ContentProvider官方教程(2)简介、Content URIs

In this document
　　Overview
　　Accessing a provider
　　Content URIs

Content Provider Basics

　　A content provider manages access to a central repository of data. A provider is part of an Android application, which often provides its own UI for working with the data. However, content providers are primarily intended to be used by other applications, which access the provider using a provider client object. Together, providers and provider clients offer a consistent, standard interface to data that also handles inter-process communication and secure data access.

　　This topic describes the basics of the following:

• How content providers work.
• The API you use retrieve data from a content provider.
• The API you use to insert, update, or delete data in a content provider.
• Other API features that facilitate working with providers.

Overview

　　A content provider presents data to external applications as one or more tables that are similar to the tables found in a relational database. A row represents an instance of some type of data the provider collects, and each column in the row represents an individual piece of data collected for an instance. 
　　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:

　　provider一般像关系型数据库一样存数据，如下：

　　Table 1: Sample user dictionary table.

　　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.

　　provider不强制要求有_ID字段当主键，但给ListView提供数据除外。

Accessing a provider 如何访问provider

　　An application accesses the data from a content provider with a ContentResolver client object. This object has methods that call identically-named methods in the provider object, an instance of one of the concrete subclasses of ContentProvider. The ContentResolver methods provide the basic "CRUD" (create, retrieve, update, and delete) functions of persistent storage.

　　ContentResolver对象可用来访问provider的数据，提供了基本的操作：创建，恢复，更新，删除。

　　The ContentResolver object in the client application's process and the ContentProvider object in the application that owns the provider automatically handle inter-process communication. ContentProvider also acts as an abstraction layer between its repository of data and the external appearance of data as tables. 
　　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 .

　　访问provider要声明权限。

　　For example, to get a list of the words and their locales from the User Dictionary Provider, you call ContentResolver.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.
　　

Content URIs

　　A content URI is a URI that identifies data in a provider. Content URIs include the symbolic name of the entire provider (its authority) and a name that points to a table (a path). When you call a client method to access a table in a provider, the content URI for the table is one of the arguments.

　　外部通过Uri访问provider内的数据。

　　In the preceding lines of code, the constant CONTENT_URI contains the content URI of the user dictionary's "words" table. The ContentResolver object parses out the URI's authority, and uses it to "resolve" the provider by comparing the authority to a system table of known providers. The ContentResolver can then dispatch the query arguments to the correct provider. 
　　The ContentProvider uses the path part of the content URI to choose the table to access. A provider usually has a path for each table it exposes. 
　　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 the words string is the table's path. The string content:// (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 class contains convenience methods for appending id values to a URI. The previous snippet uses withAppendedId() to append an id to the UserDictionary content URI.