android--Content Provider 你应该记住的一些事-1

[1].Content providers are the standard interface that connects data in one process with code running in another process.

[2].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. 

[3].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 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.

[4].

content://user_dictionary/words

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.

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.

[5].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.

[6].

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.

[7].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.

[8].* 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.

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    }} 

[9].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.

[10].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.