213 213 Chapter Using Preferences Android has many different ways for you to store data for long-term use by your activity. The simplest to use is the preferences system, which is the topic of this chapter. Android allows activities and applications to keep preferences, in the form of key/value pairs (akin to a Map), which will hang around between invocations of an activity. As the name suggests, the primary purpose is for you to store user-specified configuration details, such as the last feed the user looked at in your feed reader, the sort order to use by default on a list, or whatever. Of course, you can store in the preferences whatever you like, as long as it is keyed by a String and has a primitive value (boolean, String, etc.) Preferences can be for a single activity or shared among all activities in an application. (Eventually, preferences might be shareable across applications, but that is not supported as of the time of this writing.) Getting What You Want To get access to the preferences, you can use the following APIs:  getPreferences() from within your Activity, to access activity- specific preferences  getSharedPreferences() from within your Activity (or other application Context), to access application-level preferences  getDefaultSharedPreferences(), on PreferencesManager, to get the shared preferences that work in concert with Android’s overall preference framework The first two take a security mode parameter; for now, pass in 0. The getSharedPreferences() method also takes a name of a set of preferences. getPreferences() effectively calls getSharedPreferences() with the activity’s class name as the preference set name. The getDefaultSharedPreferences() method takes the Context for the preferences (e.g., your Activity). 21 CHAPTER 21: Using Preferences 214 All of these methods return an instance of SharedPreferences, which offers a series of getters to access named preferences, returning a suitably typed result (e.g., getBoolean() to return a Boolean preference). The getters also take a default value, which is returned if there is no preference set under the specified key. Stating Your Preference Given the appropriate SharedPreferences object, you can use edit() to get an editor for the preferences. This object has a set of setters that mirror the getters on the parent SharedPreferences object. It also has the following methods:  remove(): Deletes a single named preference.  clear(): Deletes all preferences.  commit(): Persists your changes made via the editor. The commit() method is important. If you modify preferences via the editor and fail to commit() the changes, those changes will evaporate once the editor goes out of scope. Conversely, since the preferences object supports live changes, if one part of your application (say, an activity) modifies shared preferences, another part of your application (say, a service) will have access to the changed value immediately. And Now, a Word from Our Framework Beginning with the 0.9 SDK, Android has introduced a framework for managing preferences. Ironically, this framework does not change anything shown so far. Instead, the framework is more for presenting a consistent set of preference-setting options for users, so different applications do not need to reinvent the wheel. The linchpin to the preferences framework is yet another XML data structure. You can describe your application’s preferences in an XML file stored in your project’s res/xml/ directory. Given that, Android can present a pleasant UI for manipulating those preferences, which are then stored in the SharedPreferences you get back from getDefaultSharedPreferences(). The following is the preference XML for the Prefs/Simple preferences sample project: <PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android"> <CheckBoxPreference android:key="checkbox" android:title="Checkbox Preference" android:summary="Check it on, check it off" /> <RingtonePreference android:key="ringtone" android:title="Ringtone Preference" android:showDefault="true" android:showSilent="true" CHAPTER 21: Using Preferences 215 android:summary="Pick a tone, any tone" /> </PreferenceScreen> The root of the preference XML is a PreferenceScreen element. You will see why it is named that later in this chapter. For now, take it on faith that it is a sensible name. Some of the things you can have inside a PreferenceScreen element, not surprisingly, are preference definitions. These are subclasses of Preference, such as CheckBoxPreference or RingtonePreference, as shown in the preceding XML. As you might expect, these allow you to check a check box or choose a ringtone, respectively. In the case of RingtonePreference, you have the option of allowing users to choose the system default ringtone or to choose “silence” as a ringtone. Letting Users Have Their Say Given that you have set up the preference XML, you can use a nearly built-in activity for allowing your users to set their preferences. The activity is “nearly built-in” because you merely need to subclass it and point it to your preference XML, plus hook the activity into the rest of your application. For example, here is the EditPreferences activity of the Prefs/Simple project: package com.commonsware.android.simple; import android.app.Activity; import android.os.Bundle; import android.preference.PreferenceActivity; public class EditPreferences extends PreferenceActivity { @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); addPreferencesFromResource(R.xml.preferences); } } As you can see, there is not much to see. All you need to do is call addPreferencesFromResource() and specify the XML resource containing your preferences. You will also need to add this as an activity to your AndroidManifest.xml file: <?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.commonsware.android.simple"> <application android:label="@string/app_name" android:icon="@drawable/cw"> <activity android:name=".SimplePrefsDemo" android:label="@string/app_name"> <intent-filter> <action android:name="android.intent.action.MAIN" /> CHAPTER 21: Using Preferences 216 <category android:name="android.intent.category.LAUNCHER" /> </intent-filter> </activity> <activity android:name=".EditPreferences" android:label="@string/app_name"> </activity> </application> </manifest> And you will need to arrange to invoke the activity, such as from a menu option, here pulled from SimplePrefsDemo: public boolean onCreateOptionsMenu(Menu menu) { menu.add(Menu.NONE, EDIT_ID, Menu.NONE, "Edit Prefs") .setIcon(R.drawable.misc) .setAlphabeticShortcut('e'); return(super.onCreateOptionsMenu(menu)); } @Override public boolean onOptionsItemSelected(MenuItem item) { switch (item.getItemId()) { case EDIT_ID: startActivity(new Intent(this, EditPreferences.class)); return(true); } return(super.onOptionsItemSelected(item)); } } That is all that is required, and it really is not that much code outside the preferences XML. What you get for your effort is an Android-supplied preference UI, as shown in Figure 21–1. The check box can be directly checked or unchecked. To change the ringtone preference, just select the entry in the preference list to bring up a selection dialog, as shown in Figure 21–2. CHAPTER 21: Using Preferences 217 Figure 21–1. The Simple project's preference UI Figure 21–2. Choosing a ringtone preference Notice that there is no explicit save or commit button or menu. Any changes are persisted as soon as they are made. The SimplePrefsDemo activity, beyond having the aforementioned menu, also displays the current preferences via a TableLayout: CHAPTER 21: Using Preferences 218 <?xml version="1.0" encoding="utf-8"?> <TableLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="fill_parent" android:layout_height="fill_parent" > <TableRow> <TextView android:text="Checkbox:" android:paddingRight="5px" /> <TextView android:id="@+id/checkbox" /> </TableRow> <TableRow> <TextView android:text="Ringtone:" android:paddingRight="5px" /> <TextView android:id="@+id/ringtone" /> </TableRow> </TableLayout> The fields for the table are found in onCreate(): public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); checkbox=(TextView)findViewById(R.id.checkbox); ringtone=(TextView)findViewById(R.id.ringtone); } The fields are updated on each onResume(): public void onResume() { super.onResume(); SharedPreferences prefs=PreferenceManager .getDefaultSharedPreferences(this); checkbox.setText(new Boolean(prefs .getBoolean("checkbox", false)) .toString()); ringtone.setText(prefs.getString("ringtone", "<unset>")); } This means that the fields will be updated when the activity is opened and after the preferences activity is left (e.g., via the back button), as shown in Figure 21–3. CHAPTER 21: Using Preferences 219 Figure 21–3. The Simple project's list of saved preferences Adding a Wee Bit o' Structure If you have a lot of preferences for users to set, putting them all in one big list may not be the best idea. Android’s preference framework gives you a few ways to impose a bit of structure on your bag of preferences, including categories and screens. Categories are added via a PreferenceCategory element in your preference XML and are used to group together related preferences. Rather than have your preferences all as children of the root PreferenceScreen, you can place a few PreferenceCategory elements in the PreferenceScreen, and then put your preferences in their appropriate categories. Visually, this adds a divider with the category title between groups of preferences. If you have a whole lot of preferences—more than are convenient for users to scroll through—you can also put them on separate “screens” by introducing the PreferenceScreen element. Yes, that PreferenceScreen element. Any children of PreferenceScreen go on their own screen. If you nest PreferenceScreens, the parent screen displays the screen as a placeholder entry, and tapping that entry brings up the child screen. For example, from the Prefs/Structured sample project, here is a preference XML file that contains both PreferenceCategory and nested PreferenceScreen elements: <PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android"> <PreferenceCategory android:title="Simple Preferences"> <CheckBoxPreference android:key="checkbox" . xmlns:android="http://schemas.android.com/apk/res/android"> <CheckBoxPreference android:key="checkbox" android:title="Checkbox Preference" android:summary="Check. encoding="utf-8"?> <TableLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="fill_parent" android:layout_height="fill_parent". <PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android"> <PreferenceCategory android:title="Simple Preferences"> <CheckBoxPreference android:key="checkbox"