Content Providers--怎样去访问一个基本的content provider

一个content provider 管理着怎样去访问存储在应用仓库中的数据。一个content provider是一个应用程序的一部分，它常常用来使应用程序的界面工作在它提供的数据基础上。然而，content providers 主要是用来给其它的应用程序使用的，使用一个provider客户端对象来访问使用它。总之，providers和provider 客户端对象为应用中的数据提供一个一致的、标准的访问接口，并且它也能够处理跨进程间的通信和敏感数据的访问

Accessing a provider

我们可以使用 ContentResolver 客户端对象去访问一个content provider，我们可以用 ContentResolver 实现对content provider提供的数据源进行CRUD

 ContentResolver 对象是运行在应用的主线程中的，有可能会阻塞主线程，它不用做过多的操作就可以进行跨进程间的通信，它担当着本身仓库数据和外部表数据的沟通者

当访问一个provider时，你需要在manifest文件中增加Content Provider Permissions权限声明

我们可以使用URI去定位一个content provider,使用SQL语句描述我们要进行的操作

data as table words
wordapp idfrequencylocale_IDmapreduceuser1100en_US1precompileruser14200fr_FR2appletuser2225fr_CA3constuser1255pt_BR4intuser5100en_UK5

查询操作

     

     当我们进行查询时，并不像我们用一个url去定位一个资源时那样，一股脑的把所有的数据都返回回来，而是返回一个Cursor对象，让我们可以更明智的获取数据

     SQL : select _ID,word,locale from words where word = ?

private void accessContract(String mSearchString){                          Log. e(TAG, mSearchString);                           //uri 定位             Uri uri = UserDictionary.Words. CONTENT_URI;              //access a single row in a table              //Uri singleUri = ContentUris.withAppendedId(UserDictionary.Words.CONTENT_URI, 3);                           // A "projection" defines the columns that will be returned for each row              // 要查询的字段             String[] mProjection =             {                 UserDictionary.Words. _ID,    // Contract class constant for the _ID column name                 UserDictionary.Words. WORD,   // Contract class constant for the word column name                 UserDictionary.Words. LOCALE  // Contract class constant for the locale column name             };              // Defines a string to contain the selection clause              // 查询条件             String mSelectionClause = null;              // Initializes an array to contain selection arguments              // 查询条件的值             String[] mSelectionArgs = { ""};                           // Remember to insert code here to check for invalid or malicious input.              // If the word is the empty string, gets everything              if (TextUtils.isEmpty(mSearchString)) {                 // Setting the selection clause to null will return all words                 mSelectionClause = null;                 mSelectionArgs[0] = "";             } else {                 // Constructs a selection clause that matches the word that the user entered.                 mSelectionClause = UserDictionary.Words. WORD + " = ?" ;                 // Moves the user's input string to the selection arguments.                 mSelectionArgs[0] = mSearchString;             }                           // Queries the user dictionary and returns results             Cursor mCursor = getContentResolver().query(                    uri,   // The content URI of the words table                 mProjection,                        // The columns to return for each row                 mSelectionClause,                    // Selection criteria                 //null,                 mSelectionArgs,                     // Selection criteria                 //null,                 UserDictionary.Words. WORD);                        // The sort order for the returned rows                           //last generate sql              //SELECT _ID, word, frequency, locale FROM words WHERE word = <userinput> ORDER BY word ASC;                           // Some providers return null if an error occurs, others throw an exception              if (null == mCursor) {                 /*                  * Insert code here to handle the error. Be sure not to use the cursor! You may want to                  * call android.util.Log.e() to log this error.                  *                  */              // If the Cursor is empty, the provider found no matches                                        Log. e(TAG, "mCursor is null");             } else if (mCursor.getCount() < 1) {                 /*                  * Insert code here to notify the user that the search was unsuccessful. This isn't necessarily                  * an error. You may want to offer the user the option to insert a new row, or re-type the                  * search term.                  */                    Log. e(TAG, "mCursor is empty");             } else {                 // Insert code here to do something with the results                    Log. e(TAG, "mCursor is not empty");                     //display(mCursor);             }                           //mCursor.close();       }

插入操作

     用ContentValues封装要插入的字段和值

     SQL: insert into words(_ID,locale,word,frequency) values (?,?,?,?)

       private void insert(){              // Defines a new Uri object that receives the result of the insertion             Uri mNewUri;              //...              // Defines an object to contain the new values to insert             ContentValues 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.Words. CONTENT_URI,   // the user dictionary content URI                 mNewValues                          // the values to insert             );                          Log. e(TAG, mNewUri.getPath());       }

更新操作      

SQL: update words set locale=? where locale=?

          private void update(){              // Defines an object to contain the updated values             ContentValues mUpdateValues = new ContentValues();              // Defines selection criteria for the rows you want to update             String mSelectionClause = UserDictionary.Words.LOCALE +  " LIKE ?";             String[] mSelectionArgs = { "en_%"};              // Defines a variable to contain the number of updated rows              int 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             );                    Log. e(TAG, "effect row "+mRowsUpdated);                    }

删除操作

SQL:delete from words where word=?

        private void delete(){              // Defines selection criteria for the rows you want to delete             String mSelectionClause = UserDictionary.Words. WORD + " = ?" ;             String[] mSelectionArgs = { "insert"};              // Defines a variable to contain the number of rows deleted              int mRowsDeleted = 0;              //...              // Deletes the words that match the selection criteria             mRowsDeleted = getContentResolver().delete(                 UserDictionary.Words. CONTENT_URI,   // the user dictionary content URI                 mSelectionClause,                    // the column to select on                 mSelectionArgs                      // the value to compare to             );                          Log. e(TAG, "delete effect row "+mRowsDeleted);       }

使用SimpleCursorAdapter方便的来显示一个数据源数据

             private void display(Cursor mCursor) {              // Defines a list of columns to retrieve from the Cursor and load into an output row             String[] mWordListColumns =             {                 UserDictionary.Words. WORD,   // Contract class constant containing the word column name                 UserDictionary.Words. LOCALE  // Contract class constant containing the locale column name             };              // Defines a list of View IDs that will receive the Cursor columns for each row              int[] mWordListItems = { R.id.dictWord, R.id.locale};              // Creates a new SimpleCursorAdapter             SimpleCursorAdapter mCursorAdapter = new SimpleCursorAdapter(                 getApplicationContext(),               // The application's Context object                 R.layout. wordlist_item,                  // A layout in XML for one row in the ListView                 mCursor,                               // The result from the query                 mWordListColumns,                      // A string array of column names in the cursor                 mWordListItems                        // An integer array of view IDs in the row layout                 );                                    // Flags (usually none are needed)              // Sets the adapter for the ListView              mWordList.setAdapter(mCursorAdapter);       }