创建自己的Android content provider

在以前的一篇博客中，曾将介绍了如何使用content provider, 如果需要创建你自己的cotent provider,也就是定制自己特有的，你需要做的其实很简单，那就是从抽象类ContentProvider派生出你自己的类，并且实现里面所有的抽象方法，包括：

• query(Uri, String[], String, String[], String) which returns data to the caller
• insert(Uri, ContentValues) which inserts new data into the content provider
• update(Uri, ContentValues, String, String[]) which updates existing data in the content provider
• delete(Uri, String, String[]) which deletes data from the content provider
• getType(Uri) which returns the MIME type of data in the content provider

在你的content provider里面，你有随意选择存储媒介的权利，你可以用传统文件，xml，甚至是一些特定的web service, 当然你也可以选择用的最为广泛的SQLite

 

在你自己的类中，你需要定义一个public static final Uri 命名为CONTENT_URI。

 

具体方法，还是通过一个例子来说明吧

public class BooksProvider extends ContentProvider

{
   public static final String PROVIDER_NAME =
      "MyContentProvider";  

   public static final Uri CONTENT_URI =
      Uri.parse("content://"+ PROVIDER_NAME + "/books");  

   public static final String _ID = "_id";
   public static final String TITLE = "title";
   public static final String ISBN = "isbn";
   private static final int BOOKS = 1;
   private static final int BOOK_ID = 2;     
   private static final UriMatcher uriMatcher;

   static{
      uriMatcher = new UriMatcher(UriMatcher.NO_MATCH);
      uriMatcher.addURI(PROVIDER_NAME, "books", BOOKS);
      uriMatcher.addURI(PROVIDER_NAME, "books/#", BOOK_ID);
   } 

  

   //---for database use---
   private SQLiteDatabase booksDB;
   private static final String DATABASE_NAME = "Books";
   private static final String DATABASE_TABLE = "titles";
   private static final int DATABASE_VERSION = 1;
   private static final String DATABASE_CREATE =
         "create table " + DATABASE_TABLE +
         " (_id integer primary key autoincrement, "
         + "title text not null, isbn text not null);";  

   @Override
   public boolean onCreate() {
      Context context = getContext();
      DatabaseHelper dbHelper = new DatabaseHelper(context);
      booksDB = dbHelper.getWritableDatabase();
      return (booksDB == null)? false:true;
   }
  
   private static class DatabaseHelper extends SQLiteOpenHelper
   {
      DatabaseHelper(Context context) {
         super(context, DATABASE_NAME, null, DATABASE_VERSION);
      }

 
      @Override
      public void onCreate(SQLiteDatabase db)
      {
         db.execSQL(DATABASE_CREATE);
      }
  
      @Override
      public void onUpgrade(SQLiteDatabase db, int oldVersion,
      int newVersion) {
         Log.w("Content provider database",
              "Upgrading database from version " +
              oldVersion + " to " + newVersion +
              ", which will destroy all old data");
         db.execSQL("DROP TABLE IF EXISTS titles");
         onCreate(db);
      }
   } 
  
   @Override
   public String getType(Uri uri) {
      switch (uriMatcher.match(uri)){
         //---get all books---
         case BOOKS:
            return "vnd.android.cursor.dir/vnd.learn2develop.books ";
         //---get a particular book---
         case BOOK_ID:
            return "vnd.android.cursor.item/vnd.learn2develop.books ";
         default:
            throw new IllegalArgumentException("Unsupported URI: " + uri);
      }
   }

   @Override
   public Cursor query(Uri uri, String[] projection, String selection,
      String[] selectionArgs, String sortOrder) {    

      SQLiteQueryBuilder sqlBuilder = new SQLiteQueryBuilder();
      sqlBuilder.setTables(DATABASE_TABLE);    

      if (uriMatcher.match(uri) == BOOK_ID)
         //---if getting a particular book---
         sqlBuilder.appendWhere(
            _ID + " = " + uri.getPathSegments().get(1));       

      if (sortOrder==null || sortOrder=="")
         sortOrder = TITLE;

         Cursor c = sqlBuilder.query(
         booksDB,
         projection,
         selection,
         selectionArgs,
         null,
         null,
         sortOrder);

      //---register to watch a content URI for changes---
      c.setNotificationUri(getContext().getContentResolver(), uri);
      return c;
   }
  
   @Override
   public Uri insert(Uri uri, ContentValues values) {
      //---add a new book---
      long rowID = booksDB.insert(
         DATABASE_TABLE, "", values);           

      //---if added successfully---
      if (rowID>0)
      {
         Uri _uri = ContentUris.withAppendedId(CONTENT_URI, rowID);
         getContext().getContentResolver().notifyChange(_uri,null); 
         return _uri;
      }

      throw new SQLException("Failed to insert row into " + uri);
   }
  
   @Override
   public int delete(Uri arg0, String arg1, String[] arg2) {
      // arg0 = uri 
      // arg1 = selection
      // arg2 = selectionArgs

      int count=0;
      switch (uriMatcher.match(arg0)){
         case BOOKS:
            count = booksDB.delete(
               DATABASE_TABLE,
               arg1,
               arg2);

            break;

         case BOOK_ID:
            String id = arg0.getPathSegments().get(1);
            count = booksDB.delete(
               DATABASE_TABLE,
               _ID + " = " + id +
               (!TextUtils.isEmpty(arg1) ? " AND (" +
               arg1 + ')' : ""),
               arg2);
            break;

         default: throw new IllegalArgumentException(
            "Unknown URI " + arg0); 

      } 

      getContext().getContentResolver().notifyChange(arg0, null);

      return count; 

   }

 

   @Override

   public int update(Uri uri, ContentValues values, 

      String selection, String[] selectionArgs) 

   {

      int count = 0;

      switch (uriMatcher.match(uri)){

         case BOOKS:

            count = booksDB.update(

               DATABASE_TABLE, 

               values,

               selection, 

               selectionArgs);

            break;

         case BOOK_ID: 

            count = booksDB.update(

               DATABASE_TABLE, 

               values,

               _ID + " = " + uri.getPathSegments().get(1) + 

               (!TextUtils.isEmpty(selection) ? " AND (" + 

                  selection + ')' : ""), 

                selectionArgs);

            break;

         default: throw new IllegalArgumentException(

            "Unknown URI " + uri); 

      } 

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

      return count;

   }
}

 

需要强调一点的是，如果想要建立一个自己的content provider，需要自己建立起一个UriMatcher objects 树。其中UriMatcher是一个android自带的类，用来辅助匹配当前content provider中的URI.

其中建立匹配树，需要调用UriMatcher的addURI方法：

static{
      uriMatcher = new UriMatcher(UriMatcher.NO_MATCH);
      uriMatcher.addURI(PROVIDER_NAME, "books", BOOKS);
      uriMatcher.addURI(PROVIDER_NAME, "books/#", BOOK_ID);
   }

 

当需要匹配时，需要调用uriMatcher.match方法。例如：

public String getType(Uri uri) {
      switch (uriMatcher.match(uri)){
         //---get all books---

         case BOOKS:
            return "vnd.android.cursor.dir/vnd.learn2develop.books ";
         //---get a particular book---
         case BOOK_ID:
            return "vnd.android.cursor.item/vnd.learn2develop.books ";
         default:
            throw new IllegalArgumentException("Unsupported URI: " + uri);
      }
   }