Content Provider基础(一)
目的
l 原理
l 从ContentProvider获取数据
l 插入、更新、删除数据
Content Provider 工作原理
l 概览
Content Provider对外部应用程序提供数据共享的表示形式，和关系数据库的表结构类似，例如下面用户字典的表格。每一行唯一表示了一个单词的信息。
| word |
app id |
frequency |
locale |
_ID |
| mapreduce |
user1 |
100 |
en_US |
1 |
| precompiler |
user14 |
200 |
fr_FR |
2 |
| applet |
user2 |
225 |
fr_CA |
3 |
| const |
user1 |
255 |
pt_BR |
4 |
| int |
user5 |
100 |
en_UK |
5 |
l 访问Provider
应用程序中需要访问某个Content Provider的数据是通过ContentResolver对象实现的。Content Provider对象实例的方法提供了基本的CRUD方法用于数据的持久化操作。另外，为达到这些操作的目的，需要在manifast file中提供相应的权限：Content Provider Permissions.
例如，要查询Provider所有的用户单词和locales,需要调用ContentResolver.query():
// Queries the user dictionary and returns results
mCursor = getContentResolver().query(
UserDictionary.Words.CONTENT_URI, // The content URI of the words table
mProjection, // The columns to return for each row
mSelectionClause // Selection criteria
mSelectionArgs, // Selection criteria
mSortOrder); // The sort order for the returned rows
l Content URIs
一个content URI 唯一标识了provider的数据资源。每个Content URL包括了Provider的入口和数据表格的路径（表格的某列）。当我们要去访问Provider数据资源时，也就是在调用ContentResolver的方法，URI将会作为一个参数传入。例如，访问字典中的单词时，URI描述如下：
content://user_dictionary/words
通常，我们希望获取某指定行的数据，需要给URI追加指定行的ID，例如，获取_ID为4的行：
Uri singleUri = ContentUri.withAppendedId(UserDictionary.Words.CONTENT_URI,4);//withAppendedId用于对URI追加指定的ID。
获取provider的数据
l 查询的构建
1.便于后面的处理，下面代码片段是用于访问User Dictionary Provider的一些变量定义：
// A "projection" defines the columns that will be returned for each row
String[] mProjection =
{
UserDictionary.Words._ID, // Contract class constant for the _ID column name
UserDictionary.Words.WORD, // Contract class constant for the word column name
UserDictionary.Words.LOCALE // Contract class constant for the locale column name
};
// Defines a string to contain the selection clause
String mSelectionClause = null;
// Initializes an array to contain selection arguments
String[] mSelectionArgs = {""};
2下一个片段中是介绍ContentResolver.query()的详细使用。对于provider的查询实际上与SQL有着相同的语法。如下是对word的条件查询：
/*
* This defines a one-element String array to contain the selection argument.
*/
String[] mSelectionArgs = {""};
// Gets a word from the UI
mSearchString = mSearchWord.getText().toString();
// Remember to insert code here to check for invalid or malicious input.
// If the word is the empty string, gets everything
if (TextUtils.isEmpty(mSearchString)) {
// Setting the selection clause to null will return all words
mSelectionClause = null;
mSelectionArgs[0] = "";
} else {
// Constructs a selection clause that matches the word that the user entered.
mSelectionClause = UserDictionary.Words.WORD + " = ?";
// Moves the user's input string to the selection arguments.
mSelectionArgs[0] = mSearchString;
}
// Does a query against the table and returns a Cursor object
mCursor = getContentResolver().query(
UserDictionary.Words.CONTENT_URI, // The content URI of the words table
mProjection, // The columns to return for each row
mSelectionClause // Either null, or the word the user entered
mSelectionArgs, // Either empty, or the string the user entered
mSortOrder); // The sort order for the returned rows
// Some providers return null if an error occurs, others throw an exception
if (null == mCursor) {
/*
* Insert code here to handle the error. Be sure not to use the cursor! You may want to
* call android.util.Log.e() to log this error.
*
*/
// If the Cursor is empty, the provider found no matches
} else if (mCursor.getCount() < 1) {
/*
* Insert code here to notify the user that the search was unsuccessful. This isn't necessarily
* an error. You may want to offer the user the option to insert a new row, or re-type the
* search term.
*/
} else {
// Insert code here to do something with the results
}
上面的功能与下面的查询语句是相似的：
SELECT _ID, word, frequency, locale FROM words WHERE word = <userinput> ORDER BY word ASC;
l 防止恶意攻击和SQL注入
在如下的代码片段中：
// Constructs a selection clause by concatenating the user's input to the column name String mSelectionClause = "var = " + mUserInput;
如果用户输入nothing; DROP TABLE *,那么产生的结果是数据库的表将会全部被清除。
为了避免这种结果，可以用“?”代替，如：
// Constructs a selection clause with a replaceable parameter String mSelectionClause = "var = ?";
l 数据集的展示
ContentResolver.query()查询返回一个Cursor实例，它提供按行迭代访问数据的方法。
如果没有匹配的查询结果，返回的Cursor掉用getCount()返回0；
如果出现异常，返回null.
我们知道，既然Cursor是一个按行的记录列（list of rows），比较好的展现数据的形式是通过SimpleCursorAdapter绑定数据的ListView控件。示例片段如下：
// Defines a list of columns to retrieve from the Cursor and load into an output row
String[] mWordListColumns =
{
UserDictionary.Words.WORD, // Contract class constant containing the word column name
UserDictionary.Words.LOCALE // Contract class constant containing the locale column name
};
// Defines a list of View IDs that will receive the Cursor columns for each row
int[] mWordListItems = { R.id.dictWord, R.id.locale};
// Creates a new SimpleCursorAdapter
mCursorAdapter = new SimpleCursorAdapter(
getApplicationContext(), // The application's Context object
R.layout.wordlistrow, // A layout in XML for one row in the ListView
mCursor, // The result from the query
mWordListColumns, // A string array of column names in the cursor
mWordListItems, // An integer array of view IDs in the row layout
0); // Flags (usually none are needed)
// Sets the adapter for the ListView
mWordList.setAdapter(mCursorAdapter);
l 数据集中数据的遍历
// Determine the column index of the column named "word"
int index = mCursor.getColumnIndex(UserDictionary.Words.WORD);
/*
* Only executes if the cursor is valid. The User Dictionary Provider returns null if
* an internal error occurs. Other providers may throw an Exception instead of returning null.
*/
if (mCursor != null) {
/*
* Moves to the next row in the cursor. Before the first movement in the cursor, the
* "row pointer" is -1, and if you try to retrieve data at that position you will get an
* exception.
*/
while (mCursor.moveToNext()) {
// Gets the value from the column.
newWord = mCursor.getString(index);
// Insert code here to process the retrieved word.
...
// end of while loop
}
} else {
// Insert code here to report an error if the cursor is null or the provider threw an exception.
}
Cursor提供许多“get”方法来获取不同类型的数据，如上面的getString().当然，getType()方法可以获取某一列的数据类型。
插入、更新和删除数据
l 插入数据
使用ContentResolver.insert()方法来插入一行数据，该方法返回这一行的URI，代码片段如下：
// Defines a new Uri object that receives the result of the insertion
Uri mNewUri;
...
// Defines an object to contain the new values to insert
ContentValues mNewValues = new ContentValues();
/*
* Sets the values of each column and inserts the word. The arguments to the "put"
* method are "column name" and "value"
*/
mNewValues.put(UserDictionary.Words.APP_ID, "example.user");
mNewValues.put(UserDictionary.Words.LOCALE, "en_US");
mNewValues.put(UserDictionary.Words.WORD, "insert");
mNewValues.put(UserDictionary.Words.FREQUENCY, "100");
mNewUri = getContentResolver().insert(
UserDictionary.Word.CONTENT_URI, // the user dictionary content URI
mNewValues // the values to insert
);
若某一列的数据为空，则可以赋值null或者使用ContentValues.putNull();我们注意到在插入数据的时候没有给_ID赋值，这是因为该列是自增的。在我们不需要_ID的时候完全可以不必要用(ListView 除外);如果需要或得该Uri的_ID,调用ContentUris.parseId();
l 更新数据
更新数据用到ContentResolver.update();示例如下：
// Defines an object to contain the updated values
ContentValues mUpdateValues = new ContentValues();
// Defines selection criteria for the rows you want to update
String mSelectionClause = UserDictionary.Words.LOCALE + "LIKE ?";
String[] mSelectionArgs = {"en_%"};
// Defines a variable to contain the number of updated rows
int mRowsUpdated = 0;
...
/*
* Sets the updated value and updates the selected words.
*/
mUpdateValues.putNull(UserDictionary.Words.LOCALE);
mRowsUpdated = getContentResolver().update(
UserDictionary.Words.CONTENT_URI, // the user dictionary content URI
mUpdateValues // the columns to update
mSelectionClause // the column to select on
mSelectionArgs // the value to compare to
);
l 删除数据
删除数据使用getContentResolver().delete.
// Defines selection criteria for the rows you want to delete
String mSelectionClause = UserDictionary.Words.APP_ID + " LIKE ?";
String[] mSelectionArgs = {"user"};
// Defines a variable to contain the number of rows deleted
int mRowsDeleted = 0;
...
// Deletes the words that match the selection criteria
mRowsDeleted = getContentResolver().delete(
UserDictionary.Words.CONTENT_URI, // the user dictionary content URI
mSelectionClause // the column to select on
mSelectionArgs // the value to compare to
);
Provider数据类型
Provider提供如下数据类型：
text
integer
long integer（long）
floating point（float）
long floating point（double）
Binary Large Object（BLOB）