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;
    }

1. 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" >

1. 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
   java
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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• Good Luck!