Android 四大组件之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:

• ContentProvider中的getContext和AndroidTestCast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="{@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;
       }

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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" >
   

   XML

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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"});
   }

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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();
       }
   }

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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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