Android四大组件之ContentProvider

ContentProvider

安卓应用程序默认是无法获取到其他程序的数据，这是安卓安全学的基石(沙盒原理)。但是经常我们需要给其他应用分享数据，内容提供者就是一个这种可以分享数据给其他应用的接口。可以简单的理解为，内容提供者就是一个可以在不同应用程序间共享数据的组件，相当于一个中间人，一个程序把数据暴露给这个中间人，另一个则通过这个中间人获取相应的数据.

下面的这张图片能更直观的显示：

ContentProvider中的getContext和AndroidTestCast中的getContext方法一样，都是一个模拟的上下文，必须在该类初始化之后才会调用setContext方法将context设置成自己的成员变量中记录，所以对于获取getContext的时候只能放在方法内，不能放到成员位置，因为在成员上时是null，而在方法内调用时该类就会已经初始化完了
ContentProvider中的query()后不能关闭数据库，因为其他的应用在调用该query方法时需要继续使用该返回值Cursor，所以不能关闭数据库，因为数据库关闭之后Cursor就不能用了， Cursor中保存的数据其实是数据库的一个引用，如果数据库关了Cursor就不能找到里面的数据了，Cursor.close()只是释放·Cursor·用到的资源。说到这里就多数一句 According to Dianne Hackborn (Android framework engineer) there is no need to close the database in a content provider.以为内容提供者是因为进程启动时便加载，之后就一直存在，当进程销毁释放资源时会去关闭数据库。
如果数据是SQLiteDatabase，表中必须有一个_id的列，用来表示每条记录的唯一性。

继承ContentProvider,并实现相应的方法。

public class NoteProvider extends ContentProvider {
	private static final int NOTES = 1;
	private static final int NOTE_ID = 2;
	public static final String AUTHORITY = "com.charon.demo.provider.noteprovider";
	public static final String TABLE_NAME = "note";
	// 定义一个名为`CONTENT_URI`必须为其指定一个唯一的字符串值，最好的方案是以类的全名称
	public static final Uri CONTENT_URI = Uri.parse("content://" + AUTHORITY + "/" + TABLE_NAME);
	// 声明一个路径的检查者，参数为Uri不匹配时的返回值
	// 虽然是中间人，但也不能谁要数据我们都给，所以要检查下，只有符合我们要求的人，我们才会给他数据。
	private static UriMatcher sUriMatcher = new UriMatcher(UriMatcher.NO_MATCH);
	private NoteSQLiteOpenHelper mSQLiteOpenHelper;
	private SQLiteDatabase mSQLiteDatabase;
	static {
		// 建立匹配规则，例如发现路径为ccom.charon.demo.provider.noteprovider/note/1表示要操作note表中id为1的记录
		sUriMatcher.addURI(AUTHORITY, TABLE_NAME, NOTES);
		sUriMatcher.addURI(AUTHORITY, TABLE_NAME + "/#", NOTE_ID);
	}
	/**
	 * Implement this to initialize your content provider on startup.
	 * This method is called for all registered content providers on the
	 * application main thread at application launch time.  It must not perform
	 * lengthy operations, or application startup will be delayed.
	 * <p/>
	 * <p>You should defer nontrivial initialization (such as opening,
	 * upgrading, and scanning databases) until the content provider is used
	 * (via {@link #query}, {@link #insert}, etc).  Deferred initialization
	 * keeps application startup fast, avoids unnecessary work if the provider
	 * turns out not to be needed, and stops database errors (such as a full
	 * disk) from halting application launch.
	 * <p/>
	 * <p>If you use SQLite, {@link android.database.sqlite.SQLiteOpenHelper}
	 * is a helpful utility class that makes it easy to manage databases,
	 * and will automatically defer opening until first use.  If you do use
	 * SQLiteOpenHelper, make sure to avoid calling
	 * {@link android.database.sqlite.SQLiteOpenHelper#getReadableDatabase} or
	 * {@link android.database.sqlite.SQLiteOpenHelper#getWritableDatabase}
	 * from this method.  (Instead, override
	 * {@link android.database.sqlite.SQLiteOpenHelper#onOpen} to initialize the
	 * database when it is first opened.)
	 *
	 * @return true if the provider was successfully loaded, false otherwise
	 */
	@Override
	public boolean onCreate() {
		mSQLiteOpenHelper = new NoteSQLiteOpenHelper(getContext());
		return true;
	}
	/**
	 * 内容提供者暴露的查询的方法.
	 */
	@Override
	public Cursor query(Uri uri, String[] projection, String selection,
						String[] selectionArgs, String sortOrder) {
		mSQLiteDatabase = mSQLiteOpenHelper.getReadableDatabase();
		Cursor cursor;
		// 1.重要的事情 ,检查 uri的路径.
		switch (sUriMatcher.match(uri)) {
			case NOTES:
				break;
			case NOTE_ID:
				String id = uri.getLastPathSegment();
				if (TextUtils.isEmpty(selection)) {
					selection = selection + "_id = " + id;
				} else {
					selection = selection + " and " + "_id = " + id;
				}
				break;
			default:
				throw new IllegalArgumentException("UnKnown Uri" + uri);
				break;
		}
		cursor = mSQLiteDatabase.query(TABLE_NAME, projection, selection, selectionArgs, null, null, sortOrder);
		if (cursor != null) {
			cursor.setNotificationUri(getContext().getContentResolver(), uri);
		}
		return cursor;
	}
	/**
	 * Implement this to handle requests for the MIME type of the data at the
	 * given URI.  The returned MIME type should start with
	 * <code>vnd.android.cursor.item</code> for a single record,
	 * or <code>vnd.android.cursor.dir/</code> for multiple items.
	 * This method can be called from multiple threads, as described in
	 * <a href="{@docRoot}guide/topics/fundamentals/processes-and-threads.html#Threads">Processes
	 * and Threads</a>.
	 * <p/>
	 * <p>Note that there are no permissions needed for an application to
	 * access this information; if your content provider requires read and/or
	 * write permissions, or is not exported, all applications can still call
	 * this method regardless of their access permissions.  This allows them
	 * to retrieve the MIME type for a URI when dispatching intents.
	 *
	 * @param uri the URI to query.
	 * @return a MIME type string, or {@code null} if there is no type.
	 */
	@Override
	public String getType(Uri uri) {
		// 注释说的很清楚了，下面是常用的格式
		// 单个记录的IMEI类型 vnd.android.cursor.item/vnd.<yourcompanyname>.<contenttype>
		// 多个记录的IMEI类型 vnd.android.cursor.dir/vnd.<yourcompanyname>.<contenttype>
		switch (sUriMatcher.match(uri)) {
			case NOTE_ID:
				// 如果uri为 content://com.charon.demo.noteprovider/note/1
				return "vnd.android.cursor.item/vnd.charon.note";
			case NOTES:
				return "vnd.android.cursor.dir/vnd.charon.note";
			default:
				return null;
		}
		// 这个MIME类型的作用是要匹配AndroidManifest.xml文件<activity>标签下<intent-filter>标签的子标签<data>的属性android:mimeType。
		// 如果不一致，则会导致对应的Activity无法启动。
	}
	@Override
	public Uri insert(Uri uri, ContentValues values) {
		mSQLiteDatabase = mSQLiteOpenHelper.getWritableDatabase();
		switch (sUriMatcher.match(uri)) {
			case NOTES:
				break;
			case NOTE_ID:
				break;
			default:
				throw new IllegalArgumentException("UnKnown Uri" + uri);
				break;
		}
		long rowId = mSQLiteDatabase.insert(TABLE_NAME, null, values);
		if (rowId > 0) {
			Uri noteUri = ContentUris.withAppendedId(CONTENT_URI, rowId);
			getContext().getContentResolver().notifyChange(noteUri, null);
			return noteUri;
		}
		return null;
	}
	@Override
	public int delete(Uri uri, String selection, String[] selectionArgs) {
		mSQLiteDatabase = mSQLiteOpenHelper.getWritableDatabase();
		switch (sUriMatcher.match(uri)) {
			case NOTES:
				break;
			case NOTE_ID:
				String id = uri.getLastPathSegment();
				if (TextUtils.isEmpty(selection)) {
					selection = selection + "_id = " + id;
				} else {
					selection = selection + " and " + "_id = " + id;
				}
				break;
			default:
				throw new IllegalArgumentException("UnKnown Uri" + uri);
				break;
		}
		int count = mSQLiteDatabase.delete(TABLE_NAME, selection, selectionArgs);
		getContext().getContentResolver().notifyChange(uri, null);
		return count;
	}
	@Override
	public int update(Uri uri, ContentValues values, String selection,
					  String[] selectionArgs) {
		switch (sUriMatcher.match(uri)) {
			case NOTES:
				break;
			case NOTE_ID:
				break;
			default:
				throw new IllegalArgumentException("UnKnown Uri" + uri);
				break;
		}
		mSQLiteDatabase = mSQLiteOpenHelper.getWritableDatabase();
		int update = mSQLiteDatabase.update(TABLE_NAME, values, selection, selectionArgs);
		return update;
	}

在清单文件中进行注册，并且指定其authorities

 <provider
	android:name="com.charon.demo.provider.NoteProvider"
	android:authorities="com.charon.demo.provider.noteprovider" >

使用内容提供者获取数据，使用ContentResolver去操作ContentProvider, ContentResolver用于管理ContentProvider实例，并且可实现找到指定的ContentProvider并获取里面的数据

public void query(View view){
	//得到内容提供者的解析器  中间人
	ContentResolver resolver = getContentResolver();
	Cursor cursor = resolver.query(NoteProvider.CONTENT_URI, null, null, null, null);
	while(cursor.moveToNext()){
		String name = cursor.getString(cursor.getColumnIndex("name"));
		int id = cursor.getInt(cursor.getColumnIndex("id"));
		float money = cursor.getFloat(cursor.getColumnIndex("money"));
		System.out.println("id="+id+",name="+name+",money="+money);
	}
	cursor.close();
}
public void insert(View view){
	ContentResolver resolver = getContentResolver();
	ContentValues values = new ContentValues();
	values.put("name", "买洗头膏");
	values.put("money", 22.58f);
	resolver.insert(NoteProvider.CONTENT_URI, values);
}
public void update(View view){
	ContentResolver resolver = getContentResolver();
	ContentValues values = new ContentValues();
	values.put("name", "买洗头膏");
	values.put("money", 42.58f);
	resolver.update(NoteProvider.CONTENT_URI, values, "name=?", new String[]{"买洗头膏"});
}
public void delete(View view){
	ContentResolver resolver = getContentResolver();
	resolver.delete(NoteProvider.CONTENT_URI, "name=?", new String[]{"买洗头膏"});
}

内容观察者

内容观察者的原理： How a content provider actually stores its data under the covers is up to its designer. But all content providers implement a common interface for querying the provider and returning results — as well as for adding, altering, and deleting data. It's an interface that clients use indirectly, most generally through ContentResolver objects. You get a ContentResolver by calling getContentResolver() from within the implementation of an Activity or other application component: You can then use the ContentResolver's methods to interact with whatever content providers you're interested in.

一方使用内容观察者去观察变化

protected void onCreate(Bundle savedInstanceState) {
	super.onCreate(savedInstanceState);
	setContentView(R.layout.activity_main);
	ContentResolver resolver = getContentResolver();
	resolver.registerContentObserver(NoteProvider.CONTENT_URI, true, new NoteObserver(new Handler()));
}
private class NoteObserver extends ContentObserver {
	public NoteObserver(Handler handler) {
		super(handler);
		
	}
	//当观察到数据发生变化的时候  会执行onchange方法.
	@Override
	public void onChange(boolean selfChange) {
		super.onChange(selfChange);
		Log.i(TAG,"发现有新的短信产生了...");
		//1.利用内容提供者  中间人 获取用户的短信数据.
		ContentResolver resolver  = getContentResolver();
		// .. 重新查询
		cursor = ...;
		cursor.close();
	}
}

一方在发生变化的时候去发送改变的消息对于一些系统的内容提供者内部都实现了该步骤，如果是自己写程序想要暴露就必须要加上该代码。

getContext().getContentResolver().notifyChange(uri, null);