《Android开发艺术探索》阅读笔记（七）

4.5 使用ContentProvider：

本篇是介绍如何使用ContentProvider的案例，客户端通过访问ContentProvider，对数据库进行操作。

第一步，创建数据库：
在构造函数里创建数据库，在onCreate()中创建表（使用SQL语句）

public class DbOpenHelper extends SQLiteOpenHelper {

    private static final String DB_NAME = "book_provider.db";
    
    public static final String BOOK_TABLE_NAME = "book";
    public static final String USER_TABLE_NAME = "user";

    private static final int DB_VERSION = 2;

    //创建表的语句，包括表的名称、表内的数据项
    private String CREATE_BOOK_TABLE = "CREATE TABLE IF NOT EXISTS "
            + BOOK_TABLE_NAME + "(_id INTEGER PRIMARY KEY," + "name TEXT)";

    private String CREATE_USER_TABLE = "CREATE TABLE IF NOT EXISTS "
            + USER_TABLE_NAME + "(_id INTEGER PRIMARY KEY," + "name TEXT," + "sex INT)";

    //构造函数，创建database
    public DbOpenHelper(Context context){
        super(context,DB_NAME,null,DB_VERSION);
    }

    //执行语句创建book表、user表
    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL(CREATE_BOOK_TABLE);
        db.execSQL(CREATE_USER_TABLE);
    }

    //更新数据库
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        //TODO ignored
    }
}

第二步，创建ContentProvider：
在ContentProvider里，用new DbOpenHelper(Context).getWritableDatabase()向DbOpenHelper类获取SQLiteDatabase实例，然后对此SQLiteDatabase实例进行操作。

重写的六个抽象方法：onCreate代表ContentProvider的创建，getType用来返回一个Uri请求所对应的MIME类型（媒体类型，图片，视频等），query、update、insert、delete实现数据表的增删改查功能。
（onCreate运行在主线程里，其余五个运行在Binder线程池中）

public class BookProvider extends ContentProvider {

    //Log标签
    private static final String TAG  = "aa20190131";

    //ContentProvider的唯一标识符
    public static final String AUTHORITY = "com.example.a20190130.BookProvider11";

    //Uri_Code，用于帮助getTableName()识别
    public static final int BOOK_URI_CODE = 0;
    public static final int USER_URI_CODE = 1;

    //UriMatcher很重要，它将客户端传过来的Uri转换为其他可识别的标志
    private static final UriMatcher sUriMatcher = new UriMatcher(UriMatcher.NO_MATCH);

    private Context mContext;
    private SQLiteDatabase mDb;

    //将Uri和的Uri_Code相关联
    static {
        sUriMatcher.addURI(AUTHORITY,"book",BOOK_URI_CODE);
        sUriMatcher.addURI(AUTHORITY,"user",USER_URI_CODE);
    }

    //初始化
    @Override
    public boolean onCreate() {
        Log.d(TAG,"onCreate, current thread:" + Thread.currentThread().getName());
        mContext = getContext();
        //onCreate()在主线程中运行，实际使用中不推荐进行耗时的数据库操作
        initProviderData();//初始化数据表
        return true;
    }

    //客户端执行query时会跳到这里
    @Override
    public Cursor query(Uri uri,String[] projection,String selection, String[] selectionArgs, String sortOrder) {
        Log.d(TAG,"query, current thread:" + Thread.currentThread().getName());
        String table = getTableName(uri);
        if (table == null){
            throw new IllegalArgumentException("Unsupported URI:" + uri);
        }
        return mDb.query(table,projection,selection,selectionArgs,null,null,sortOrder,null);
    }

    @Override
    public String getType(Uri uri) {
        Log.d(TAG,"getType");
        return null;
    }

    //客户端执行insert时会跳到这里
    @Override
    public Uri insert(Uri uri,ContentValues values) {
        Log.d(TAG,"insert");
        String table = getTableName(uri);
        if (table == null){
            throw new IllegalArgumentException("Unsupport URI:" + uri);
        }
        mDb.insert(table,null,values);
        //通知外界，当前ContentProvider中的数据已经发生改变，不设置观察者
        mContext.getContentResolver().notifyChange(uri,null);
        return uri;
    }

    //客户端执行delete时会跳到这里
    @Override
    public int delete(Uri uri,String selection, String[] selectionArgs) {
        Log.d(TAG,"delete");
        String table = getTableName(uri);
        if (table == null){
            throw new IllegalArgumentException("Unsupport URI:" + uri);
        }
        int count = mDb.delete(table,selection,selectionArgs);
        if (count > 0){
            getContext().getContentResolver().notifyChange(uri,null);
        }
        return count;
    }

    //客户端执行update时会跳到这里
    @Override
    public int update(Uri uri, ContentValues values,String selection,String[] selectionArgs) {
        Log.d(TAG,"update");
        String table = getTableName(uri);
        if (table == null){
            throw new IllegalArgumentException("Unsupport URI:" + uri);
        }
        int row = mDb.update(table,values,selection,selectionArgs);
        if (row > 0){
            getContext().getContentResolver().notifyChange(uri,null);
        }
        return row;
    }

    //执行SQL语句，添加数据
    private void initProviderData(){
        mDb = new DbOpenHelper(mContext).getWritableDatabase();//向DbOpenHelper类获取SQLiteDatabase！
        mDb.execSQL("delete from " + DbOpenHelper.BOOK_TABLE_NAME);//清空这张表
        mDb.execSQL("delete from " + DbOpenHelper.USER_TABLE_NAME);
        mDb.execSQL("insert into book values(3,'Android');");
        mDb.execSQL("insert into book values(4,'IOS');");
        mDb.execSQL("insert into book values(5,'Html5');");
        mDb.execSQL("insert into user(_id,name,sex) values(1,'jake',1);");
        mDb.execSQL("insert into user(_id,name,sex) values(2,'jasmine',0);");
    }

    //把客户端传来的Uri转为数据库中表的名称
    private String getTableName(Uri uri){
        String tableName = null;
        switch (sUriMatcher.match(uri)){//将Uri转换为对应的Uri_Code
            case BOOK_URI_CODE:
                tableName = DbOpenHelper.BOOK_TABLE_NAME;
                break;
            case USER_URI_CODE:
                tableName = DbOpenHelper.USER_TABLE_NAME;
                break;
            default:
                break;
        }
        return tableName;
    }
}

在AndroidManifest.xml文件要对ContentProvider进行设置：

<provider
	android:authorities="com.example.a20190130.BookProvider11"
	android:name=".BookProvider"
	android:permission="com.example.a20190130.PROVIDER"
	android:process=":provider"/>

• android:authorities是ContentProvider的唯一标识；
• android:permission指定了外界应用如果要访问ContentProvider，必须声明这个权限；
• android:name与自己创建的ContentProvider对应；
• android:process为ContentProvider开辟独立运行的进程；

第三步，在客户端访问ContentProvider：
用Uri bookUri = Uri.parse("content://" + AUTHORITY + "/book")获取ContentProvider里的表，通过getContentResolver().insert()等对此表进行操作。
insert、query、delete、update等在ContentProvider中进行了重写。客户端对表的操作会对应到ContentProvider里重写的四大方法。

public class MainActivity extends AppCompatActivity {

    //ContentProvider的唯一标识符
    public static final String AUTHORITY = "com.example.a20190130.BookProvider11";
    private static final String TAG  = "aa20190131";
    Button button;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

//                //BookProvider中的query被调用了三次，这三次调用不在同一个线程中，它们运行在一个Binder线程中
//                Uri uri = Uri.parse("content://com.example.a20190130.BookProvider");
//                getContentResolver().query(uri,null,null,null,null);
//                getContentResolver().query(uri,null,null,null,null);
//                getContentResolver().query(uri,null,null,null,null);

                //要访问的是ContentProvider中的book表
                Uri bookUri = Uri.parse("content://" + AUTHORITY + "/book");
                ContentValues values = new ContentValues();
                values.put("_id",6);
                values.put("name","程序设计的艺术");
                getContentResolver().insert(bookUri,values);

                Cursor bookCursor = getContentResolver().query(bookUri,new String[]{"_id","name"},
                        null,null,null);
                while (bookCursor.moveToNext()){
                    Book book = new Book();
                    book.bookId = bookCursor.getInt(0);
                    book.bookName = bookCursor.getString(1);
                    Log.d(TAG, "query book: " + "bookId:" + book.bookId + " bookName:" + book.bookName);
                }
                bookCursor.close();

                Uri userUri = Uri.parse("content://" + AUTHORITY + "/user");
                Cursor userCursor = getContentResolver().query(userUri,new String[]{"_id","name","sex"},
                        null,null,null);
                while (userCursor.moveToNext()){
                    User user = new User();
                    user.userId = userCursor.getInt(0);
                    user.userName = userCursor.getString(1);
                    user.isMale = userCursor.getInt(2) == 1;
                    Log.d(TAG, "query user: " + "userId:" + user.userId + " userName:" + user.userName + " isMale:" + user.isMale);
                }
                userCursor.close();
            }
}