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  • Content Provider Basics(内容提供者的基本操作)

    Content Provider Basics（基本操作）

    Content provider 管理访问中心库的数据。provider 是android 应用的一部分，它提供自己的UI操作数据。当然，content provider 主要作用是被其他的应用使用。provider 和provider clients是不可分割的，标准的数据接口保证了访问数据的安全性和处理跨进程调用。

    This topic describes the basics of the following:

    • How content providers work. (Content provider 是的原理)
    • The API you use retrieve data from a content provider.(通过content provider API 取得数据)
    • The API you use to insert, update, or delete data in a content provider.(通过Content provider 插入，删除，更新数据)
    • Other API features that facilitate working with providers.(其他的API 特征可以有效的和Provider 共用)

    Overview

    Content provider 把数据暴露给其他的应用，就相当于是其他应用的表。一行数据是provider 集合里面的一个实例。每行里面的一列数据是数据实例的一小片数据。


    For example, one of the built-in providers in the Android platform is the user dictionary, which stores the spellings of non-standard words that the user wants to keep. Table 1 illustrates what the data might look like in this provider's table:

    Table 1: Sample user dictionary table.

    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

    In table 1, each row represents an instance of a word that might not be found in a standard dictionary. Each column represents some data for that word, such as the locale in which it was first encountered. The column headers are column names that are stored in the provider. To refer to a row's locale, you refer to its locale column. For this provider, the _ID column serves as a "primary key" column that the provider automatically maintains.

    Note: A provider isn't required to have a primary key, and it isn't required to use _ID as the column name of a primary key if one is present. However, if you want to bind data from a provider to a ListView, one of the column names has to be _ID. This requirement is explained in more detail in the section Displaying query results.

    Accessing a provider

    一个应用通过ContentResolver 访问数据。用ContentResolver调用的方法，与provider里面的方法是一致，是ContentProvider的一个具体的子类。ContentResolver提供最基本的CRUD方法处理数据。

    ContentResolver 对象是在客户应用进程中中的，ContentProvider对象在数据源应用中处理内部进程的通讯。ContentProvider在自己的数据库和外部应用数据表中作为一个抽象层。

    Note: To access a provider, your application usually has to request specific permissions in its manifest file. This is described in more detail in the section Content Provider Permissions

    For example, to get a list of the words and their locales from the User Dictionary Provider, you callContentResolver.query(). The query() method calls the ContentProvider.query() method defined by the User Dictionary Provider. The following lines of code show a ContentResolver.query() call:

    // 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

    Table 2 shows how the arguments to query(Uri,projection,selection,selectionArgs,sortOrder)match an SQL SELECT statement:

    Table 2: Query() compared to SQL query.

    query() argument SELECT keyword/parameter Notes
    Uri FROM table_name Uri maps to the table in the provider named table_name.
    projection col,col,col,... projection is an array of columns that should be included for each row retrieved.
    selection WHERE col =value selection specifies the criteria for selecting rows.
    selectionArgs (No exact equivalent. Selection arguments replace? placeholders in the selection clause.)
    sortOrder ORDER BYcol,col,... sortOrder specifies the order in which rows appear in the returnedCursor.

    Content URIs

    Content URI 是为Provider 定义数据。 Content URIs 包括符号名称(authority) 和名字(path).当用provider调用数据，content URI 就是必须的参数。

    前面的代码，CONTENT_URI 的常量包括用户字典表content uri。ContentResolver 对象解析URI的authority，它通过解析authority，通过查询系统表查找到具体provider。ContentResolver 能将查询参数正确的分发到正确的provider。

    ContentProvider通过Content URI的path找到目标表。provider通常为每个表对应一个table。

    In the previous lines of code, the full URI for the "words" table is:

    content://user_dictionary/words

    where the user_dictionary string is the provider's authority, and words string is the table's path. The stringcontent:// (the scheme) is always present, and identifies this as a content URI.

    Many providers allow you to access a single row in a table by appending an ID value to the end of the URI. For example, to retrieve a row whose _ID is 4 from user dictionary, you can use this content URI:

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

    You often use id values when you've retrieved a set of rows and then want to update or delete one of them.

    Note: The Uri and Uri.Builder classes contain convenience methods for constructing well-formed Uri objects from strings. The ContentUris contains convenience methods for appending id values to a URI. The previous snippet uses withAppendedId() to append an id to the UserDictionary content URI.

    provider 返回结果

    这部分描述如何从provider取得数据，还是把Dictionary provider 作为例子。

    For the sake of clarity, the code snippets in this section call ContentResolver.query() on the "UI thread"". In actual code, however, you should do queries asynchronously on a separate thread. One way to do this is to use the CursorLoader class, which is described in more detail in the Loaders guide. Also, the lines of code are snippets only; they don't show a complete application.

    为了从provider中获得数据，按照下面两部:

    1. 请求读权限。

    2. 定义好请求数据的provider。

    Requesting read access permission(请求读权限)

    要通过provider获取数据，您的应用需要为provider提供"读访问权限"。您不能再运行期的时候请求权限；反而，您需要在manifest文件中详细说明读权限，用<uses-permission>元素和准确的权限名字为provider定义。When you specify this element in your manifest, you are in effect "requesting" this permission for your application. When users install your application, they implicitly grant this request.

    To find the exact name of the read access permission for the provider you're using, as well as the names for other access permissions used by the provider, look in the provider's documentation.

    The role of permissions in accessing providers is described in more detail in the section Content Provider Permissions.

    The User Dictionary Provider defines the permission android.permission.READ_USER_DICTIONARY in its manifest file, so an application that wants to read from the provider must request this permission.

    Constructing the query

    The next step in retrieving data a provider is to construct a query. This first snippet defines some variables for accessing the User Dictionary Provider:

    
// 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 = {""};

    The next snippet shows how to use ContentResolver.query(), using the User Dictionary Provider as an example. A provider client query is similar to an SQL query, and it contains a set of columns to return, a set of selection criteria, and a sort order.

    The set of columns that the query should return is called a projection (the variable mProjection).

    The expression that specifies the rows to retrieve is split into a selection clause and selection arguments. The selection clause is a combination of logical and Boolean expressions, column names, and values (the variablemSelectionClause). If you specify the replaceable parameter ? instead of a value, the query method retrieves the value from the selection arguments array (the variable mSelectionArgs).

    In the next snippet, if the user doesn't enter a word, the selection clause is set to null, and the query returns all the words in the provider. If the user enters a word, the selection clause is set to UserDictionary.Words.WORD + " = ?" and the first element of selection arguments array is set to the word the user enters.

    /*
 * 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

}

    This query is analogous to the SQL statement:

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

    In this SQL statement, the actual column names are used instead of contract class constants.

    Protecting against malicious input(防止恶意的输入)

    If the data managed by the content provider is in an SQL database, including external untrusted data into raw SQL statements can lead to SQL injection.

    Consider this selection clause:

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

    If you do this, you're allowing the user to concatenate(连接) malicious(恶意的) SQL onto your SQL statement. For example, the user could enter "nothing; DROP TABLE *;" for mUserInput, which would result in the selection clause var = nothing; DROP TABLE *;. Since the selection clause is treated as an SQL statement, this might cause the provider to erase all of the tables in the underlying SQLite database (unless the provider is set up to catch SQL injection attempts).

    To avoid this problem, use a selection clause that uses ? as a replaceable parameter and a separate array of selection arguments. When you do this, the user input is bound directly to the query rather than being interpreted as part of an SQL statement. Because it's not treated as SQL, the user input can't inject malicious SQL. Instead of using concatenation to include the user input, use this selection clause:

    // Constructs a selection clause with a replaceable parameter
String mSelectionClause =  "var = ?";

    Set up the array of selection arguments like this:

    // Defines an array to contain the selection arguments
String[] selectionArgs = {""};

    Put a value in the selection arguments array like this:

    // Sets the selection argument to the user's input
selectionArgs[0] = mUserInput;

    A selection clause that uses ? as a replaceable parameter and an array of selection arguments array are preferred way to specify a selection, even if the provider isn't based on an SQL database.

    Displaying query results

    The ContentResolver.query() client method always returns a Cursor containing the columns specified by the query's projection for the rows that match the query's selection criteria. A Cursor object provides random read access to the rows and columns it contains. Using Cursor methods, you can iterate over the rows in the results, determine the data type of each column, get the data out of a column, and examine other properties of the results. Some Cursor implementations automatically update the object when the provider's data changes, or trigger methods in an observer object when the Cursor changes, or both.

    Note: A provider may restrict access to columns based on the nature of the object making the query. For example, the Contacts Provider restricts access for some columns to sync adapters, so it won't return them to an activity or service.

    If no rows match the selection criteria, the provider returns a Cursor object for which Cursor.getCount() is 0 (an empty cursor).

    If an internal error occurs, the results of the query depend on the particular provider. It may choose to returnnull, or it may throw an Exception.

    Since a Cursor is a "list" of rows, a good way to display the contents of a Cursor is to link it to a ListView via a SimpleCursorAdapter.

    The following snippet continues the code from the previous snippet. It creates a SimpleCursorAdapter object containing the Cursor retrieved by the query, and sets this object to be the adapter for a ListView:

    // 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);

    Note: To back a ListView with a Cursor, the cursor must contain a column named _ID. Because of this, the query shown previously retrieves the _ID column for the "words" table, even though the ListView doesn't display it. This restriction also explains why most providers have a _ID column for each of their tables.

    Getting data from query results

    Rather than simply displaying query results, you can use them for other tasks. For example, you can retrieve spellings from the user dictionary and then look them up in other providers. To do this, you iterate over the rows in the Cursor:

    
// 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 implementations contain several "get" methods for retrieving different types of data from the object. For example, the previous snippet uses getString(). They also have a getType() method that returns a value indicating the data type of the column.

    Content Provider Permissions

    A provider's application can specify permissions that other applications must have in order to access the provider's data. These permissions ensure that the user knows what data an application will try to access. Based on the provider's requirements, other applications request the permissions they need in order to access the provider. End users see the requested permissions when they install the application.

    If a provider's application doesn't specify any permissions, then other applications have no access to the provider's data. However, components in the provider's application always have full read and write access, regardless of the specified permissions.

    As noted previously(正如上述), the User Dictionary Provider requires the android.permission.READ_USER_DICTIONARYpermission to retrieve data from it. The provider has the separateandroid.permission.WRITE_USER_DICTIONARY permission for inserting, updating, or deleting data.

    To get the permissions needed to access a provider, an application requests them with a <uses-permission>element in its manifest file. When the Android Package Manager installs the application, a user must approve all of the permissions the application requests. If the user approves all of them, Package Manager continues the installation; if the user doesn't approve them, Package Manager aborts the installation.

    The following <uses-permission> element requests read access to the User Dictionary Provider:

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

    The impact of permissions on provider access is explained in more detail in the Security and Permissions guide.

    Inserting, Updating, and Deleting Data

    In the same way that you retrieve data from a provider, you also use the interaction between a provider client and the provider's ContentProvider to modify data. You call a method of ContentResolver with arguments that are passed to the corresponding method of ContentProvider. The provider and provider client automatically handle security and inter-process communication.

    Inserting data

    To insert data into a provider, you call the ContentResolver.insert() method. This method inserts a new row into the provider and returns a content URI for that row. This snippet shows how to insert a new word into the User Dictionary Provider:

    // 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
);

    The data for the new row goes into a single ContentValues object, which is similar in form to a one-row cursor. The columns in this object don't need to have the same data type, and if you don't want to specify a value at all, you can set a column to null using ContentValues.putNull().

    The snippet doesn't add the _ID column, because this column is maintained automatically. The provider assigns a unique value of _ID to every row that is added. Providers usually use this value as the table's primary key.

    The content URI returned in newUri identifies the newly-added row, with the following format:

    content://user_dictionary/words/<id_value>

    The <id_value> is the contents of _ID for the new row. Most providers can detect this form of content URI automatically and then perform the requested operation on that particular row.

    To get the value of _ID from the returned Uri, call ContentUris.parseId().

    Updating data

    To update a row, you use a ContentValues object with the updated values just as you do with an insertion, and selection criteria just as you do with a query. The client method you use is ContentResolver.update(). You only need to add values to the ContentValues object for columns you're updating. If you want to clear the contents of a column, set the value to null.

    The following snippet changes all the rows whose locale has the language "en" to a have a locale of null. The return value is the number of rows that were updated:

    // 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
);

    You should also sanitize user input when you call ContentResolver.update(). To learn more about this, read the section Protecting against malicious input.

    Deleting data

    Deleting rows is similar to retrieving row data: you specify selection criteria for the rows you want to delete and the client method returns the number of deleted rows. The following snippet deletes rows whose appid matches "user". The method returns the number of deleted rows.

    
// 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
);

    You should also sanitize user input when you call ContentResolver.delete(). To learn more about this, read the section Protecting against malicious input.

    Provider Data Types

    Content providers can offer many different data types. The User Dictionary Provider offers only text, but providers can also offer the following formats:

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

    Another data type that providers often use is Binary Large OBject (BLOB) implemented as a 64KB byte array. You can see the available data types by looking at the Cursor class "get" methods.

    The data type for each column in a provider is usually listed in its documentation. The data types for the User Dictionary Provider are listed in the reference documentation for its contract class UserDictionary.Words(contract classes are described in the section Contract Classes). You can also determine the data type by callingCursor.getType().

    Providers also maintain MIME data type information for each content URI they define. You can use the MIME type information to find out if your application can handle data that the provider offers, or to choose a type of handling based on the MIME type. You usually need the MIME type when you are working with a provider that contains complex data structures or files. For example, the ContactsContract.Data table in the Contacts Provider uses MIME types to label the type of contact data stored in each row. To get the MIME type corresponding to a content URI, call ContentResolver.getType().

    The section MIME Type Reference describes the syntax of both standard and custom MIME types.

    Alternative Forms of Provider Access

    Three alternative forms of provider access are important in application development:

    • Batch access: You can create a batch of access calls with methods in the ContentProviderOperationclass, and then apply them with ContentResolver.applyBatch().
    • Asynchronous queries: You should do queries in a separate thread. One way to do this is to use aCursorLoader object. The examples in the Loaders guide demonstrate how to do this.
    • Data access via intents: Although you can't send an intent directly to a provider, you can send an intent to the provider's application, which is usually the best-equipped to modify the provider's data.

    Batch access and modification via intents are described in the following sections.

    Batch access

    Batch access to a provider is useful for inserting a large number of rows, or for inserting rows in multiple tables in the same method call, or in general for performing a set of operations across process boundaries as a transaction (an atomic operation).

    To access a provider in "batch mode", you create an array of ContentProviderOperation objects and then dispatch them to a content provider with ContentResolver.applyBatch(). You pass the content provider'sauthority to this method, rather than a particular content URI. This allows each ContentProviderOperationobject in the array to work against a different table. A call to ContentResolver.applyBatch() returns an array of results.

    The description of the ContactsContract.RawContacts contract class includes a code snippet that demonstrates batch insertion. The Contact Manager sample application contains an example of batch access in its ContactAdder.java source file.

    Displaying data using a helper app

    If your application does have access permissions, you still may want to use an intent to display data in another application. For example, the Calendar application accepts an ACTION_VIEWintent, which displays a particular date or event. This allows you to display calendar information without having to create your own UI. To learn more about this feature, see the Calendar Provider guide.

    The application to which you send the intent doesn't have to be the application associated with the provider. For example, you can retrieve a contact from the Contact Provider, then send anACTION_VIEW intent containing the content URI for the contact's image to an image viewer.

    Data access via intents

    Intents can provide indirect access to a content provider. You allow the user to access data in a provider even if your application doesn't have access permissions, either by getting a result intent back from an application that has permissions, or by activating an application that has permissions and letting the user do work in it.

    Getting access with temporary permissions

    You can access data in a content provider, even if you don't have the proper access permissions, by sending an intent to an application that does have the permissions and receiving back a result intent containing "URI" permissions. These are permissions for a specific content URI that last until the activity that receives them is finished. The application that has permanent permissions grants temporary permissions by setting a flag in the result intent:

    Note: These flags don't give general read or write access to the provider whose authority is contained in the content URI. The access is only for the URI itself.

    A provider defines URI permissions for content URIs in its manifest, using the android:grantUriPermissionattribute of the <provider> element, as well as the <grant-uri-permission> child element of the<provider> element. The URI permissions mechanism is explained in more detail in the Security and Permissions guide, in the section "URI Permissions".

    For example, you can retrieve data for a contact in the Contacts Provider, even if you don't have theREAD_CONTACTS permission. You might want to do this in an application that sends e-greetings to a contact on his or her birthday. Instead of requesting READ_CONTACTS, which gives you access to all of the user's contacts and all of their information, you prefer to let the user control which contacts are used by your application. To do this, you use the following process:

    1. Your application sends an intent containing the action ACTION_PICK and the "contacts" MIME typeCONTENT_ITEM_TYPE, using the method startActivityForResult().
    2. Because this intent matches the intent filter for the People app's "selection" activity, the activity will come to the foreground.
    3. In the selection activity, the user selects a contact to update. When this happens, the selection activity callssetResult(resultcode, intent) to set up a intent to give back to your application. The intent contains the content URI of the contact the user selected, and the "extras" flags FLAG_GRANT_READ_URI_PERMISSION. These flags grant URI permission to your app to read data for the contact pointed to by the content URI. The selection activity then calls finish() to return control to your application.
    4. Your activity returns to the foreground, and the system calls your activity's onActivityResult() method. This method receives the result intent created by the selection activity in the People app.
    5. With the content URI from the result intent, you can read the contact's data from the Contacts Provider, even though you didn't request permanent read access permission to the provider in your manifest. You can then get the contact's birthday information or his or her email address and then send the e-greeting.

    Using another application

    A simple way to allow the user to modify data to which you don't have access permissions is to activate an application that has permissions and let the user do the work there.

    For example, the Calendar application accepts an ACTION_INSERT intent, which allows you to activate the application's insert UI. You can pass "extras" data in this intent, which the application uses to pre-populate the UI. Because recurring events have a complex syntax, the preferred way of inserting events into the Calendar Provider is to activate the Calendar app with an ACTION_INSERT and then let the user insert the event there.

    Contract Classes

    A contract class defines constants that help applications work with the content URIs, column names, intent actions, and other features of a content provider. Contract classes are not included automatically with a provider; the provider's developer has to define them and then make them available to other developers. Many of the providers included with the Android platform have corresponding contract classes in the packageandroid.provider.

    For example, the User Dictionary Provider has a contract class UserDictionary containing content URI and column name constants. The content URI for the "words" table is defined in the constantUserDictionary.Words.CONTENT_URI. The UserDictionary.Words class also contains column name constants, which are used in the example snippets in this guide. For example, a query projection can be defined as:

    String[] mProjection =
{
    UserDictionary.Words._ID,
    UserDictionary.Words.WORD,
    UserDictionary.Words.LOCALE
};

    Another contract class is ContactsContract for the Contacts Provider. The reference documentation for this class includes example code snippets. One of its subclasses, ContactsContract.Intents.Insert, is a contract class that contains constants for intents and intent data.

    MIME Type Reference

    Content providers can return standard MIME media types, or custom MIME type strings, or both.

    MIME types have the format

    type/subtype

    For example, the well-known MIME type text/html has the text type and the html subtype. If the provider returns this type for a URI, it means that a query using that URI will return text containing HTML tags.

    Custom MIME type strings, also called "vendor-specific" MIME types, have more complex type and subtypevalues. The type value is always

    vnd.android.cursor.dir

    for multiple rows, or

    vnd.android.cursor.item

    for a single row.

    The subtype is provider-specific. The Android built-in providers usually have a simple subtype. For example, the when the Contacts application creates a row for a telephone number, it sets the following MIME type in the row:

    vnd.android.cursor.item/phone_v2

    Notice that the subtype value is simply phone_v2.

    Other provider developers may create their own pattern of subtypes based on the provider's authority and table names. For example, consider a provider that contains train timetables. The provider's authority iscom.example.trains, and it contains the tables Line1, Line2, and Line3. In response to the content URI

    content://com.example.trains/Line1

    for table Line1, the provider returns the MIME type

    vnd.android.cursor.dir/vnd.example.line1

    In response to the content URI

    content://com.example.trains/Line2/5

    for row 5 in table Line2, the provider returns the MIME type

    vnd.android.cursor.item/vnd.example.line2

    Most content providers define contract class constants for the MIME types they use. The Contacts Provider contract class ContactsContract.RawContacts, for example, defines the constant CONTENT_ITEM_TYPE for the MIME type of a single raw contact row.

    Content URIs for single rows are described in the section Content URIs.
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  • 原文地址：https://www.cnblogs.com/java20130722/p/3207293.html
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