Android四大组件之一ContentProvider(内容提供者)

ContentProvider为存储和获取数据提供了统一的接口。ContentProvide对数据进行封装，不用关心数据存储的细节，虽然使用其他方法也可以对外共享数据，但数据访问方式

使用ContentProvider可以在不同的应用程序之间共享数据。Android为常见的一些数据提供了默认的ContentProvider（包括音频、视频、图片和通讯录等）。ContentProvider所提供的函数：query(),insert(),update(),delete(),getType(),onCreate()等。

 

For example, to get a list of the words and their locales from the User Dictionary Provider,    you callContentResolver.query().    Thequery() method calls the   ContentProvider.query() method defined by the     User Dictionary Provider. The following lines of code show a   ContentResolver.query() call:

 

 

// Queries the user dictionary and returns resultsmCursor = getContentResolver().query(    UserDictionary.Words.CONTENT_URI,   // The content URI of the words table    mProjection,                        // The columns to return for each row    mSelectionClause                    // Selection criteria    mSelectionArgs,                     // Selection criteria    mSortOrder);                        // The sort order for the returned rows

Table 2 shows how the arguments to    query(Uri,projection,selection,selectionArgs,sortOrder) match an SQL SELECT statement:

在Content Provider中使用的查询字符串有别于标准的SQL查询。

Table 2: Query() compared to SQL query.

URI：

A content URI is a URI that identifies data in a provider. Content URIs    include the symbolic name of the entire provider (itsauthority) and a    name that points to a table (apath). When you call    a client method to access a table in a provider, the content URI for the table is one of    the arguments.

    In the preceding lines of code, the constant    CONTENT_URI contains the content URI of    the user dictionary's "words" table. TheContentResolver    object parses out the URI's authority, and uses it to "resolve" the provider by    comparing the authority to a system table of known providers. The   ContentResolver can then dispatch the query arguments to the correct    provider.

    The ContentProvider uses the path part of the content URI to choose the    table to access. A provider usually has apath for each table it exposes.

    In the previous lines of code, the full URI for the "words" table is:

content://user_dictionary/words

    where the user_dictionary string is the provider's authority, and   words string is the table's path. The string   content:// (thescheme) is always present,    and identifies this as a content URI.

    Many providers allow you to access a single row in a table by appending an ID value    to the end of the URI. For example, to retrieve a row whose_ID is   4 from user dictionary, you can use this content URI:

Uri singleUri = ContentUris.withAppendedId(UserDictionary.Words.CONTENT_URI,4);

 

Inserting, Updating, and Deleting Data

Inserting data

    To insert data into a provider, you call the    ContentResolver.insert()    method. This method inserts a new row into the provider and returns a content URI for that row.    This snippet shows how to insert a new word into the User Dictionary Provider:

 

// Defines a new Uri object that receives the result of the insertionUri mNewUri;...// Defines an object to contain the new values to insertContentValues mNewValues = new ContentValues();/* * Sets the values of each column and inserts the word. The arguments to the "put" * method are "column name" and "value" */mNewValues.put(UserDictionary.Words.APP_ID, "example.user");mNewValues.put(UserDictionary.Words.LOCALE, "en_US");mNewValues.put(UserDictionary.Words.WORD, "insert");mNewValues.put(UserDictionary.Words.FREQUENCY, "100");mNewUri = getContentResolver().insert(    UserDictionary.Word.CONTENT_URI,   // the user dictionary content URI    mNewValues                          // the values to insert);

 

Updating data

    To update a row, you use a ContentValues object with the updated    values just as you do with an insertion, and selection criteria just as you do with a query.    The client method you use is    ContentResolver.update(). You only need to add     values to the ContentValues object for columns you're updating. If you     want to clear the contents of a column, set the value tonull.

    The following snippet changes all the rows whose locale has the language "en" to a    have a locale ofnull. The return value is the number of rows that were updated:

// Defines an object to contain the updated valuesContentValues mUpdateValues = new ContentValues();// Defines selection criteria for the rows you want to updateString mSelectionClause = UserDictionary.Words.LOCALE +  "LIKE ?";String[] mSelectionArgs = {"en_%"};// Defines a variable to contain the number of updated rowsint mRowsUpdated = 0;.../* * Sets the updated value and updates the selected words. */mUpdateValues.putNull(UserDictionary.Words.LOCALE);mRowsUpdated = getContentResolver().update(    UserDictionary.Words.CONTENT_URI,   // the user dictionary content URI    mUpdateValues                       // the columns to update    mSelectionClause                    // the column to select on    mSelectionArgs                      // the value to compare to);

 

 

 

The data for the new row goes into a single ContentValues object, which    is similar in form to a one-row cursor. The columns in this object don't need to have the    same data type, and if you don't want to specify a value at all, you can set a column    tonull usingContentValues.putNull().

    The snippet doesn't add the _ID column, because this column is maintained    automatically. The provider assigns a unique value of_ID to every row that is    added. Providers usually use this value as the table's primary key.

    The content URI returned in newUri identifies the newly-added row, with    the following format:

content://user_dictionary/words/<id_value>

    The <id_value> is the contents of _ID for the new row.    Most providers can detect this form of content URI automatically and then perform the requested    operation on that particular row.

    To get the value of _ID from the returned Uri, call   ContentUris.parseId().

 

Deleting data

    Deleting rows is similar to retrieving row data: you specify selection criteria for the rows    you want to delete and the client method returns the number of deleted rows.    The following snippet deletes rows whose appid matches "user". The method returns the    number of deleted rows.

// Defines selection criteria for the rows you want to deleteString mSelectionClause = UserDictionary.Words.APP_ID + " LIKE ?";String[] mSelectionArgs = {"user"};// Defines a variable to contain the number of rows deletedint mRowsDeleted = 0;...// Deletes the words that match the selection criteriamRowsDeleted = getContentResolver().delete(    UserDictionary.Words.CONTENT_URI,   // the user dictionary content URI   mSelectionClause                    // the column to select on     mSelectionArgs                      // the value to compare to);