferences.Editor class. Get the Editor object by calling edit on the SharedPreferences object you want to change. To save edits, call commit on the Editor, as shown in the code snippet below.

public static final String MYPREFS = “mySharedPreferences”;

protected void savePreferences(){

Create or retrieve the shared preference object. int mode = Activity.MODE_PRIVATE;

SharedPreferences mySharedPreferences = getSharedPreferences(MYPREFS,

mode);

Retrieve an editor to modify the shared preferences. SharedPreferences.Editor editor = mySharedPreferences.edit();

Store new primitive types in the shared preferences object. editor.putBoolean(“isTrue”, true); editor.putFloat(“lastFloat”, 1f); editor.putInt(“wholeNumber”, 2); editor.putLong(“aNumber”, 3l); editor.putString(“textEntryValue”, “Not Empty”);

//Commit the changes.

editor.commit();

}

Retrieving Shared Preferences

Accessing saved Shared Preferences is also done with the getSharedPreferences method. Pass in the name of the Shared Preference you want to access, and use the type-safe get<type> methods to extract saved values.

Each getter takes a key and a default value (used when no value is available for that key), as shown in the skeleton code below:

public void loadPreferences() {

// Get the stored preferences

int mode = Activity.MODE_PRIVATE;

SharedPreferences mySharedPreferences = getSharedPreferences(MYPREFS,

mode);

// Retrieve the saved values.

boolean isTrue = mySharedPreferences.getBoolean(“isTrue”, false); float lastFloat = mySharedPreferences.getFloat(“lastFloat”, 0f); int wholeNumber = mySharedPreferences.getInt(“wholeNumber”, 1); long aNumber = mySharedPreferences.getLong(“aNumber”, 0); String stringPreference;

stringPreference = mySharedPreferences.getString(“textEntryValue”, “”);

}

Saving the Activity State

If you want to save Activity information that doesn’t need to be shared with other components (e.g., class instance variables), you can call Activity.getPreferences() without specifying a preferences name. Access to the Shared Preferences map returned is restricted to the calling Activity; each Activity supports a single unnamed SharedPreferences object.

The following skeleton code shows how to use the Activity’s private Shared Preferences:

protected void saveActivityPreferences(){

//Create or retrieve the activity preferences object. SharedPreferences 

activityPreferences =

getPreferences(Activity.MODE_PRIVATE);

//Retrieve an editor to modify the shared preferences. 

SharedPreferences.Editor editor = activityPreferences.edit();

Retrieve the View

TextView myTextView = (TextView)findViewById(R.id.myTextView);

Store new primitive types in the shared preferences object. editor.putString(“currentTextValue”,

myTextView.getText().toString());

Commit changes. editor.commit();

}

Saving and Restoring Instance State

To save Activity instance variables, Android offers a specialized alternative to Shared Preferences.

By overriding an Activity’s onSaveInstanceState event handler, you can use its Bundle parameter to save instance values. Store values using the same get and put methods as shown for Shared Pref-erences, before passing the modified Bundle into the superclass’s handler, as shown in the following code snippet:

private static final String TEXTVIEW_STATE_KEY = “TEXTVIEW_STATE_KEY”;

@Override

public void onSaveInstanceState(Bundle outState) { // Retrieve the View

TextView myTextView = (TextView)findViewById(R.id.myTextView);

Save its state outState.putString(TEXTVIEW_STATE_KEY,

myTextView.getText().toString()); super.onSaveInstanceState(outState);

}

This handler will be triggered whenever an Activity completes its Active life cycle, but only when it’s not being explicitly finished. As a result, it’s used to ensure a consistent Activity state between active life cycles of a single user session.

The saved Bundle is passed in to the onRestoreInstanceState and onCreate methods if the applica-tion is forced to restart during a session. The following snippet shows how to extract values from the Bundle and use them to update the Activity instance state:

@Override

public void onCreate(Bundle icicle) {

super.onCreate(icicle);

setContentView(R.layout.main);

TextView myTextView = (TextView)findViewById(R.id.myTextView);

String text = “”;

if (icicle != null && icicle.containsKey(TEXTVIEW_STATE_KEY))

text = icicle.getString(TEXTVIEW_STATE_KEY);

myTextView.setText(text);

}

It’s important to remember that onSaveInstanceState is called only when an Activity becomes inactive, but not when it is being closed by a call to finish or by the user pressing the Back button.

Saving the To-Do List Activity State

Currently, each time the To-Do List example application is restarted, all the to-do items are lost and any text entered into the text entry box is cleared. In this example, you’ll start to save the application state of the To-Do list application across sessions.

The instance state in the ToDoList Activity consists of three variables:

❑Is a new item being added?

❑ What text exists in the new item entry textbox?

❑ What is the currently selected item?

Using the Activity’s default Shared Preference, you can store each of these values and update the UI when the Activity is restarted.

Later in this chapter, you’ll learn how to use the SQLite database to persist the to-do items as well. This example is a first step that shows how to ensure a seamless experience by saving Activity instance details.

Start by adding static String variables to use as preference keys.

private static final String TEXT_ENTRY_KEY = “TEXT_ENTRY_KEY”; private static final String ADDING_ITEM_KEY = “ADDING_ITEM_KEY”; private static final String SELECTED_INDEX_KEY = “SELECTED_INDEX_KEY”;

Next, override the onPause method. Get the Activity’s private Shared Preference object, and get its Editor object.

Using the keys you created in Step 1, store the instance values based on whether a new item is being added and any text in the “new item” Edit Box.

@Override

protected void onPause(){

super.onPause();

Get the activity preferences object. SharedPreferences uiState = getPreferences(0);

Get the preferences editor. SharedPreferences.Editor editor = uiState.edit();

Add the UI state preference values.

editor.putString(TEXT_ENTRY_KEY, myEditText.getText().toString()); editor.putBoolean(ADDING_ITEM_KEY, addingNew);

//Commit the preferences. editor.commit();

}

Write a restoreUIState method that applies the instance values you recorded in Step 2 when the application restarts.

Modify the onCreate method to add a call to the restoreUIState method at the very end.

@Override

public void onCreate(Bundle icicle) {

... existing onCreate logic ... ] restoreUIState();

}

private void restoreUIState() {

// Get the activity preferences object.

SharedPreferences settings = getPreferences(Activity.MODE_PRIVATE);

Read the UI state values, specifying default values. String text = settings.getString(TEXT_ENTRY_KEY, “”); Boolean adding = settings.getBoolean(ADDING_ITEM_KEY, false);

Restore the UI to the previous state.

if (adding) {

addNewItem();

myEditText.setText(text);

}

}

Record the index of the selected item using the onSaveInstanceState / onRestore InstanceState mechanism. It’s then only saved and applied if the application is killed without the user’s explicit instruction.

@Override

public void onSaveInstanceState(Bundle outState) { outState.putInt(SELECTED_INDEX_KEY,

myListView.getSelectedItemPosition());

super.onSaveInstanceState(outState);

}

@Override

public void onRestoreInstanceState(Bundle savedInstanceState) { int pos = -1;

if (savedInstanceState != null)

if (savedInstanceState.containsKey(SELECTED_INDEX_KEY)) pos = savedInstanceState.getInt(SELECTED_INDEX_KEY, -1);

myListView.setSelection(pos);

}

When you run the To-Do List application, you should now see the UI state persisted across sessions. That said, it still won’t persist the to-do list items — you’ll add this essential piece of functionality later in the chapter.