Android Content Provider在应用程序之间共享数据的原理分析

       本文參考Android应用程序组件Content Provider在应用程序之间共享数据的原理分析http://blog.csdn.net/luoshengyang/article/details/6967204和《Android系统源码情景分析》,作者罗升阳。

       0、总图流程图例如以下:

       整体类图:



     1、MainActivity进程向AriticlesProvider进程发送IContentProvider.QUERY_TRANSACTION



       如图:第一步

       ~/Android/frameworks/base/core/java/android/content

       ----ContentProviderNative.java

final class ContentProviderProxy implements IContentProvider {
	......

	public Cursor query(Uri url, String[] projection, String selection,
			String[] selectionArgs, String sortOrder) throws RemoteException {
		//TODO make a pool of windows so we can reuse memory dealers
		CursorWindow window = new CursorWindow(false /* window will be used remotely */);
		BulkCursorToCursorAdaptor adaptor = new BulkCursorToCursorAdaptor();
		IBulkCursor bulkCursor = bulkQueryInternal(
			url, projection, selection, selectionArgs, sortOrder,
			adaptor.getObserver(), window,
			adaptor);
		if (bulkCursor == null) {
			return null;
		}
		return adaptor;
	}

	......
      (1)创建了CursorWindow对象。

      (2)创建类BulkCursorToCursorAdaptor对象。


      (3)调用bulkQueryInternal。


       ~/Android/frameworks/base/core/java/android/content

       ----ContentProviderNative.java

final class ContentProviderProxy implements IContentProvider
{
	......

	private IBulkCursor bulkQueryInternal(
			Uri url, String[] projection,
			String selection, String[] selectionArgs, String sortOrder,
			IContentObserver observer, CursorWindow window,
			BulkCursorToCursorAdaptor adaptor) throws RemoteException {
		Parcel data = Parcel.obtain();
		Parcel reply = Parcel.obtain();

		data.writeInterfaceToken(IContentProvider.descriptor);

		url.writeToParcel(data, 0);
		int length = 0;
		if (projection != null) {
			length = projection.length;
		}
		data.writeInt(length);
		for (int i = 0; i < length; i++) {
			data.writeString(projection[i]);
		}
		data.writeString(selection);
		if (selectionArgs != null) {
			length = selectionArgs.length;
		} else {
			length = 0;
		}
		data.writeInt(length);
		for (int i = 0; i < length; i++) {
			data.writeString(selectionArgs[i]);
		}
		data.writeString(sortOrder);
		data.writeStrongBinder(observer.asBinder());
		window.writeToParcel(data, 0);

		// Flag for whether or not we want the number of rows in the
		// cursor and the position of the "_id" column index (or -1 if
		// non-existent).  Only to be returned if binder != null.
		final boolean wantsCursorMetadata = (adaptor != null);
		data.writeInt(wantsCursorMetadata ?

1 : 0); mRemote.transact(IContentProvider.QUERY_TRANSACTION, data, reply, 0); DatabaseUtils.readExceptionFromParcel(reply); IBulkCursor bulkCursor = null; IBinder bulkCursorBinder = reply.readStrongBinder(); if (bulkCursorBinder != null) { bulkCursor = BulkCursorNative.asInterface(bulkCursorBinder); if (wantsCursorMetadata) { int rowCount = reply.readInt(); int idColumnPosition = reply.readInt(); if (bulkCursor != null) { adaptor.set(bulkCursor, rowCount, idColumnPosition); } } } data.recycle(); reply.recycle(); return bulkCursor; } ...... }

       我们这里仅仅关注window.writeToParcel(data, 0)。详解请看相应的博客或者图书。



       如图:第二步。省略binder_transaction传输过程,由于上面已经分析过了。


       如图:第三步

       ~/Android/frameworks/base/core/java/android/content

       ----ContentProviderNative.java

abstract public class ContentProviderNative extends Binder implements IContentProvider {
	......

	@Override
	public boolean onTransact(int code, Parcel data, Parcel reply, int flags)
	throws RemoteException {
		try {
			switch (code) {
			case QUERY_TRANSACTION:
				{
					data.enforceInterface(IContentProvider.descriptor);

					Uri url = Uri.CREATOR.createFromParcel(data);

					// String[] projection
					int num = data.readInt();
					String[] projection = null;
					if (num > 0) {
						projection = new String[num];
						for (int i = 0; i < num; i++) {
							projection[i] = data.readString();
						}
					}

					// String selection, String[] selectionArgs...
					String selection = data.readString();
					num = data.readInt();
					String[] selectionArgs = null;
					if (num > 0) {
						selectionArgs = new String[num];
						for (int i = 0; i < num; i++) {
							selectionArgs[i] = data.readString();
						}
					}

					String sortOrder = data.readString();
					IContentObserver observer = IContentObserver.Stub.
						asInterface(data.readStrongBinder());
					CursorWindow window = CursorWindow.CREATOR.createFromParcel(data);

					// Flag for whether caller wants the number of
					// rows in the cursor and the position of the
					// "_id" column index (or -1 if non-existent)
					// Only to be returned if binder != null.
					boolean wantsCursorMetadata = data.readInt() != 0;

					IBulkCursor bulkCursor = bulkQuery(url, projection, selection,
						selectionArgs, sortOrder, observer, window);
					reply.writeNoException();
					if (bulkCursor != null) {
						reply.writeStrongBinder(bulkCursor.asBinder());

						if (wantsCursorMetadata) {
							reply.writeInt(bulkCursor.count());
							reply.writeInt(BulkCursorToCursorAdaptor.findRowIdColumnIndex(
								bulkCursor.getColumnNames()));
						}
					} else {
						reply.writeStrongBinder(null);
					}

					return true;
				}
			......
			}
		} catch (Exception e) {
			DatabaseUtils.writeExceptionToParcel(reply, e);
			return true;
		}

		return super.onTransact(code, data, reply, flags);
	}

	......
}
       当中,CursorWindow window = CursorWindow.CREATOR.createFromParcel(data);详解请看博客或者书。


       如图:第四步
       ~/Android/frameworks/base/core/java/android/content

       ----ContentProvider.java

public abstract class ContentProvider implements ComponentCallbacks {
	......

	class Transport extends ContentProviderNative {
		......

		public IBulkCursor bulkQuery(Uri uri, String[] projection,
				String selection, String[] selectionArgs, String sortOrder,
				IContentObserver observer, CursorWindow window) {
			......

			Cursor cursor = ContentProvider.this.query(uri, projection,
				selection, selectionArgs, sortOrder);
			......

			return new CursorToBulkCursorAdaptor(cursor, observer,
				ContentProvider.this.getClass().getName(),
				hasWritePermission(uri), window);
		}

		......
	}

	......
}
       主要做了以下几件事:

     (1)调用AriticlesProvider的query方法。获取了SQLiteCursor对象。

     (2)由cursor和window对象,形成CursorToBulkCursorAdaptor对象。


       如图。第五步

if (bulkCursor != null) {
	reply.writeStrongBinder(bulkCursor.asBinder());

	if (wantsCursorMetadata) {
		reply.writeInt(bulkCursor.count());
		reply.writeInt(BulkCursorToCursorAdaptor.findRowIdColumnIndex(
			bulkCursor.getColumnNames()));
	}
}
      传递CursorToBulkCursorAdaptor对象,例如以下图:

      

       如图:第六步。省略binder_transaction传输过程,由于上面已经分析过了。


       如图:第七步

       ~/Android/frameworks/base/core/java/android/content

       ----ContentProviderNative.java

IBulkCursor bulkCursor = null;
IBinder bulkCursorBinder = reply.readStrongBinder();
if (bulkCursorBinder != null) {
	bulkCursor = BulkCursorNative.asInterface(bulkCursorBinder);

	if (wantsCursorMetadata) {
		int rowCount = reply.readInt();
		int idColumnPosition = reply.readInt();
		if (bulkCursor != null) {
			adaptor.set(bulkCursor, rowCount, idColumnPosition);
		}
	}
}
       bulkCursor为BulkCursorProxy对象例如以下:

adaptor.set(bulkCursor, rowCount, idColumnPosition);
       把BulkCursorProxy对象放入到BulkCursorToCursorAdaptor对象的句柄变量mBulkCursor中。


       如图:第八步

return new CursorWrapperInner(qCursor, provider);
       最后返回了这个对象,qCursor是 BulkCursorToCursorAdaptor对象,provider为ContentProviderProxy对象。

     

       至此,我们形成了下图:



       进程间通信结束了,以下我们分析怎样应用匿名共享内存来数据传输

       在前面的一篇文章Android Content Provider的启动过程源码分析http://blog.csdn.net/jltxgcy/article/details/37725749。最后获取了ContentProviderProxy对象,通过进程间通信来传递数据

public class ArticlesAdapter {
	......

	private ContentResolver resolver = null;

	public ArticlesAdapter(Context context) {
		resolver = context.getContentResolver();
	}

	......

	public Article getArticleByPos(int pos) {
		Uri uri = ContentUris.withAppendedId(Articles.CONTENT_POS_URI, pos);

		String[] projection = new String[] {
			Articles.ID,
			Articles.TITLE,
			Articles.ABSTRACT,
			Articles.URL
		};

		Cursor cursor = resolver.query(uri, projection, null, null, Articles.DEFAULT_SORT_ORDER);
		if (!cursor.moveToFirst()) {
			return null;
		}

		int id = cursor.getInt(0);
		String title = cursor.getString(1);
		String abs = cursor.getString(2);
		String url = cursor.getString(3);

		return new Article(id, title, abs, url);
	}
}
  

        我们不分析具体过程,首先BulkCursorToCursorAdaptor对象里面的成员变量mBulkCursor通过进程间通信的方式,找到CursorToBulkCursorAdaptor对象。通过里面的成员函数mCursor查询出数据,而且保存在mWindows所指向的匿名共享内存。

        而BulkCursorToCursorAdaptor通过成员变量mWindow来訪问同样的匿名共享内存的。这样MainActivity就获取了数据。

       假设是删除数据,可能不须要读写匿名共享内存,仅仅要通过mBulkCursor通过进程间通信的方式的操作和读取返回结果。

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转载自www.cnblogs.com/mqxnongmin/p/10870853.html
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