Content Providers--怎样去访问一个基本的content provider
一个content provider 管理着怎样去访问存储在应用仓库中的数据。一个content provider是一个应用程序的一部分，它常常用来使应用程序的界面工作在它提供的数据基础上。然而，content providers 主要是用来给其它的应用程序使用的，使用一个provider客户端对象来访问使用它。总之，providers和provider 客户端对象为应用中的数据提供一个一致的、标准的访问接口，并且它也能够处理跨进程间的通信和敏感数据的访问
Accessing a provider
我们可以使用
ContentResolver
客户端对象去访问一个content provider，我们可以用
ContentResolver
实现对content provider提供的数据源进行CRUD
ContentResolver
对象是运行在应用的主线程中的，有可能会阻塞主线程，它
不用做过多的操作就可以
进行跨进程间的通信，它担当着本身仓库数据和外部表数据的沟通者
当访问一个provider时，你需要在manifest文件中增加 Content Provider Permissions权限声明
我们可以使用URI去定位一个content provider,使用SQL语句描述我们要进行的操作
data as table words
| 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 |
查询操作
当我们进行查询时，并不像我们用一个url去定位一个资源时那样，一股脑的把所有的数据都返回回来，而是返回一个Cursor对象，让我们可以更明智的获取数据
SQL : select _ID,word,locale from words where word = ?
private void accessContract(String mSearchString){
Log. e(TAG, mSearchString);
//uri 定位
Uri uri = UserDictionary.Words. CONTENT_URI;
//access a single row in a table
//Uri singleUri = ContentUris.withAppendedId(UserDictionary.Words.CONTENT_URI, 3);
// A "projection" defines the columns that will be returned for each row
// 要查询的字段
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
// 查询条件
String mSelectionClause = null;
// Initializes an array to contain selection arguments
// 查询条件的值
String[] mSelectionArgs = { ""};
// 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;
}
// Queries the user dictionary and returns results
Cursor mCursor = getContentResolver().query(
uri, // The content URI of the words table
mProjection, // The columns to return for each row
mSelectionClause, // Selection criteria
//null,
mSelectionArgs, // Selection criteria
//null,
UserDictionary.Words. WORD); // The sort order for the returned rows
//last generate sql
//SELECT _ID, word, frequency, locale FROM words WHERE word = <userinput> ORDER BY word ASC;
// 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
Log. e(TAG, "mCursor is null");
} 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.
*/
Log. e(TAG, "mCursor is empty");
} else {
// Insert code here to do something with the results
Log. e(TAG, "mCursor is not empty");
//display(mCursor);
}
//mCursor.close();
}
插入操作
用ContentValues封装要插入的字段和值
SQL: insert into words(_ID,locale,word,frequency) values (?,?,?,?)
private void insert(){
// 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.Words. CONTENT_URI, // the user dictionary content URI
mNewValues // the values to insert
);
Log. e(TAG, mNewUri.getPath());
}
更新操作
SQL: update words set locale=? where locale=?
private void update(){
// 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
);
Log. e(TAG, "effect row "+mRowsUpdated);
}
删除操作
SQL:delete from words where word=?
private void delete(){
// Defines selection criteria for the rows you want to delete
String mSelectionClause = UserDictionary.Words. WORD + " = ?" ;
String[] mSelectionArgs = { "insert"};
// 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
);
Log. e(TAG, "delete effect row "+mRowsDeleted);
}
使用SimpleCursorAdapter方便的来显示一个数据源数据
private void display(Cursor mCursor) {
// 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
SimpleCursorAdapter mCursorAdapter = new SimpleCursorAdapter(
getApplicationContext(), // The application's Context object
R.layout. wordlist_item, // 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
); // Flags (usually none are needed)
// Sets the adapter for the ListView
mWordList.setAdapter(mCursorAdapter);
}