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[Android开发]Android 组件ContentProvider

ContentProvider

安卓应用程序默认是无法获取到其他程序的数据,这是安卓安全学的基石(沙盒原理)。但是经常我们需要给其他应用分享数据,内容提供者就是一个这种可以分享数据给其他应用的接口。
可以简单的理解为,内容提供者就是一个可以在不同应用程序间共享数据的组件,相当于一个中间人,一个程序把数据暴露给这个中间人,另一个则通过这个中间人获取相应的数据.

下面的这张图片能更直观的显示:

Image

  • ContentProvider中的getContextAndroidTestCast中的getContext方法一样,都是一个模拟的上下文,必须在该类初始化之后才会调用setContext方法将context设置成自己的成员变量中记录,
    所以对于获取getContext的时候只能放在方法内,不能放到成员位置,因为在成员上时是null,而在方法内调用时该类就会已经初始化完了
  • ContentProvider中的query()后不能关闭数据库,因为其他的应用在调用该query方法时需要继续使用该返回值Cursor,所以不能关闭数据库,因为数据库关闭之后Cursor就不能用了,
    Cursor中保存的数据其实是数据库的一个引用,如果数据库关了Cursor就不能找到里面的数据了,Cursor.close()只是释放Cursor用到的资源。说到这里就多数一句
    According to Dianne Hackborn (Android framework engineer) there is no need to close the database in a content provider.以为内容提供者是因为进程启动时便加载,之后就一直存在,当进程销毁
    释放资源时会去关闭数据库。
  • 如果数据是SQLiteDatabase,表中必须有一个_id的列,用来表示每条记录的唯一性。
  1. 继承ContentProvider,并实现相应的方法。
    public class NoteProvider extends ContentProvider {
    private static final int NOTES = 1;
    private static final int NOTE_ID = 2;
    public static final String AUTHORITY = "com.charon.demo.provider.noteprovider";
    public static final String TABLE_NAME = "note";
    // 定义一个名为`CONTENT_URI`必须为其指定一个唯一的字符串值,最好的方案是以类的全名称
    public static final Uri CONTENT_URI = Uri.parse("content://" + AUTHORITY + "/" + TABLE_NAME);
    // 声明一个路径的检查者,参数为Uri不匹配时的返回值
    // 虽然是中间人,但也不能谁要数据我们都给,所以要检查下,只有符合我们要求的人,我们才会给他数据。
    private static UriMatcher sUriMatcher = new UriMatcher(UriMatcher.NO_MATCH);
    private NoteSQLiteOpenHelper mSQLiteOpenHelper;
    private SQLiteDatabase mSQLiteDatabase;
    static {
    // 建立匹配规则,例如发现路径为ccom.charon.demo.provider.noteprovider/note/1表示要操作note表中id为1的记录
    sUriMatcher.addURI(AUTHORITY, TABLE_NAME, NOTES);
    sUriMatcher.addURI(AUTHORITY, TABLE_NAME + "/#", NOTE_ID);
    }
    /**
    * Implement this to initialize your content provider on startup.
    * This method is called for all registered content providers on the
    * application main thread at application launch time.  It must not perform
    * lengthy operations, or application startup will be delayed.
    * <p/>
    * <p>You should defer nontrivial initialization (such as opening,
    * upgrading, and scanning databases) until the content provider is used
    * (via {@link #query}, {@link #insert}, etc).  Deferred initialization
    * keeps application startup fast, avoids unnecessary work if the provider
    * turns out not to be needed, and stops database errors (such as a full
    * disk) from halting application launch.
    * <p/>
    * <p>If you use SQLite, {@link android.database.sqlite.SQLiteOpenHelper}
    * is a helpful utility class that makes it easy to manage databases,
    * and will automatically defer opening until first use.  If you do use
    * SQLiteOpenHelper, make sure to avoid calling
    * {@link android.database.sqlite.SQLiteOpenHelper#getReadableDatabase} or
    * {@link android.database.sqlite.SQLiteOpenHelper#getWritableDatabase}
    * from this method.  (Instead, override
    * {@link android.database.sqlite.SQLiteOpenHelper#onOpen} to initialize the
    * database when it is first opened.)
    *
    * @return true if the provider was successfully loaded, false otherwise
    */
    @Override
    public boolean onCreate() {
    mSQLiteOpenHelper = new NoteSQLiteOpenHelper(getContext());
    return true;
    }
    /**
    * 内容提供者暴露的查询的方法.
    */
    @Override
    public Cursor query(Uri uri, String[] projection, String selection,
    String[] selectionArgs, String sortOrder) {
    mSQLiteDatabase = mSQLiteOpenHelper.getReadableDatabase();
    Cursor cursor;
    // 1.重要的事情 ,检查 uri的路径.
    switch (sUriMatcher.match(uri)) {
    case NOTES:
    break;
    case NOTE_ID:
    String id = uri.getLastPathSegment();
    if (TextUtils.isEmpty(selection)) {
    selection = selection + "_id = " + id;
    } else {
    selection = selection + " and " + "_id = " + id;
    }
    break;
    default:
    throw new IllegalArgumentException("UnKnown Uri" + uri);
    break;
    }
    cursor = mSQLiteDatabase.query(TABLE_NAME, projection, selection, selectionArgs, null, null, sortOrder);
    if (cursor != null) {
    cursor.setNotificationUri(getContext().getContentResolver(), uri);
    }
    return cursor;
    }
    /**
    * Implement this to handle requests for the MIME type of the data at the
    * given URI.  The returned MIME type should start with
    * <code>vnd.android.cursor.item</code> for a single record,
    * or <code>vnd.android.cursor.dir/</code> for multiple items.
    * This method can be called from multiple threads, as described in
    * <a href="https://www.xyhtml5.com/android/android-tutorial/{@docRoot}guide/topics/fundamentals/processes-and-threads.html#Threads">Processes
    * and Threads</a>.
    * <p/>
    * <p>Note that there are no permissions needed for an application to
    * access this information; if your content provider requires read and/or
    * write permissions, or is not exported, all applications can still call
    * this method regardless of their access permissions.  This allows them
    * to retrieve the MIME type for a URI when dispatching intents.
    *
    * @param uri the URI to query.
    * @return a MIME type string, or {@code null} if there is no type.
    */
    @Override
    public String getType(Uri uri) {
    // 注释说的很清楚了,下面是常用的格式
    // 单个记录的IMEI类型 vnd.android.cursor.item/vnd.<yourcompanyname>.<contenttype>
    // 多个记录的IMEI类型 vnd.android.cursor.dir/vnd.<yourcompanyname>.<contenttype>
    switch (sUriMatcher.match(uri)) {
    case NOTE_ID:
    // 如果uri为 content://com.charon.demo.noteprovider/note/1
    return "vnd.android.cursor.item/vnd.charon.note";
    case NOTES:
    return "vnd.android.cursor.dir/vnd.charon.note";
    default:
    return null;
    }
    // 这个MIME类型的作用是要匹配AndroidManifest.xml文件<activity>标签下<intent-filter>标签的子标签<data>的属性android:mimeType。
    // 如果不一致,则会导致对应的Activity无法启动。
    }
    @Override
    public Uri insert(Uri uri, ContentValues values) {
    mSQLiteDatabase = mSQLiteOpenHelper.getWritableDatabase();
    switch (sUriMatcher.match(uri)) {
    case NOTES:
    break;
    case NOTE_ID:
    break;
    default:
    throw new IllegalArgumentException("UnKnown Uri" + uri);
    break;
    }
    long rowId = mSQLiteDatabase.insert(TABLE_NAME, null, values);
    if (rowId > 0) {
    Uri noteUri = ContentUris.withAppendedId(CONTENT_URI, rowId);
    getContext().getContentResolver().notifyChange(noteUri, null);
    return noteUri;
    }
    return null;
    }
    @Override
    public int delete(Uri uri, String selection, String[] selectionArgs) {
    mSQLiteDatabase = mSQLiteOpenHelper.getWritableDatabase();
    switch (sUriMatcher.match(uri)) {
    case NOTES:
    break;
    case NOTE_ID:
    String id = uri.getLastPathSegment();
    if (TextUtils.isEmpty(selection)) {
    selection = selection + "_id = " + id;
    } else {
    selection = selection + " and " + "_id = " + id;
    }
    break;
    default:
    throw new IllegalArgumentException("UnKnown Uri" + uri);
    break;
    }
    int count = mSQLiteDatabase.delete(TABLE_NAME, selection, selectionArgs);
    getContext().getContentResolver().notifyChange(uri, null);
    return count;
    }
    @Override
    public int update(Uri uri, ContentValues values, String selection,
    String[] selectionArgs) {
    switch (sUriMatcher.match(uri)) {
    case NOTES:
    break;
    case NOTE_ID:
    break;
    default:
    throw new IllegalArgumentException("UnKnown Uri" + uri);
    break;
    }
    mSQLiteDatabase = mSQLiteOpenHelper.getWritableDatabase();
    int update = mSQLiteDatabase.update(TABLE_NAME, values, selection, selectionArgs);
    return update;
    }
    
  2. 在清单文件中进行注册,并且指定其authorities
    <provider
    android:name="com.charon.demo.provider.NoteProvider"
    android:authorities="com.charon.demo.provider.noteprovider" >
    
  3. 使用内容提供者获取数据,使用ContentResolver去操作ContentProvider, ContentResolver用于管理ContentProvider实例,
    并且可实现找到指定的ContentProvider并获取里面的数据

    public void query(View view){
    //得到内容提供者的解析器  中间人
    ContentResolver resolver = getContentResolver();
    Cursor cursor = resolver.query(NoteProvider.CONTENT_URI, null, null, null, null);
    while(cursor.moveToNext()){
    String name = cursor.getString(cursor.getColumnIndex("name"));
    int id = cursor.getInt(cursor.getColumnIndex("id"));
    float money = cursor.getFloat(cursor.getColumnIndex("money"));
    System.out.println("id="+id+",name="+name+",money="+money);
    }
    cursor.close();
    }
    public void insert(View view){
    ContentResolver resolver = getContentResolver();
    ContentValues values = new ContentValues();
    values.put("name", "买洗头膏");
    values.put("money", 22.58f);
    resolver.insert(NoteProvider.CONTENT_URI, values);
    }
    public void update(View view){
    ContentResolver resolver = getContentResolver();
    ContentValues values = new ContentValues();
    values.put("name", "买洗头膏");
    values.put("money", 42.58f);
    resolver.update(NoteProvider.CONTENT_URI, values, "name=?", new String[]{"买洗头膏"});
    }
    public void delete(View view){
    ContentResolver resolver = getContentResolver();
    resolver.delete(NoteProvider.CONTENT_URI, "name=?", new String[]{"买洗头膏"});
    }
    

内容观察者

内容观察者的原理:
How a content provider actually stores its data under the covers is up to its designer. But all content providers implement a common interface for
querying the provider and returning results — as well as for adding, altering, and deleting data.
It's an interface that clients use indirectly, most generally through ContentResolver objects.
You get a ContentResolver by calling getContentResolver() from within the implementation of an Activity or other application component:
You can then use the ContentResolver's methods to interact with whatever content providers you're interested in.

  1. 一方使用内容观察者去观察变化
        protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    ContentResolver resolver = getContentResolver();
    resolver.registerContentObserver(NoteProvider.CONTENT_URI, true, new NoteObserver(new Handler()));
    }
    private class NoteObserver extends ContentObserver {
    public NoteObserver(Handler handler) {
    super(handler);
    }
    //当观察到数据发生变化的时候  会执行onchange方法.
    @Override
    public void onChange(boolean selfChange) {
    super.onChange(selfChange);
    Log.i(TAG,"发现有新的短信产生了...");
    //1.利用内容提供者  中间人 获取用户的短信数据.
    ContentResolver resolver  = getContentResolver();
    // .. 重新查询
    cursor = ...;
    cursor.close();
    }
    }
    
  2. 一方在发生变化的时候去发送改变的消息
    对于一些系统的内容提供者内部都实现了该步骤,如果是自己写程序想要暴露就必须要加上该代码。

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

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