Android 4 学习（17）：使用Content Resolver

Content Resolver简介

每个应用程序都有一个ContentResolver实例，通过getContentResolver()方法可以获取：

ContentResolver cr = getContentResolver();

与Content Provider对应，Content Resolver用于使用Content Provider发布的数据。使用ContentResolver查询ConentProvider提供的数据：

// Get the Content Resolver.

ContentResolver cr = getContentResolver();

// Specify the result column projection. Return the minimum set of columns required to satisfy your requirements.

String[] result_columns = new String[] {

　　MyHoardContentProvider.KEY_ID,

　　MyHoardContentProvider.KEY_GOLD_HOARD_ACCESSIBLE_COLUMN,

　　MyHoardContentProvider.KEY_GOLD_HOARDED_COLUMN };

// Specify the where clause that will limit your results.

String where = MyHoardContentProvider.KEY_GOLD_HOARD_ACCESSIBLE_COLUMN + “=” + 1;

// Replace these with valid SQL statements as necessary.

String whereArgs[] = null;

String order = null;

// Return the specified rows.

Cursor resultCursor = cr.query(MyHoardContentProvider.CONTENT_URI, result_columns, where, whereArgs, order);

从Cursor中解析数据的方法和上一篇博文中介绍的一样，首先要移动到指定行，然后从指定列中获取对应类型的数据：

float largestHoard = 0f;

String hoardName = “No Hoards”;

// Find the index to the column(s) being used.

int GOLD_HOARDED_COLUMN_INDEX = resultCursor.getColumnIndexOrThrow(

MyHoardContentProvider.KEY_GOLD_HOARDED_COLUMN);

int HOARD_NAME_COLUMN_INDEX = resultCursor.getColumnIndexOrThrow(

MyHoardContentProvider.KEY_GOLD_HOARD_NAME_COLUMN);

// Iterate over the cursors rows.

// The Cursor is initialized at before first, so we can

// check only if there is a “next” row available. If the

// result Cursor is empty, this will return false.

while (resultCursor.moveToNext()) {

float hoard = resultCursor.getFloat(GOLD_HOARDED_COLUMN_INDEX);

if (hoard > largestHoard) {

largestHoard = hoard;

hoardName = resultCursor.getString(HOARD_NAME_COLUMN_INDEX);

}

}

// Close the Cursor when you’ve finished with it.

resultCursor.close();

使用Cursor Loader进行异步查询

Cursor Loader存在于在每个Activity和Fragment中，可以实现异步查询和监听底层数据的变化。例如管理Cursor的生命周期，确保在Activity退出之前Cursor被关闭。

实现LoaderCallbacks接口

若要使用Cursor Loader，首先要实现LoaderManager.LoaderCallbacks接口：

LoaderManager.LoaderCallbacks<Cursor> loaderCallback = new LoaderManager.LoaderCallbacks<Cursor>()

LoaderCallback主要有这几个回调方法：

onCreateLoader
onLoadFinished
onLoaderReset

public Loader<Cursor> onCreateLoader(int id, Bundle args) {

　　// Construct the new query in the form of a Cursor Loader. Use the id

　　// parameter to construct and return different loaders.

　　String[] projection = null;

　　String where = null;

　　String[] whereArgs = null;

　　String sortOrder = null;

　　// Query URI

　　Uri queryUri = MyContentProvider.CONTENT_URI;

　　// Create the new Cursor loader.

　　return new CursorLoader(DatabaseSkeletonActivity.this, queryUri,projection, where, whereArgs, sortOrder);

}

public void onLoadFinished(Loader<Cursor> loader, Cursor cursor) {

　　// Replace the result Cursor displayed by the Cursor Adapter with the new result set.

　　adapter.swapCursor(cursor);

　　// This handler is not synchronized with the UI thread, so you will need to synchronize it before modifying any UI elements directly.

}

public void onLoaderReset(Loader<Cursor> loader) {

　　// Remove the existing result Cursor from the List Adapter.

　　adapter.swapCursor(null);

　　// This handler is not synchronized with the UI thread, so you

　　// will need to synchronize it before modifying any UI elements directly.

}

初始化和重启Cursor Loader

在Activity和Fragment中，可以调用getLoaderManager获得Cursor Loader

LoaderManager loaderManager = getLoaderManager();

获取Cursor Loader：

Bundle args = null;

loaderManager.initLoader(LOADER_ID, args, myLoaderCallbacks);

LOADER_ID用于标识获得的loader，args是可选参数，myLoaderCallbacks则是前面LoaderManager.LoaderCallbacks接口的实现。如果对应LOADER_ID的Cursor Loader不存在，那么系统会调用onCreateLoader方法来创建一个Loader。

使用Content Resolver增、改、删

插入数据：

// Create a new row of values to insert.

ContentValues newValues = new ContentValues();

// Assign values for each row.

newValues.put(MyHoardContentProvider.KEY_GOLD_HOARD_NAME_COLUMN, hoardName);

newValues.put(MyHoardContentProvider.KEY_GOLD_HOARDED_COLUMN, hoardValue);

newValues.put(MyHoardContentProvider.KEY_GOLD_HOARD_ACCESSIBLE_COLUMN, hoardAccessible);

// [ ... Repeat for each column / value pair ... ]

// Get the Content Resolver

ContentResolver cr = getContentResolver();

// Insert the row into your table

Uri myRowUri = cr.insert(MyHoardContentProvider.CONTENT_URI, newValues);

删除数据：

// Specify a where clause that determines which row(s) to delete.

// Specify where arguments as necessary.

String where = MyHoardContentProvider.KEY_GOLD_HOARDED_COLUMN + “=” + 0;

String whereArgs[] = null;

// Get the Content Resolver.

ContentResolver cr = getContentResolver();

// Delete the matching rows

int deletedRowCount = cr.delete(MyHoardContentProvider.CONTENT_URI, where, whereArgs);

修改数据：

// Create the updated row content, assigning values for each row.

ContentValues updatedValues = new ContentValues();

updatedValues.put(MyHoardContentProvider.KEY_GOLD_HOARDED_COLUMN, newHoardValue);

// [ ... Repeat for each column to update ... ]

// Create a URI addressing a specific row.

Uri rowURI = ContentUris.withAppendedId(MyHoardContentProvider.CONTENT_URI, hoardId);

// Specify a specific row so no selection clause is required.

String where = null;

String whereArgs[] = null;

// Get the Content Resolver.

ContentResolver cr = getContentResolver();

// Update the specified row.

int updatedRowCount = cr.update(rowURI, updatedValues, where, whereArgs);

使用Content Resolver读写Content Provider中的数据

public void addNewHoardWithImage(String hoardName, float hoardValue, boolean hoardAccessible, Bitmap bitmap) {

　　// Create a new row of values to insert.

　　ContentValues newValues = new ContentValues();

　　// Assign values for each row.

　　newValues.put(MyHoardContentProvider.KEY_GOLD_HOARD_NAME_COLUMN, hoardName);

　　newValues.put(MyHoardContentProvider.KEY_GOLD_HOARDED_COLUMN, hoardValue);

　　newValues.put(MyHoardContentProvider.KEY_GOLD_HOARD_ACCESSIBLE_COLUMN, hoardAccessible);

　　// Get the Content Resolver

　　ContentResolver cr = getContentResolver();

　　// Insert the row into your table

　　Uri myRowUri = cr.insert(MyHoardContentProvider.CONTENT_URI, newValues);

　　try {

　　　　// Open an output stream using the new row’s URI.

　　　　OutputStream outStream = cr.openOutputStream(myRowUri);

　　　　// Compress your bitmap and save it into your provider.

　　　　bitmap.compress(Bitmap.CompressFormat.JPEG, 80, outStream);

　　}catch (FileNotFoundException e) {

　　　　Log.d(TAG, “No file found for this record.”);

　　}

}

public Bitmap getHoardImage(long rowId) {

　　Uri myRowUri = ContentUris.withAppendedId(MyHoardContentProvider.CONTENT_URI, rowId);

　　try {

　　　　// Open an input stream using the new row’s URI.

　　　　InputStream inStream = getContentResolver().openInputStream(myRowUri);

　　　　// Make a copy of the Bitmap.

　　　　Bitmap bitmap = BitmapFactory.decodeStream(inStream);

　　　　return bitmap;

　　}catch (FileNotFoundException e) {

　　　　Log.d(TAG, “No file found for this record.”);

　　}

　　return null;

}

使用c