Tag: ContentResolver

SQLite Databases

SQLite databases are commonly used for on-device storage of data. Databases can be created and manipulated using the standard SQL commands. The following example uses a Contract class file to define the names of the table columns and the URI for the Content Authority, which would allow a third-party app access to the data in your database if you wished it to be a Content Provider:

Create a Contract class:
import android.content.ContentResolver;
import android.content.ContentUris;
import android.net.Uri;
import android.provider.BaseColumns;

public class ExampleContract implements BaseColumns {

    // This is the content authority for our app Content Provider.
    public static final String CONTENT_AUTHORITY = "com.example.android.ourapp";

    // This is the {@link Uri} on which all other Uris for our app are built.
    public static final Uri BASE_CONTENT_URI = Uri.parse("content://" + CONTENT_AUTHORITY);

    // The path for a particular database of data e.g. 'data'
    public static final String PATH_DATA = "data";

    // This is the {@link Uri} used to get a full list of names and ages.
    public static final Uri CONTENT_URI =
            BASE_CONTENT_URI.buildUpon().appendPath(PATH_DATA).build();


    // This is a String type that denotes a Uri references a list or directory.
    public static final String CONTENT_TYPE =
            ContentResolver.CURSOR_DIR_BASE_TYPE + "/" + CONTENT_AUTHORITY + "/" + PATH_TERMS;

    // This is a String type that denotes a Uri references a single item.
    public static final String CONTENT_ITEM_TYPE =
            ContentResolver.CURSOR_ITEM_BASE_TYPE + "/" + CONTENT_AUTHORITY + "/" + PATH_TERMS;


    // Declaring all these as constants makes code a lot more readable.
    // It also looks a more like SQL.

    // This is the version of the database
    public static final int DATABASE_VERSION = 1;

    // This is the name of the SQL table for data.
    public static final String DATA_TABLE = "data_entries";
    // This is the name of the SQL database for data.
    public static final String DATABASE_NAME = "data";

    // This is the column name in the SQLiteDatabase for our first column, 'name'.
    public static final String COLUMN_NAME = "name";
    // This is the column name in the SQLiteDatabase for our first column, 'age'.
    public static final String COLUMN_AGE = "age";

    // This is an array containing all the column headers in the data table.
    public static final String[] COLUMNS =
            {_ID, COLUMN_NAME, COLUMN_AGE};

    // This is the index of the ID in the data table
    public static final int COLUMN_INDEX_ID = 0;
    // This is the index of the name in the data table
    public static final int COLUMN_INDEX_NAME = 1;
    // This is the index of the age in the data table
    public static final int COLUMN_INDEX_AGE = 2;

    /**
     * This method creates a {@link Uri} for a single piece of data, referenced by id.
     * @param id The id of the term.
     * @return The Uri with the appended id.
     */
    public static Uri buildDataUriWithId(long id) {
        return ContentUris.withAppendedId(CONTENT_URI, id);
    }
}



Create a helper class to handle the SQL commands (in separate class file):


public class DataDbHelper extends SQLiteOpenHelper {
    public static final String DATABASE_NAME = "data.db";
    private static final int DATABASE_VERSION = 1;

    public DataDbHelper(@Nullable Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        /*
         * This String will contain a simple SQL statement that will create a table that will
         * cache our data.
         */
        final String SQL_CREATE_DATA_TABLE =

                "CREATE TABLE " + ExampleContract.DATA_TABLE + " (" +

                        /*
                         * Our contract did not explicitly declare a column called "_ID". However,
                         * it implements the interface "BaseColumns" which does have a field
                         * named "_ID". We use that here to designate our table's primary key.
                         */
                        ExampleContract._ID               + " INTEGER PRIMARY KEY AUTOINCREMENT, " +
                        ExampleContract.COLUMN_NAME       + " VARCHAR, "                 +
                        ExampleContract.COLUMN_AGE + " VARCHAR" + ");";

        /*
         * After we've spelled out our SQLite table creation statement above, we actually execute
         * that SQL with the execSQL method of our SQLite database object.
         */
        db.execSQL(SQL_CREATE_DATA_TABLE);
    }

    /**
     * This example database is only a cache for online data, so its upgrade policy is simply to discard
     * the data and call through to onCreate to recreate the table. Note that this only fires if
     * you change the version number for your database (in our case, DATABASE_VERSION). It does NOT
     * depend on the version number for your application found in your app/build.gradle file. If
     * you want to update the schema without wiping data, commenting out the current body of this
     * method should be your top priority before modifying this method.
     *
     * @param sqLiteDatabase Database that is being upgraded
     * @param oldVersion     The old database version
     * @param newVersion     The new database version
     */

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("DROP TABLE IF EXISTS " + ExampleContract.DATA_TABLE);
        onCreate(db);
    }
}



To make use of this database use a Cursor

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Cursors

This interface provides random read-write access to the result set returned by a database query. 

The cursor begins at position -1 (before the first row). On moveToNext() it will point to the first row and return either true (next row exists) or false (already at end of rows). Other options include moveToPrevious(), moveToFirst() and moveToLast(). getColumnIndex (String columnName) will return the index of the column with the given name (columns are numbered beginning with 0). Use one of the get methods to retrieve the actual value (e.g. getString(int columnIndex), getInt(int columnIndex)), passing in the column index as an argument. getCount() returns the number of rows that are in the Cursor. close() should be called when you are finished interacting with the Cursor to prevent memory leaks leading to app crashes.
The following code snippet iterates through all of the rows in the Cursor and prints out what was in the table:
int wordCol = cursor.getColumnIndex(ExampleContract.COLUMN_WORD);
int defCol = cursor.getColumnIndex(ExampleContract.COLUMN_DEFINITION);

while (cursor.moveToNext()) {
  String word = cursor.getString(wordCol);
  String definition = cursor.getString(defCol);
  Log.v("Cursor Example", word + " - " + definition);
}

cursor.close();



This code could be run in the postExecute() method of an AsyncTask so it has access to the data from the Cursor.

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Content Resolver

Use a Content Resolver to handle the SQL commands (query(), insert(), update() or delete()) to a Content Provider’s data source. Since this can be a demanding task it should not be run on the main thread (e.g. AsyncTask):

Content URI (e.g. content://com.example.android.exampleapp/data) is comprised of:
Content Provider Prefix: content://
Content Authority: com.example.android.exampleapp
Path to Specific Data: data

Against each argument is the SQL equivalent. For query():
ContentResolver resolver = getContentResolver();
Cursor cursor = resolver.query // returns a Cursor object which contains all of the information requested. query() can be replaced with insert/update/delete()
    (ExampleAppContract.CONTENT_URI, // FROM table_name. Developer for Content Provider should have specified this constant in their app
     null, // Projection - col,col,col,... The columns to return for each row
     null, // selection - WHERE col = value. specifies the criteria for selecting rows.
     null, // selectionArgs - No exact equivalent. Selection arguments replace ? placeholders in the selection clause.
     null); // sortOrder - ORDER BY col,col,...



For insert():


ContentResolver resolver = getContentResolver();
Cursor cursor = resolver.insert
    (ExampleAppContract.CONTENT_URI, // FROM table_name. Developer for Content Provider should have specified this constant in their app
     null); // ContentValues - the values to be inserted in array of key-value pairings (key is column name)



For update():


ContentResolver resolver = getContentResolver();
Cursor cursor = resolver.update
    (ExampleAppContract.CONTENT_URI, // FROM table_name. Developer for Content Provider should have specified this constant in their app
     null, // ContentValues - the values to be updated in array of key-value pairings (key is column name)
     null, // selection - WHERE filter specifies the criteria for selecting rows.
     null); // selectionArgs - No exact equivalent. Selection arguments replace ? placeholders in the selection clause.



For delete():


ContentResolver resolver = getContentResolver();
Cursor cursor = resolver.delete
    (ExampleAppContract.CONTENT_URI, // FROM table_name. Developer for Content Provider should have specified this constant in their app
     null, // selection - WHERE filter specifies the criteria for selecting rows.
     null); // selectionArgs - No exact equivalent. Selection arguments replace ? placeholders in the selection clause.



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