创建Content Provider
Content Provider为公布数据提供了一个接口，别的APP使用Content Resolver来使用该接口所提供的数据。
作为4大组件之一，创建一个新的Content Provider需要继承一个抽象类ContentProvider：
public class MyContentProvider extends ContentProvider
就像之前描述的数据库的Contract/Helper类，也同样最好是去包含静态数据库常量---尤其是列名等。
你需要重写onCreate方法去初始化底层数据源，同样还有重写query,update,delete,insert和getType这些方法，使Content Resolver可以与数据进行交互。
注册Content Provider
像Activity,Service,Content Provider,必须在Mainfest中注册。使用一个provider标签：包含一个name和authorities属性。
authorities属性 去定义Provider的authority的基础URI。Content Provider的authority代表一个数据库。
每个Content Provider的authority必须是唯一的，所以定义基础URI通常以包名作为路径。如：
com.<CompanyName>.provider.<ApplicationName>例子：
公布你的Content ProviderURI地址
每个Content Provider应该使用公有静态常量 CONTENT_URI ,使其更容易被访问。
如：
public static final Uri CONTENT_URI = Uri.parse(“content://com.paad.skeletondatabaseprovider/elements”);
类似与上述这例子表示请求表中所有的记录，而：
表示某行记录。（接数字）
所有这些形式都支持你去访问你的Provider。这么做最简单方式是去使用UriMatcher,一个非常有用得类，用来解析URI和决定它的格式。
// Create the constants used to differentiate between the different URI // requests. private static final int ALLROWS = 1; private static final int SINGLE_ROW = 2;private static final UriMatcher uriMatcher;// Populate the UriMatcher object, where a URI ending in //‘elements’ will correspond to a request for all items, // and ‘elements/[rowID]’ represents a single row. static { uriMatcher = new UriMatcher(UriMatcher.NO_MATCH); uriMatcher.addURI(“com.paad.skeletondatabaseprovider”, “elements”, ALLROWS); uriMatcher.addURI(“com.paad.skeletondatabaseprovider”, “elements/#”, SINGLE_ROW); }
你可能使用类似的技术去暴露某个Content Provider更多的URI，这些URI可以代表不同的数据集,或是不同的表。
创建Content Provider的数据库
private MySQLiteOpenHelper myOpenHelper;@Override public boolean onCreate() { // Construct the underlying database. // Defer opening the database until you need to perform // a query or transaction. myOpenHelper = new MySQLiteOpenHelper(getContext(), MySQLiteOpenHelper.DATABASE_NAME, null, MySQLiteOpenHelper.DATABASE_VERSION); return true; }
注意：Content Provider的onCreate是在主线程中执行的，数据库一旦打开后，若程序还在运行，没必要马上又把它关掉,这出于效率问题。你可能担心资源问题，其实事实是系统如果真需要额外资源，你的APP会被杀死，然后相关的数据库也会被关。
实现Content Provider查询
为了支持Content Provider相关的查询，你必须实现query和getType方法。Content Resolver使用这些方法去获取低沉的数据而无需关心它的细节和实现。
注意：UriMatcher对象通常用来完善事务和查询的请求，SQLite Query Builder更方便作为执行基于行查询的帮手。
例子：实现Content Provider查询的框架代码
@Override public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) { // Open the database. SQLiteDatabase db; try { db = myOpenHelper.getWritableDatabase(); } catch (SQLiteException ex) { db = myOpenHelper.getReadableDatabase(); } // Replace these with valid SQL statements if necessary. String groupBy = null; String having = null; // Use an SQLite Query Builder to simplify constructing the // database query. SQLiteQueryBuilder queryBuilder = new SQLiteQueryBuilder(); // If this is a row query, limit the result set to the passed in row. switch (uriMatcher.match(uri)) { case SINGLE_ROW : String rowID = uri.getPathSegments().get(1); queryBuilder.appendWhere(KEY_ID + “=” + rowID); default: break; } // Specify the table on which to perform the query. This can // be a specific table or a join as required. queryBuilder.setTables(MySQLiteOpenHelper.DATABASE_TABLE); // Execute the query. Cursor cursor = queryBuilder.query(db, projection, selection, selectionArgs, groupBy, having, sortOrder); // Return the result Cursor. return cursor; }
实现查询后，你必须还要指定一个MIME类型去鉴定返回的数据。重写getType方法，返回一个唯一描述你数据的字符串。
此类型返回必须包含2种形式，其一为单个条目，另外一个为所有的条目：
Single item:
vnd.android.cursor.item/vnd.<companyname>.<contenttype>
All items:
vnd.android.cursor.dir/vnd.<companyname>.<contenttype>
例子：
@Override public String getType(Uri uri) { // Return a string that identifies the MIME type // for a Content Provider URI switch (uriMatcher.match(uri)) { case ALLROWS: return “vnd.android.cursor.dir/vnd.paad.elemental”; case SINGLE_ROW: return “vnd.android.cursor.item/vnd.paad.elemental”; default: throw new IllegalArgumentException(“Unsupported URI: “ + uri); } }
Content Provider事务
像query方法，当然还有delete,insert,update方法，实现后，由Content Resolver调用，从而其它APP就可以使用。
当执行修改数据集的事务，比较好的方式是去调用Content Resolver的notifyChange方法。这个方法会通知所有内容观察者。
怎么用？ 直接看一些框架代码：
@Override public int delete(Uri uri, String selection, String[] selectionArgs) { // Open a read / write database to support the transaction. SQLiteDatabase db = myOpenHelper.getWritableDatabase(); // If this is a row URI, limit the deletion to the specified row. switch (uriMatcher.match(uri)) { case SINGLE_ROW : String rowID = uri.getPathSegments().get(1); selection = KEY_ID + “=” + rowID + (!TextUtils.isEmpty(selection) ? “ AND (“ + selection + ‘)’ : “”); default: break; } // To return the number of deleted items you must specify a where // clause. To delete all rows and return a value pass in “1”. if (selection == null) selection = “1”; // Perform the deletion. int deleteCount = db.delete(MySQLiteOpenHelper.DATABASE_TABLE, selection, selectionArgs); // Notify any observers of the change in the data set. getContext().getContentResolver().notifyChange(uri, null); // Return the number of deleted items. return deleteCount; }@Override public Uri insert(Uri uri, ContentValues values) { // Open a read / write database to support the transaction. SQLiteDatabase db = myOpenHelper.getWritableDatabase(); // To add empty rows to your database by passing in an empty // Content Values object you must use the null column hack // parameter to specify the name of the column that can be // set to null. String nullColumnHack = null; // Insert the values into the table long id = db.insert(MySQLiteOpenHelper.DATABASE_TABLE, nullColumnHack, values); // Construct and return the URI of the newly inserted row. if (id > -1) { // Construct and return the URI of the newly inserted row. Uri insertedId = ContentUris.withAppendedId(CONTENT_URI, id); // Notify any observers of the change in the data set. getContext().getContentResolver().notifyChange(insertedId, null); return insertedId; } else return null; }@Override public int update(Uri uri, ContentValues values, String selection, String[] selectionArgs) { // Open a read / write database to support the transaction. SQLiteDatabase db = myOpenHelper.getWritableDatabase();// If this is a row URI, limit the deletion to the specified row. switch (uriMatcher.match(uri)) { case SINGLE_ROW : String rowID = uri.getPathSegments().get(1); selection = KEY_ID + “=” + rowID + (!TextUtils.isEmpty(selection) ? “ AND (“ + selection + ‘)’ : “”); default: break; }// Perform the update. int updateCount = db.update(MySQLiteOpenHelper.DATABASE_TABLE, values, selection, selectionArgs);// Notify any observers of the change in the data set. getContext().getContentResolver().notifyChange(uri, null); return updateCount; }
Tip:ContentUris类包含一个withAppendedId方法，此方法很方便得帮助构建一个附加指定行ID的Uri。
Content Provider 存储文件
以前提到过，数据库里保存文件，通常建议保存的是路径->一个合适的Uri.
通常表中文件类型的数据列名取名_data形式。
重写openFile方法
@Override public ParcelFileDescriptor openFile(Uri uri, String mode) throws FileNotFoundException { // Find the row ID and use it as a filename. String rowID = uri.getPathSegments().get(1); // Create a file object in the application’s external // files directory. String picsDir = Environment.DIRECTORY_PICTURES; File file = new File(getContext().getExternalFilesDir(picsDir), rowID); // If the file doesn’t exist, create it now. if (!file.exists()) { try { file.createNewFile(); } catch (IOException e) { Log.d(TAG, “File creation failed: “ + e.getMessage()); } } // Translate the mode parameter to the corresponding Parcel File // Descriptor open mode. int fileMode = 0; if (mode.contains(“w”)) fileMode |= ParcelFileDescriptor.MODE_WRITE_ONLY; if (mode.contains(“r”)) fileMode |= ParcelFileDescriptor.MODE_READ_ONLY; if (mode.contains(“+”)) fileMode |= ParcelFileDescriptor.MODE_APPEND; // Return a Parcel File Descriptor that represents the file. return ParcelFileDescriptor.open(file, fileMode); }
完整的Content Provider 实现框架代码：
public class MyContentProvider extends ContentProvider { public static final Uri CONTENT_URI = Uri.parse(“content://com.paad.skeletondatabaseprovider/elements”); // Create the constants used to differentiate between // the different URI requests. private static final int ALLROWS = 1; private static final int SINGLE_ROW = 2; private static final UriMatcher uriMatcher; // Populate the UriMatcher object, where a URI ending // in ‘elements’ will correspond to a request for all // items, and ‘elements/[rowID]’ represents a single row. static { uriMatcher = new UriMatcher(UriMatcher.NO_MATCH); uriMatcher.addURI(“com.paad.skeletondatabaseprovider”, “elements”, ALLROWS); uriMatcher.addURI(“com.paad.skeletondatabaseprovider”, “elements/#”, SINGLE_ROW); }// The index (key) column name for use in where clauses. public static final String KEY_ID = “_id”;// The name and column index of each column in your database. // These should be descriptive. public static final String KEY_COLUMN_1_NAME = “KEY_COLUMN_1_NAME”; // TODO: Create public field for each column in your table.// SQLite Open Helper variable private MySQLiteOpenHelper myOpenHelper;@Override public boolean onCreate() { // Construct the underlying database. // Defer opening the database until you need to perform // a query or transaction. myOpenHelper = new MySQLiteOpenHelper(getContext(), MySQLiteOpenHelper.DATABASE_NAME, null, MySQLiteOpenHelper.DATABASE_VERSION); return true; }@Override public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) { // Open the database. SQLiteDatabase db = myOpenHelper.getWritableDatabase(); // Replace these with valid SQL statements if necessary. String groupBy = null; String having = null; SQLiteQueryBuilder queryBuilder = new SQLiteQueryBuilder(); queryBuilder.setTables(MySQLiteOpenHelper.DATABASE_TABLE); // If this is a row query, limit the result set to the // passed in row. switch (uriMatcher.match(uri)) { case SINGLE_ROW : String rowID = uri.getPathSegments().get(1); queryBuilder.appendWhere(KEY_ID + “=” + rowID); default: break; } Cursor cursor = queryBuilder.query(db, projection, selection, selectionArgs, groupBy, having, sortOrder); return cursor; }@Override public int delete(Uri uri, String selection, String[] selectionArgs) { // Open a read / write database to support the transaction. SQLiteDatabase db = myOpenHelper.getWritableDatabase(); // If this is a row URI, limit the deletion to the specified row. switch (uriMatcher.match(uri)) { case SINGLE_ROW : String rowID = uri.getPathSegments().get(1); selection = KEY_ID + “=” + rowID + (!TextUtils.isEmpty(selection) ? “ AND (“ + selection + ‘)’ : “”); default: break; } // To return the number of deleted items, you must specify a where // clause. To delete all rows and return a value, pass in “1”. if (selection == null) selection = “1”; // Execute the deletion. int deleteCount = db.delete(MySQLiteOpenHelper.DATABASE_TABLE, selection, selectionArgs); // Notify any observers of the change in the data set. getContext().getContentResolver().notifyChange(uri, null); return deleteCount; }@Override public Uri insert(Uri uri, ContentValues values) { // Open a read / write database to support the transaction. SQLiteDatabase db = myOpenHelper.getWritableDatabase(); // To add empty rows to your database by passing in an empty // Content Values object, you must use the null column hack // parameter to specify the name of the column that can be // set to null. String nullColumnHack = null; // Insert the values into the table long id = db.insert(MySQLiteOpenHelper.DATABASE_TABLE, nullColumnHack, values); if (id > -1) { // Construct and return the URI of the newly inserted row. Uri insertedId = ContentUris.withAppendedId(CONTENT_URI, id); // Notify any observers of the change in the data set. getContext().getContentResolver().notifyChange(insertedId, null); return insertedId; } else return null; } @Override public int update(Uri uri, ContentValues values, String selection, String[] selectionArgs) { // Open a read / write database to support the transaction. SQLiteDatabase db = myOpenHelper.getWritableDatabase(); // If this is a row URI, limit the deletion to the specified row. switch (uriMatcher.match(uri)) { case SINGLE_ROW : String rowID = uri.getPathSegments().get(1); selection = KEY_ID + “=” + rowID + (!TextUtils.isEmpty(selection) ? “ AND (“ + selection + ‘)’ : “”); default: break; } // Perform the update. int updateCount = db.update(MySQLiteOpenHelper.DATABASE_TABLE, values, selection, selectionArgs); // Notify any observers of the change in the data set. getContext().getContentResolver().notifyChange(uri, null); return updateCount; } @Override public String getType(Uri uri) { // Return a string that identifies the MIME type // for a Content Provider URI switch (uriMatcher.match(uri)) { case ALLROWS: return “vnd.android.cursor.dir/vnd.paad.elemental”; case SINGLE_ROW: return “vnd.android.cursor.item/vnd.paad.elemental”; default: throw new IllegalArgumentException(“Unsupported URI: “ + uri); } } private static class MySQLiteOpenHelper extends SQLiteOpenHelper { // [ ... SQLite Open Helper Implementation ... ] }}