Android-Content Provide

内容提供者 是为了不同的应用程序共享数据的

主要有以下几个方面 1.为其他程序提供数据 2.获取其他程序提供的数据

类似于服务器客户端这样的程序 提供ContentProvider的是服务器 接收ContentProvider的是客户端

一.如何获得ContentProvider的数据

看如下的几行代码

// 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

客户端获取一个ContentResolver的类 然后调用其增删该查方法获取数据 返回一个Cursor的数据

参数理解如下

URI：唯一的标识 应该和服务器端的URI相同

比如这样的形式：

content://user_dictionary/words

其中 content://是固定的 后面的服务器约定的字段 一般后面是表明

Projection：投影 学过数据库的都知道 投影就是挑选某几列 这是一个数组

SelectionClause:选择的添加 相当于数据库查询语句的where字句的条件

SelectionArgs：是SelectionClause的占位符？所代表的参数

SortOrder：按照什么排序

比如如下代码：

/*
* 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

}

接下来如何显示查询出来的Cursor结果呢
1.通过Adapter

// Sets the adapter for the ListView
mWordList.setAdapter(mCursorAdapter);

2.或者是通过一个While循环

// 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.
}增删该查如下：

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
);

然后返回的是插入的数据的id

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
);

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
);