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Storing and retrieving data
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case WIDGETS: count = db.update(WidgetProvider.DB_TABLE, values, selection, selectionArgs); break; case WIDGET: String segment = uri.getPathSegments().get(1); String where = ""; if (!TextUtils.isEmpty(selection)) { where = " AND (" + selection + ")"; } count = db.update(WidgetProvider.DB_TABLE, values, "_id=" + segment + where, selectionArgs); break; default: throw new IllegalArgumentException("Unknown URI " + uri); } getContext().getContentResolver().notifyChange(uri, null); return count; } @Override public int delete( Uri uri, String selection, String[] selectionArgs) { int count; switch (WidgetProvider.URI_MATCHER.match(uri)) { case WIDGETS: count = db.delete(WidgetProvider.DB_TABLE, selection, selectionArgs); break; case WIDGET: String segment = uri.getPathSegments().get(1); String where = ""; if (!TextUtils.isEmpty(selection)) { where = " AND (" + selection + ")"; } count = db.delete(WidgetProvider.DB_TABLE, "_id=" + segment + where, selectionArgs); break; default: throw new IllegalArgumentException("Unknown URI " + uri); } getContext().getContentResolver().notifyChange(uri, null); return count; } }
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The last part of our WidgetProvider class shows how to implement the ContentProvider methods. First, we use a SQLQueryBuilder inside the query method to append the projection map passed in B and any SQL clauses, along with the correct URI based on our matcher C, before we make the actual query and get a handle on a Cursor to return D. At the end of the query method, we use the setNotificationUri method to watch the returned Uri for changes E. This event-based mechanism keeps track of when Cursor data items change, regardless of who changes them.
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Working with ContentProvider classes
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Next, you see the insert method, where we validate the passed-in ContentValues object and populate it with default values, if the values aren t present F. After we have the values, we call the database insert method G and get the resulting Uri to return with the appended ID of the new record H. After the insert is complete, we use another notification system, this time for ContentResolver. Because we ve made a data change, we inform the ContentResolver what happened so that any registered listeners can be updated I. After completing the insert method, we come to the update J and delete 1) methods. These methods repeat many of the previous concepts. First, they match the Uri passed in to a single element or the set, and then they call the respective update and delete methods on the database object. Again, at the end of these methods, we notify listeners that the data has changed. Implementing the needed provider methods completes our class. After we register this provider with the platform, any application can use it to query, insert, update, or delete data. Registration occurs in the application manifest, which we ll look at next.
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PROVIDER MANIFESTS
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Content providers must be defined in an application manifest file and installed on the platform so the platform can learn that they re available and what data types they offer. The following listing shows the manifest for our provider.
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WidgetProvider AndroidManifest.xml file
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< xml version="1.0" encoding="utf-8" > <manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.msi.manning.chapter5.widget"> <application android:icon="@drawable/icon" android:label="@string/app_short_name"> <activity android:name=".WidgetExplorer" android:label="@string/app_name"> <intent-filter> <action android:name="android.intent.action.MAIN" /> <category android:name= "android.intent.category.LAUNCHER" /> </intent-filter> </activity> <provider android:name="WidgetProvider" Declare android:authorities= provider s authority "com.msi.manning.chapter5.Widget" /> </application> </manifest>
The <provider> element B defines the class that implements the provider and associates a particular authority with that class. A completed project that supports inserting, retrieving, updating, and deleting records rounds out our exploration of using and building ContentProvider classes. And with that, we ve also now demonstrated the ways to locally store and retrieve data on the Android platform.
Storing and retrieving data
Additional ContentProvider manifest properties
The properties of a ContentProvider can configure several important settings beyond the basics, such as specific permissions, initialization order, multiprocess capability, and more. Though most ContentProvider implementations won t need to delve into these details, you should still keep them in mind. For complete and upto-date ContentProvider properties, see the SDK documentation.
Summary
From a simple SharedPreferences mechanism to file storage, databases, and finally the concept of a ContentProvider, Android provides myriad ways for applications to retrieve and store data. As we discussed in this chapter, several storage types can share data across application and process boundaries, and several can t. You can create SharedPreferences with a permissions mode, allowing the flexibility to keep things private, or to share data globally with read-only or read-write permissions. The filesystem provides more flexible and powerful data storage for a single application. Android also provides a relational database system based on SQLite. Use this lightweight, speedy, and capable system for local data persistence within a single application. To share data, you can still use a database, but you need to expose an interface through a ContentProvider. Providers expose data types and operations through a URI-based approach. In this chapter, we examined each of the data paths available to an Android application. You built several small, focused sample applications to use preferences and the filesystem, and you expanded the WeatherReporter sample application that you began in the last chapter. This Android application uses a SQLite database to access and persist data. You also built your own custom content provider from the ground up. To expand your Android horizons beyond data, we ll move on to general networking in the next chapter. We ll cover networking basics and the networking APIs Android provides. We ll also expand on the data concepts we ve covered in this chapter to use the network itself as a data source.
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