Lập trình Androi part 32 pptx

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.. Preferences can

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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)

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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"

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" />

<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

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:

<?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

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"