Add a CrossProcessCursorWrapper.

Bug: 5220669

The CrossProcessCursorWrapper is a useful building block
for wrapping cursors that will be sent to processes remotely.
It can also transform normal Cursors into CrossProcessCursors
transparently.

The new class fixes common performance problems and bugs
that applications have encountered when implementing
ContentProviders.

Change-Id: Icc59fec10add3f7d8cfbd0495447860c1872c752

Author: Jeff Brown

api/current.txt

@@ -6782,6 +6782,13 @@ package android.database {
     method public abstract boolean onMove(int, int);
   }
 
+  public class CrossProcessCursorWrapper extends android.database.CursorWrapper implements android.database.CrossProcessCursor {
+    ctor public CrossProcessCursorWrapper(android.database.Cursor);
+    method public void fillWindow(int, android.database.CursorWindow);
+    method public android.database.CursorWindow getWindow();
+    method public boolean onMove(int, int);
+  }
+
   public abstract interface Cursor {
     method public abstract void close();
     method public abstract void copyStringToBuffer(int, android.database.CharArrayBuffer);

core/java/android/content/ContentResolver.java

@@ -26,6 +26,7 @@
 import android.content.res.AssetFileDescriptor;
 import android.content.res.Resources;
 import android.database.ContentObserver;
+import android.database.CrossProcessCursorWrapper;
 import android.database.Cursor;
 import android.database.CursorWrapper;
 import android.database.IContentObserver;
@@ -1562,7 +1563,7 @@ private void maybeLogUpdateToEventLog(
             samplePercent);
     }
 
-    private final class CursorWrapperInner extends CursorWrapper {
+    private final class CursorWrapperInner extends CrossProcessCursorWrapper {
         private final IContentProvider mContentProvider;
         public static final String TAG="CursorWrapperInner";
 

core/java/android/database/CrossProcessCursor.java

@@ -16,27 +16,63 @@
 
 package android.database;
 
+/**
+ * A cross process cursor is an extension of a {@link Cursor} that also supports
+ * usage from remote processes.
+ * <p>
+ * The contents of a cross process cursor are marshalled to the remote process by
+ * filling {@link CursorWindow} objects using {@link #fillWindow}.  As an optimization,
+ * the cursor can provide a pre-filled window to use via {@link #getWindow} thereby
+ * obviating the need to copy the data to yet another cursor window.
+ */
 public interface CrossProcessCursor extends Cursor {
     /**
-     * returns a pre-filled window, return NULL if no such window
+     * Returns a pre-filled window that contains the data within this cursor.
+     * <p>
+     * In particular, the window contains the row indicated by {@link Cursor#getPosition}.
+     * The window's contents are automatically scrolled whenever the current
+     * row moved outside the range covered by the window.
+     * </p>
+     *
+     * @return The pre-filled window, or null if none.
      */
     CursorWindow getWindow();
 
     /**
-     * copies cursor data into the window start at pos
+     * Copies cursor data into the window.
+     * <p>
+     * Clears the window and fills it with data beginning at the requested
+     * row position until all of the data in the cursor is exhausted
+     * or the window runs out of space.
+     * </p><p>
+     * The filled window uses the same row indices as the original cursor.
+     * For example, if you fill a window starting from row 5 from the cursor,
+     * you can query the contents of row 5 from the window just by asking it
+     * for row 5 because there is a direct correspondence between the row indices
+     * used by the cursor and the window.
+     * </p><p>
+     * The current position of the cursor, as returned by {@link #getPosition},
+     * is not changed by this method.
+     * </p>
+     *
+     * @param position The zero-based index of the first row to copy into the window.
+     * @param window The window to fill.
      */
-    void fillWindow(int pos, CursorWindow winow);
+    void fillWindow(int position, CursorWindow window);
 
     /**
      * This function is called every time the cursor is successfully scrolled
      * to a new position, giving the subclass a chance to update any state it
-     * may have. If it returns false the move function will also do so and the
+     * may have.  If it returns false the move function will also do so and the
      * cursor will scroll to the beforeFirst position.
+     * <p>
+     * This function should be called by methods such as {@link #moveToPosition(int)},
+     * so it will typically not be called from outside of the cursor class itself.
+     * </p>
      *
-     * @param oldPosition the position that we're moving from
-     * @param newPosition the position that we're moving to
-     * @return true if the move is successful, false otherwise
+     * @param oldPosition The position that we're moving from.
+     * @param newPosition The position that we're moving to.
+     * @return True if the move is successful, false otherwise.
      */
     boolean onMove(int oldPosition, int newPosition); 
-    
 }

core/java/android/database/CrossProcessCursorWrapper.java

@@ -0,0 +1,75 @@
+/*
+ * Copyright (C) 2011 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License
+ */
+
+package android.database;
+
+import android.database.CrossProcessCursor;
+import android.database.Cursor;
+import android.database.CursorWindow;
+import android.database.CursorWrapper;
+
+/**
+ * Cursor wrapper that implements {@link CrossProcessCursor}.
+ * <p>
+ * If the wrapper cursor implemented {@link CrossProcessCursor}, then delegates
+ * {@link #fillWindow}, {@link #getWindow()} and {@link #onMove} to it.  Otherwise,
+ * provides default implementations of these methods that traverse the contents
+ * of the cursor similar to {@link AbstractCursor#fillWindow}.
+ * </p><p>
+ * This wrapper can be used to adapt an ordinary {@link Cursor} into a
+ * {@link CrossProcessCursor}.
+ * </p>
+ */
+public class CrossProcessCursorWrapper extends CursorWrapper implements CrossProcessCursor {
+    /**
+     * Creates a cross process cursor wrapper.
+     * @param cursor The underlying cursor to wrap.
+     */
+    public CrossProcessCursorWrapper(Cursor cursor) {
+        super(cursor);
+    }
+
+    @Override
+    public void fillWindow(int position, CursorWindow window) {
+        if (mCursor instanceof CrossProcessCursor) {
+            final CrossProcessCursor crossProcessCursor = (CrossProcessCursor)mCursor;
+            crossProcessCursor.fillWindow(position, window);
+            return;
+        }
+
+        DatabaseUtils.cursorFillWindow(mCursor, position, window);
+    }
+
+    @Override
+    public CursorWindow getWindow() {
+        if (mCursor instanceof CrossProcessCursor) {
+            final CrossProcessCursor crossProcessCursor = (CrossProcessCursor)mCursor;
+            return crossProcessCursor.getWindow();
+        }
+
+        return null;
+    }
+
+    @Override
+    public boolean onMove(int oldPosition, int newPosition) {
+        if (mCursor instanceof CrossProcessCursor) {
+            final CrossProcessCursor crossProcessCursor = (CrossProcessCursor)mCursor;
+            return crossProcessCursor.onMove(oldPosition, newPosition);
+        }
+
+        return true;
+    }
+}

core/java/android/database/CursorToBulkCursorAdaptor.java

@@ -89,11 +89,10 @@ public void onChange(boolean selfChange) {
 
     public CursorToBulkCursorAdaptor(Cursor cursor, IContentObserver observer,
             String providerName) {
-        try {
-            mCursor = (CrossProcessCursor) cursor;
-        } catch (ClassCastException e) {
-            throw new UnsupportedOperationException(
-                    "Only CrossProcessCursor cursors are supported across process for now", e);
+        if (cursor instanceof CrossProcessCursor) {
+            mCursor = (CrossProcessCursor)cursor;
+        } else {
+            mCursor = new CrossProcessCursorWrapper(cursor);
         }
         mProviderName = providerName;
 

core/java/android/database/CursorWrapper.java

@@ -25,9 +25,13 @@
  * use for this class is to extend a cursor while overriding only a subset of its methods.
  */
 public class CursorWrapper implements Cursor {
+    /** @hide */
+    protected final Cursor mCursor;
 
-    private final Cursor mCursor;
-
+    /**
+     * Creates a cursor wrapper.
+     * @param cursor The underlying cursor to wrap.
+     */
     public CursorWrapper(Cursor cursor) {
         mCursor = cursor;
     }