3.App Components-Content Providers

1. Content Providers

　　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.

2. OverView

　　2.1 Accessing a 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.

　　For example, read contrats

Cursor cursor = getContentResolver().query(ContactsContract.Contacts.CONTENT_URI, 
                null, 
                null, 
                null, 
                null);
        while(cursor.moveToNext()){  //cursor is like a point to every element
            Log.d("Contract Name",cursor.getString(cursor.getColumnIndex((ContactsContract.Contacts.DISPLAY_NAME))));
        }

　　2.2 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(参数).

　　　　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 string content:// (the scheme) is always present, and identifies this as a content URI, like the http:// indentifies HTTP internet

　　　　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); 

3. Retrieving Data from the Provider

　　To retrieve data from a provider, follow these basic steps:

　　　　<1> Request the read access permission for the provider.

　　　　<2> Define the code that sends a query to the provider

　　3.1 Requesting read access Permission

　　　　To retrieve data from a provider, your application needs "read access permission" for the provider

　　3.2 Constructing the query

　　　　

　　3.3 Displaying qurey results

　　3.4 Getting data from qurey results