简单探索ContentProviderOperation

前面一片文章中用到了ContentProviderOperation，那么我们就来看看ContentProviderOperation到底是怎么工作的。

1. ContentProviderOperation实现了Parcelable，构造器是私有的，因此不能直接创建该对象，代码如下：

 public class ContentProviderOperation implements Parcelable {
     /** @hide exposed for unit tests */
     public final static int TYPE_INSERT = 1;
     /** @hide exposed for unit tests */
     public final static int TYPE_UPDATE = 2;
     /** @hide exposed for unit tests */
     public final static int TYPE_DELETE = 3;
     /** @hide exposed for unit tests */
     public final static int TYPE_ASSERT = 4;

     private final int mType;
     private final Uri mUri;
     private final String mSelection;
     private final String[] mSelectionArgs;
     private final ContentValues mValues;
     private final Integer mExpectedCount;
     private final ContentValues mValuesBackReferences;
     private final Map<Integer, Integer> mSelectionArgsBackReferences;
     private final boolean mYieldAllowed;

     private final static String TAG = "ContentProviderOperation";
     private ContentProviderOperation(Builder builder) {
         mType = builder.mType;
         mUri = builder.mUri;
         mValues = builder.mValues;
         mSelection = builder.mSelection;
         mSelectionArgs = builder.mSelectionArgs;
         mExpectedCount = builder.mExpectedCount;
         mSelectionArgsBackReferences = builder.mSelectionArgsBackReferences;
         mValuesBackReferences = builder.mValuesBackReferences;
         mYieldAllowed = builder.mYieldAllowed;
     }

提供了一些方法来进行RUID操作，代码如下：

     public static Builder newInsert(Uri uri) {
         return new Builder(TYPE_INSERT, uri);
     }

     /**
      * Create a {@link Builder} suitable for building an update
      * {@link ContentProviderOperation}.
      * @param uri The {@link Uri} that is the target of the update.
      * @return a {@link Builder}
      */
     public static Builder newUpdate(Uri uri) {
         return new Builder(TYPE_UPDATE, uri);
     }

     /**
      * Create a {@link Builder} suitable for building a delete
      * {@link ContentProviderOperation}.
      * @param uri The {@link Uri} that is the target of the delete.
      * @return a {@link Builder}
      */
     public static Builder newDelete(Uri uri) {
         return new Builder(TYPE_DELETE, uri);
     }

     /**
      * Create a {@link Builder} suitable for building a
      * {@link ContentProviderOperation} to assert a set of values as provided
      * through {@link Builder#withValues(ContentValues)}.
      */
     public static Builder newAssertQuery(Uri uri) {
         return new Builder(TYPE_ASSERT, uri);
     }

其中比较难理解的是newAssertQuery方法，该方法并不是用来查询数据的，可以理解为断点查询，也就是查询有没有符合条件的数据，如果没有，会抛出一个OperationApplicationException异常。

 public void onClick(View v) {
     final ArrayList<ContentProviderOperation> operationList =
              new ArrayList<ContentProviderOperation>();
     ContentProviderOperation.Builder builder = null;
     int rawContactIndex = 0;

     builder = ContentProviderOperation
             .newAssertQuery(RawContacts.CONTENT_URI);
     builder.withSelection("version=?", new String[]{String.valueOf(3)})
             .withExpectedCount(3);
     operationList.add(builder.build());

     try {
         ContentProviderResult[] res = getContentResolver().
             applyBatch(ContactsContract.AUTHORITY, operationList);
         Log.e(TAG, "res.length = " + res.length);
         for (int i = 0; i < res.length; i++) {
             Log.e(TAG, i + "res.toString() = " + res[i].toString());
             Log.e(TAG, i + "res.uri = " + res[i].uri);
         }
     } catch (RemoteException e) {
     } catch (OperationApplicationException e) {
     }
 }

以上这个onClick()方法的作用是查询version==3的数据是否有三条uri对应的表里面，如果为真，res ！= null，反之会抛出OperationApplicationException异常，其他使用情形可以参照：http://blog.sina.com.cn/s/blog_a387d6d2010114mp.html

2. 我们在使用ContentProviderOperation时，通常是采用这样的方式

builder = ContentProviderOperation.newInsert(RawContacts.CONTENT_URI); 然后我们可以给这个builder添加一些条件，最后调用他的build()方法返回一个ContentProviderOperation对象。看它的

build()方法， 代码如下：

         /** Create a ContentProviderOperation from this {@link Builder}. */
         public ContentProviderOperation build() {
             if (mType == TYPE_UPDATE) {
                 if ((mValues == null || mValues.size() == 0) &&
                     (mValuesBackReferences == null ||
                      mValuesBackReferences.size() == 0)) {
                         throw new IllegalArgumentException("Empty values");
                 }
             }
             if (mType == TYPE_ASSERT) {
                 if ((mValues == null || mValues.size() == 0) &&
                     (mValuesBackReferences == null ||
                      mValuesBackReferences.size() == 0) &&
                      (mExpectedCount == null)) {
                        throw new IllegalArgumentException("Empty values");
                 }
             }
             return new ContentProviderOperation(this);
         }

可以看到new ContentProviderOperation()方法里做了赋值的操作。

3. 最后调用 getContentResolver().applyBatch(ContactsContract.AUTHORITY, operationList) 进行批处理操作，可以这样理解，之前构造ContentProviderOperation的过程就像是在组合SQL语句，而applyBatch相当于执行SQL语句。具体流程是： ContentResolver.java（applyBatch（））--> ContentProviderClient.java(applyBatch()) --> ContentProvider.java, ContentProvider的applyBatch（）方法如下：

 public ContentProviderResult[] applyBatch(
         ArrayList<ContentProviderOperation> operations)
         throws OperationApplicationException {
     final int numOperations = operations.size();
     final ContentProviderResult[] results =
           new ContentProviderResult[numOperations];
     for (int i = 0; i < numOperations; i++) {
         results[i] = operations.get(i).apply(this, results, i);
     }
     return results;
 }

可以看到，用了一个循环，但最终调用的还是ContentProviderOperation的apply方法：

 public ContentProviderResult apply(ContentProvider provider,
           ContentProviderResult[] backRefs, int numBackRefs) throws OperationApplicationException {
     ContentValues values =
           resolveValueBackReferences(backRefs, numBackRefs);

首先调用了resolveValueBackReferences()方法， 他会返回mValues或者加工后的mValues：

 public ContentValues resolveValueBackReferences(ContentProviderResult[] backRefs, int numBackRefs) {
     if () {
         return mValues;
     }
     final ContentValues values;
     if (mValues == null) {
         values = new ContentValues();
     } else {
         values = new ContentValues(mValues);
     }
     for (Map.Entry<String, Object> entry :
         mValuesBackReferences.valueSet()) {
         String key = entry.getKey();
         Integer backRefIndex = mValuesBackReferences.getAsInteger(key);
         if (backRefIndex == null) {
             Log.e(TAG, this.toString());
             throw new IllegalArgumentException("
                    values backref " + key + " is not an integer");
         }
         values.put(key, backRefToValue(backRefs, numBackRefs, backRefIndex));
     }
     return values;
 }

为了理解mValuesBackReferences == null这个判断，我们先得知道一个问题，在前面一篇文章中，从第二个ContentProviderOperation开始使用了builder.withValueBackReference(Email.RAW_CONTACT_ID, rawContactIndex)，其实就是给mValuesBackReferences赋了值，代码如下：

 public Builder withValueBackReference(String key, int previousResult) {
     if (mType != TYPE_INSERT && mType != TYPE_UPDATE
                              && mType != TYPE_ASSERT) {
         throw new IllegalArgumentException("only inserts, updates, and asserts can have value back-references");
     }
     if (mValuesBackReferences == null) {
         mValuesBackReferences = new ContentValues();
     }
     mValuesBackReferences.put(key, previousResult);
     return this;
 }

可以看到这个方法不能用于TYPE_DELETE。

继续看resolveValueBackReferences方法，如果mValuesBackReferences == null，直接返回mValues，mValues就是通过withValues（）方法填进去的值所组装的ContentValue对象，比如要更新或要插入的值。如果mValuesBackReferences ！= null，那么会将前一个ContentProviderResult的uri里面的id取出来赋给Email.RAW_CONTACT_ID，就是builder.withValueBackReference(Email.RAW_CONTACT_ID, rawContactIndex)的第一个变量，现在withValueBackReference方法的作用我们也该清楚了。具体的可以参照：http://blog.sina.com.cn/s/blog_a387d6d2010114mp.html

继续看apply()方法：

 public ContentProviderResult apply(ContentProvider provider, ContentProviderResult[] backRefs,
             int numBackRefs) throws OperationApplicationException {
         ContentValues values = resolveValueBackReferences(backRefs, numBackRefs);
         String[] selectionArgs =
                 resolveSelectionArgsBackReferences(backRefs, numBackRefs);

         if (mType == TYPE_INSERT) {
             Uri newUri = provider.insert(mUri, values);
             if (newUri == null) {
                 throw new OperationApplicationException("insert failed");
             }
             return new ContentProviderResult(newUri);
         }

         int numRows;
         if (mType == TYPE_DELETE) {
             numRows = provider.delete(mUri, mSelection, selectionArgs);
         } else if (mType == TYPE_UPDATE) {
             numRows = provider.update(mUri, values, mSelection, selectionArgs);
         } else if (mType == TYPE_ASSERT) {
             // Assert that all rows match expected values
             String[] projection =  null;
             if (values != null) {
                 // Build projection map from expected values
                 final ArrayList<String> projectionList = new ArrayList<String>();
                 for (Map.Entry<String, Object> entry : values.valueSet()) {
                     projectionList.add(entry.getKey());
                 }
                 projection = projectionList.toArray(new String[projectionList.size()]);
             }
             final Cursor cursor = provider.query(mUri, projection, mSelection, selectionArgs, null);
             try {
                 numRows = cursor.getCount();
                 if (projection != null) {
                     while (cursor.moveToNext()) {
                         for (int i = 0; i < projection.length; i++) {
                             final String cursorValue = cursor.getString(i);
                             final String expectedValue = values.getAsString(projection[i]);
                             if (!TextUtils.equals(cursorValue, expectedValue)) {
                                 // Throw exception when expected values don't match
                                 Log.e(TAG, this.toString());
                                 throw new OperationApplicationException("Found value " + cursorValue
                                         + " when expected " + expectedValue + " for column "
                                         + projection[i]);
                             }
                         }
                     }
                 }
             } finally {
                 cursor.close();
             }
         } else {
             Log.e(TAG, this.toString());
             throw new IllegalStateException("bad type, " + mType);
         }

         if (mExpectedCount != null && mExpectedCount != numRows) {
             Log.e(TAG, this.toString());
             throw new OperationApplicationException("wrong number of rows: " + numRows);
         }

         return new ContentProviderResult(numRows);
     }

我们可以发现，ContentProviderOperation的apply()方法才是真正的执行RUID操作的地方。

同时我们在上面的代码中并未发现使用事务，如果我们要求操作失败时需回滚，那么就应该添加事务，经过上面的分析，可以发现一个比较好的思路就是在我们自己的ContentProvider里面重写appltBatch()方法，并在其中添加事务，后面会有专门分析ContactsProvider文章，我们会看到，其实联系人的ContactsProcider中采用的就是这个思路。