Lập trình Androi part 33 ppsx

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" CHAPTER 21: Using Preferences 220 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" /> </PreferenceCategory> <PreferenceCategory android:title="Detail Screens"> <PreferenceScreen android:key="detail" android:title="Detail Screen" android:summary="Additional preferences held in another page"> <CheckBoxPreference android:key="checkbox2" android:title="Another Checkbox" android:summary="On. Off. It really doesn't matter." /> </PreferenceScreen> </PreferenceCategory> </PreferenceScreen> The result, when you use this preference XML with your PreferenceActivity implementation, is a categorized list of elements, as shown in Figure 21–4. Figure 21–4. The Structured project's preference UI, showing categories and a screen placeholder If you tap the Detail Screen entry, you are taken to the child preference screen, as shown in Figure 21–5. CHAPTER 21: Using Preferences 221 Figure 21–5. The child preference screen of the Structured project's preference UI The Kind of Pop-Ups You Like Of course, not all preferences are check boxes and ringtones. For others, like entry fields and lists, Android uses pop-up dialogs. Users do not enter their preference directly in the preference UI activity, but rather tap a preference, fill in a value, and click OK to commit the change. Structurally, in the preference XML, fields and lists are not significantly different from other preference types, as seen in this preference XML from the Prefs/Dialogs sample project: <PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android"> <PreferenceCategory android:title="Simple Preferences"> <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" /> </PreferenceCategory> <PreferenceCategory android:title="Detail Screens"> <PreferenceScreen android:key="detail" CHAPTER 21: Using Preferences 222 android:title="Detail Screen" android:summary="Additional preferences held in another page"> <CheckBoxPreference android:key="checkbox2" android:title="Another Checkbox" android:summary="On. Off. It really doesn't matter." /> </PreferenceScreen> </PreferenceCategory> <PreferenceCategory android:title="Simple Preferences"> <EditTextPreference android:key="text" android:title="Text Entry Dialog" android:summary="Click to pop up a field for entry" android:dialogTitle="Enter something useful" /> <ListPreference android:key="list" android:title="Selection Dialog" android:summary="Click to pop up a list to choose from" android:entries="@array/cities" android:entryValues="@array/airport_codes" android:dialogTitle="Choose a Pennsylvania city" /> </PreferenceCategory> </PreferenceScreen> With the field (EditTextPreference), in addition to the title and summary you put on the preference itself, you can also supply the title to use for the dialog. With the list (ListPreference), you supply both a dialog title and two string-array resources: one for the display names and one for the values. These need to be in the same order, because the index of the chosen display name determines which value is stored as the preference in the SharedPreferences. For example, here are the arrays for use by the ListPreference shown in the preceding example: <?xml version="1.0" encoding="utf-8"?> <resources> <string-array name="cities"> <item>Philadelphia</item> <item>Pittsburgh</item> <item>Allentown/Bethlehem</item> <item>Erie</item> <item>Reading</item> <item>Scranton</item> <item>Lancaster</item> <item>Altoona</item> <item>Harrisburg</item> </string-array> <string-array name="airport_codes"> <item>PHL</item> <item>PIT</item> <item>ABE</item> <item>ERI</item> <item>RDG</item> <item>AVP</item> <item>LNS</item> <item>AOO</item> CHAPTER 21: Using Preferences 223 <item>MDT</item> </string-array> </resources> When you bring up the preference UI, you start with another category with another pair of preference entries, as shown in Figure 21–6. Figure 21–6. The preference screen of the Dialogs project's preference UI Tapping the Text Entry Dialog entry brings up a text-entry dialog with the prior preference entry already filled in, as shown in Figure 21–7. Figure 21–7. Editing a text preference CHAPTER 21: Using Preferences 224 Tapping Selection Dialog brings up a selection dialog showing the display names from the one array, as shown in Figure 21–8. Figure 21–8. Editing a list preference . <PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android"> <PreferenceCategory android:title="Simple Preferences"> <CheckBoxPreference android:key="checkbox". <PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android"> <PreferenceCategory android:title="Simple Preferences"> <CheckBoxPreference android:key="checkbox". android:showDefault="true" android:showSilent="true" android:summary="Pick a tone, any tone" /> </PreferenceCategory> <PreferenceCategory android:title="Detail