notes/docs/lectures/mdp/12_content_providers.md

Content Providers - Storage

Access to data is restricted to the app that owns it

• Database is usually located in internal app-specific storage
• If we want other apps to access our data, or we want to access other apps’ data, or we want to be notified when data has changed, we must use a content provider

Content providers expose data / content to other applications in a structured manner

• Fundamentally IPC via Binder and ashmem (Android shared memory) with a well-defined (database-like) interface

System Content Providers

Content providers manage data for:

Browser: bookmarks, history

Call Log: Telephone usage

Contacts: Contact data, contacts for other apps, e.g. WhatsApp

Media: Media metadata database

User Dictionary: database for predictive spelling

Storage Access Framework

Is an abstraction of documents

• Restricted access to external storage
  • Enumerated by the system content object picker

A content provider is mostly used for making content available outside your application, as using it to get data is now done by the repository.

How to use it?

• Create our own by sub-classing ContentProvider
  • Must add it in the manifest
• Or add/query data via an existing content provider

Data Model

Very similar to a relational database table

• A collection of records
• Support for read/write
• Support for typical database operations
  • CRUD - Create Read Update Delete

Records are stored in rows, with each column providing different data fields

• Each record has a numeric id (in the field ID) that uniquely identifies it
• Tables exposed via URI
  • This is a further abstraction

Applications query the content provider, which maps the requests to the underlying database. This can be SQLite or Room (doesn’t really matter to the application)

Querying a Content Provider

Content Providers identify data sets through URIs

• content://authority/path/id
• content: Data managed by a content provider
• authority: ID for the Content provider
  • A fully qualified class name, e.g. com.example.project
• path: 0 or more segments indicating the subset of data to be requested
  • e.g. table name
  • A RESTful resource philosophy
• id: specific record (row) being accessed

/email/2021/Nov/17 -> would retrieve all emails received on Nov 17th 📅

Content Resolver

• Manages and supports content provider access
• How to service a request for content
  • Similar to ServiceManager
• Enables Content Providers to be used across multiple applications
• Can observe a content provider to be informed of real-time modifications
  • e.g. contentResolver monitors the path and can notify if a new email pops up ✉

Full URI: ContactsContract.Contact.CONTENT_URI = "content://com.android.contacts/contacts/"

ContentResolver.query(...)

• Returns a cursor instance for accessing results
• Cursor is a pointer to a cursorWindow
  • A read-only reference to shared memory
  • Allocated by ashmem
  • Retrieved via Binder
• Max CursorWindow size is 2 Mb

Example of Querying Contacts

• To access / modify requires a permission
  • android.permission.READ_CONTACTS
  • android.permission.WRITE_CONTACTS
• Contacts has 3 components
  • Data
    • Rows (mime-typed) that can hold personal info
  • Raw Contacts
    • A contact for a given person from a given system
      • Gmail contact, Facebook contact, etc.
      • Associated with data entries
  • Contacts
    • Aggregated raw contacts
      • Single view of a person

Modifying a Content Provider

Uri insert (Uri uri, ContentValues values)

Returns the URI of the newly inserted item

int update(Uri uri, ContentValues values, String where, String[] selectionArgs)

Parameters

uri: the table we want to modify

content values: values for the new row or key/value pairs where the key is the column name

Creating a content Provider

• Implement a storage system for the data
  • Structured data, can be SQLite or Room
  • Media & blobs
• Implement a content provider
  • query, add, update, insert, etc.
  • onCreate
    • When content provider is instantiated, needs to instantiate the internal storage (create the table)
  • getType
    • single items, multiple items, mime type
• Tell Android we are a provider
  • Declare in the manifest

Contract

• Defines metadata pertaining to the provider
• Constant definitions that are exposed to developers via a compiled .jar file
  • Authority
  • URI
  • Meta-data
  • Column names

URI Matching

All of these methods take a URI as the first parameter

• The object will need to parse it to know what to return

android.content.UriMatcher

• Provides mapping between abstraction of contract class to concrete db implementation
• Does the calling app want all the data from the table or just a row?
  • content://authority/tablename/
    • whole table
  • content://authority/tablename/1
    • row
  • Or a virtual table (joins across various underlying resources)