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;
}