Content Providers是Android中四大组件之一，用于管理应用程序访问结构化的数据。Content Providers可以压缩数据（They encapsulate the data），并保护访问数据的安全。Content Providers是应用程序访问跨进程数据的标准接口（standard interface that connects data in one process with code running in another process）。

Content Providers

Content Provider 基础（Content Provider Basics）

• content providers 是如何工作的；
• 从content provider中检索数据API的使用；
• content provider中增删改查API的使用；
• 其他content provider的API；

概述（Overview）

访问provider（Accessing a provider）

// 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

内容URIs（Content URIs）

所以，如上所述，words表应具有如下URI：

content://user_dictionary/words

Uri singleUri = ContentUris.withAppendedId(UserDictionary.Words.CONTENT_URI,4);

这表示希望查询单词表中_ID为4 的那一行数据。当您需要删除或更新某一行数据时，应当使用这种加入了_ID的URI。

从Provider中查询数据（Retrieving Data from the Provider）

1. 声明读取数据的访问权限（Request the read access permission for the provider）；
2. 发送查询请求（Define the code that sends a query to the provider） ；

声明权限（Requesting read access permission）

构建查询条件（Constructing the query）

// 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 = {""};

/*
 * 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

}

SELECT _ID, word, locale FROM words WHERE word = <userinput> ORDER BY word ASC

防止用户恶意输入（Protecting against malicious input）

// Constructs a selection clause by concatenating the user‘s input to the column name
String mSelectionClause =  "var = " + mUserInput;

// Constructs a selection clause with a replaceable parameter
String mSelectionClause =  "var = ?";
// Defines an array to contain the selection arguments
String[] selectionArgs = {""};
// Sets the selection argument to the user‘s input
selectionArgs[0] = mUserInput;

显示查询结果（Displaying query results）

/ 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);

从Cursor中获取数据（Getting data from query results）

// 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.
}

Content Provider 权限（Content Provider Permissions）

<uses-permission android:name="android.permission.READ_USER_DICTIONARY">

增删改数据（Inserting, Updating, and Deleting Data）

插入数据（Inserting data）

// 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
);

content://user_dictionary/words/<id_value>

更新数据（Updating data）

// 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
);

删除数据（Deleting data）

// 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管理的数据类型（Provider Data Types）

• integer；
• long integer (long)；
• floating point；
• long floating point (double)。

访问Provider的其他方式（Alternative Forms of Provider Access）

• 批量访问（Batch access）：您可以使用ContentProviderOperation类对provider进行批量访问。并在client调用ContentResolver.applyBatch()方法实现批量操作。

• 异步查询（Asynchronous queries）：您应当在子线程中执行查询操作，可以使用CursorLoader类来实现。

• 通过Intent访问数据（Data access via intents）：虽然不能直接向provider传递intent，但是可以向provider所在的应用传递。这种方式提供了修改provider中数据的最齐全的方法（which is usually the best-equipped to modify the provider’s data）。

批量访问（Batch access）

使用Intent访问数据（Data access via intents）

获取临时访问权限（Getting access with temporary permissions）

• 读取权限：FLAG_GRANT_READ_URI_PERMISSION；
• 写入权限：FLAG_GRANT_WRITE_URI_PERMISSION

• 调用startActivityForResult()方法，传入Intent，Intent应包含ACTION_PICK的action，和contact的MIME类型CONTENT_ITEM_TYPE；
• 这会启动通讯录应用的selectionactivity，该activity会切至前台；
• 在selectionactivity中，用户会选择某个联系人并修改其信息，此时，setResult(resultcode, intent)方法将被回调，并将intent回传至您的应用中，intent中包含了用户选择的联系人的content URI，并在extra中包含了FLAG_GRANT_READ_URI_PERMISSIONflag。该flag提供了您的应用程序访问联系人通讯中指定数据的权限（These flags grant URI permission to your app to read data for the contact pointed to by the content URI）。最后，selectionactivity调用finish()destroy。
• 您的activity切回前台并回调onActivityResult()方法获取回传的intent，从Intent中获取的content URI可以直接访问联系人应用中的provider数据而无需在manifest中声明权限。

MIME类型引用（MIME Type Reference）

type/subtype

vnd.android.cursor.dir表示URI指向的数据包含多行。而vnd.android.cursor.item表示包含一行。

vnd.android.cursor.item/phone_v2