从Notepad实例学习Content Providers

Content providers是程序间共享数据的唯一方法。Android在 android.provider 包下提供了一些访问音频，视频，图像，个人联系信息等常用数据的Content providers，我们可以直接使用这些类来访问这些数据。如果想要分享自己的数据给别人，需要实现自定义的Content Provider，这通过继承类 ContentProvider 来实现。通过学习SDK下面Notepad的源码来学习如何自定义Content Provider。

 创建Content Provider分三个步骤：

1. 建立一个存储数据的系统，android中的大多数都是使用SQLite数据库（本文不涉及其它方式）

2. 扩展 ContentProvider 类访问数据

3. 在程序的AndroidManifest.xml文件中声明。

继承ContentProvider需要实现这六个方法：

query()
insert()
update()
delete()
getType()
onCreate()

注意：因为ContentProvider可能被不同的进程和线程调用，所以这些方法必须是线程安全的。

package com.example.android.notepad;

import android.net.Uri;
import android.provider.BaseColumns;

/**
 * Convenience definitions for NotePadProvider
 * Content Provider的基础类，该类定义了每一列的名字，内容Uri，默认排序方式等常量
 */
public final class NotePad {
    public static final String AUTHORITY = "com.google.provider.NotePad";

    // This class cannot be instantiated
    private NotePad() {}
    
    /**
     * Notes table
     * BaseColumns定义了两个基本字段_ID(每一行的ID)和_COUNT(每个目录下的行数)
     */
    public static final class Notes implements BaseColumns {
        // This class cannot be instantiated
        private Notes() {}

        /**
         * The content:// style URL for this table
         */
        public static final Uri CONTENT_URI = Uri.parse("content://" + AUTHORITY + "/notes");

        /**
         * The MIME type of {@link #CONTENT_URI} providing a directory of notes.
         * 定义新的MIME类型对应notes的目录
         */
        public static final String CONTENT_TYPE = "vnd.android.cursor.dir/vnd.google.note";

        /**
         * The MIME type of a {@link #CONTENT_URI} sub-directory of a single note.
         * 定义新的MIME类型对应一个note
         */
        public static final String CONTENT_ITEM_TYPE = "vnd.android.cursor.item/vnd.google.note";

        /**
         * The default sort order for this table
         */
        public static final String DEFAULT_SORT_ORDER = "modified DESC";

        /**
         * The title of the note
         * <P>Type: TEXT</P>
         */
        public static final String TITLE = "title";

        /**
         * The note itself
         * <P>Type: TEXT</P>
         */
        public static final String NOTE = "note";

        /**
         * The timestamp for when the note was created
         * <P>Type: INTEGER (long from System.curentTimeMillis())</P>
         */
        public static final String CREATED_DATE = "created";

        /**
         * The timestamp for when the note was last modified
         * <P>Type: INTEGER (long from System.curentTimeMillis())</P>
         */
        public static final String MODIFIED_DATE = "modified";
    }
}

 

package com.example.android.notepad;

import com.example.android.notepad.NotePad.Notes;

import android.content.ContentProvider;
import android.content.ContentUris;
import android.content.ContentValues;
import android.content.Context;
import android.content.UriMatcher;
import android.content.res.Resources;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.database.sqlite.SQLiteQueryBuilder;
import android.net.Uri;
import android.text.TextUtils;
import android.util.Log;

import java.util.HashMap;

/**
 * Provides access to a database of notes. Each note has a title, the note
 * itself, a creation date and a modified data.
 */
public class NotePadProvider extends ContentProvider {

    private static final String TAG = "NotePadProvider";
		//数据库名
    private static final String DATABASE_NAME = "note_pad.db";
    private static final int DATABASE_VERSION = 2;
    //表名
    private static final String NOTES_TABLE_NAME = "notes";

    private static HashMap<String, String> sNotesProjectionMap;

    private static final int NOTES = 1;
    private static final int NOTE_ID = 2;

    private static final UriMatcher sUriMatcher;

    /**
     * This class helps open, create, and upgrade the database file.
     */
    private static class DatabaseHelper extends SQLiteOpenHelper {

        DatabaseHelper(Context context) {
            super(context, DATABASE_NAME, null, DATABASE_VERSION);
        }

        @Override
        public void onCreate(SQLiteDatabase db) {
            db.execSQL("CREATE TABLE " + NOTES_TABLE_NAME + " ("
                    + Notes._ID + " INTEGER PRIMARY KEY,"
                    + Notes.TITLE + " TEXT,"
                    + Notes.NOTE + " TEXT,"
                    + Notes.CREATED_DATE + " INTEGER,"
                    + Notes.MODIFIED_DATE + " INTEGER"
                    + ");");
        }

        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
            Log.w(TAG, "Upgrading database from version " + oldVersion + " to "
                    + newVersion + ", which will destroy all old data");
            db.execSQL("DROP TABLE IF EXISTS notes");
            onCreate(db);
        }
    }

    private DatabaseHelper mOpenHelper;

    @Override
    public boolean onCreate() {
        mOpenHelper = new DatabaseHelper(getContext());
        return true;
    }

    @Override
    public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs,
            String sortOrder) {
        SQLiteQueryBuilder qb = new SQLiteQueryBuilder();

        switch (sUriMatcher.match(uri)) {
        case NOTES:
            qb.setTables(NOTES_TABLE_NAME);
            qb.setProjectionMap(sNotesProjectionMap);
            break;

        case NOTE_ID:
            qb.setTables(NOTES_TABLE_NAME);
            qb.setProjectionMap(sNotesProjectionMap);
            qb.appendWhere(Notes._ID + "=" + uri.getPathSegments().get(1));
            break;

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

        // If no sort order is specified use the default
        String orderBy;
        if (TextUtils.isEmpty(sortOrder)) {
            orderBy = NotePad.Notes.DEFAULT_SORT_ORDER;
        } else {
            orderBy = sortOrder;
        }

        // Get the database and run the query
        SQLiteDatabase db = mOpenHelper.getReadableDatabase();
        Cursor c = qb.query(db, projection, selection, selectionArgs, null, null, orderBy);

        // Tell the cursor what uri to watch, so it knows when its source data changes
        c.setNotificationUri(getContext().getContentResolver(), uri);
        return c;
    }

    @Override
    //如果有自定义类型，必须实现该方法
    public String getType(Uri uri) {
        switch (sUriMatcher.match(uri)) {
        case NOTES:
            return Notes.CONTENT_TYPE;

        case NOTE_ID:
            return Notes.CONTENT_ITEM_TYPE;

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

    @Override
    public Uri insert(Uri uri, ContentValues initialValues) {
        // Validate the requested uri
        if (sUriMatcher.match(uri) != NOTES) {
            throw new IllegalArgumentException("Unknown URI " + uri);
        }

        ContentValues values;
        if (initialValues != null) {
            values = new ContentValues(initialValues);
        } else {
            values = new ContentValues();
        }

        Long now = Long.valueOf(System.currentTimeMillis());

        // Make sure that the fields are all set
        if (values.containsKey(NotePad.Notes.CREATED_DATE) == false) {
            values.put(NotePad.Notes.CREATED_DATE, now);
        }

        if (values.containsKey(NotePad.Notes.MODIFIED_DATE) == false) {
            values.put(NotePad.Notes.MODIFIED_DATE, now);
        }

        if (values.containsKey(NotePad.Notes.TITLE) == false) {
            Resources r = Resources.getSystem();
            values.put(NotePad.Notes.TITLE, r.getString(android.R.string.untitled));
        }

        if (values.containsKey(NotePad.Notes.NOTE) == false) {
            values.put(NotePad.Notes.NOTE, "");
        }

        SQLiteDatabase db = mOpenHelper.getWritableDatabase();
        long rowId = db.insert(NOTES_TABLE_NAME, Notes.NOTE, values);
        if (rowId > 0) {
            Uri noteUri = ContentUris.withAppendedId(NotePad.Notes.CONTENT_URI, rowId);
            getContext().getContentResolver().notifyChange(noteUri, null);
            return noteUri;
        }

        throw new SQLException("Failed to insert row into " + uri);
    }

    @Override
    public int delete(Uri uri, String where, String[] whereArgs) {
        SQLiteDatabase db = mOpenHelper.getWritableDatabase();
        int count;
        switch (sUriMatcher.match(uri)) {
        case NOTES:
            count = db.delete(NOTES_TABLE_NAME, where, whereArgs);
            break;

        case NOTE_ID:
            String noteId = uri.getPathSegments().get(1);
            count = db.delete(NOTES_TABLE_NAME, Notes._ID + "=" + noteId
                    + (!TextUtils.isEmpty(where) ? " AND (" + where + ')' : ""), whereArgs);
            break;

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

        getContext().getContentResolver().notifyChange(uri, null);
        return count;
    }

    @Override
    public int update(Uri uri, ContentValues values, String where, String[] whereArgs) {
        SQLiteDatabase db = mOpenHelper.getWritableDatabase();
        int count;
        switch (sUriMatcher.match(uri)) {
        case NOTES:
            count = db.update(NOTES_TABLE_NAME, values, where, whereArgs);
            break;

        case NOTE_ID:
            String noteId = uri.getPathSegments().get(1);
            count = db.update(NOTES_TABLE_NAME, values, Notes._ID + "=" + noteId
                    + (!TextUtils.isEmpty(where) ? " AND (" + where + ')' : ""), whereArgs);
            break;

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

        getContext().getContentResolver().notifyChange(uri, null);
        return count;
    }

    static {
        sUriMatcher = new UriMatcher(UriMatcher.NO_MATCH);
        sUriMatcher.addURI(NotePad.AUTHORITY, "notes", NOTES);
        sUriMatcher.addURI(NotePad.AUTHORITY, "notes/#", NOTE_ID);

        sNotesProjectionMap = new HashMap<String, String>();
        sNotesProjectionMap.put(Notes._ID, Notes._ID);
        sNotesProjectionMap.put(Notes.TITLE, Notes.TITLE);
        sNotesProjectionMap.put(Notes.NOTE, Notes.NOTE);
        sNotesProjectionMap.put(Notes.CREATED_DATE, Notes.CREATED_DATE);
        sNotesProjectionMap.put(Notes.MODIFIED_DATE, Notes.MODIFIED_DATE);
    }
}