[Android] Content Provider

• Content providers are the standard interface that connects data in one process with code running in another process.
• The provider and provider client automatically handle security and inter-process communication.

ContentResolver

• In the client application's process.
• Owns the provider automatically handle inter-process communication.
• Acts as an abstraction layer between its repository of data and the external appearance of data as tables.

query(Uri, projection, selection, selectionArgs, sortOrder)
Should do queries asynchronously on a separate thread. (ex: CursorLoader class)

• projection: the columns that will be returned for each row
• The expression that specifies the rows to retrieve is split into a selection clause and selection arguments.
• selection: 
• The selection clause is a combination of logical and Boolean expressions, column names, and values (the variablemSelection).
• selection 中的值應用 ? 取代，實際的值放在 selectionArgs(the selection arguments array )，作為對應.
• The selection clause is set to null, and the query returns all the words in the provider

Query result:

• If no rows match the selection criteria
• Cursor.getCount() is 0 (an empty cursor).
• If an internal error occurs
• Return null, 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.

Content providers can offer the following formats:
You can see the available data types by looking at the Cursor class "get" methods.

• integer
• long integer (long)
• floating point
• long floating point (double)
• Binary Large OBject (BLOB) implemented as a 64KB byte array

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 ContentProviderOperation class, and then apply them with ContentResolver.applyBatch().
• Useful for: 
• Inserting a large number of rows.
• Inserting rows in multiple tables in the same method call
• Performing a set of operations across process boundaries as a transaction (an atomic operation).
• Asynchronous queries: 
• You should do queries in a separate thread. 
• One way to do this is to use a CursorLoader 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.

* Reference
- Content Providers
- Content Provider Basics