165 lines
5.2 KiB
Markdown
165 lines
5.2 KiB
Markdown
# Content Providers - Storage
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Access to data is restricted to the app that owns it
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- Database is usually located in internal app-specific storage
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- 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
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Content providers expose data / content to other applications in a structured manner
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- Fundamentally IPC via Binder and ashmem (Android shared memory) with a well-defined (database-like) interface
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#### System Content Providers
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Content providers manage data for:
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**Browser**: bookmarks, history
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**Call Log**: Telephone usage
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**Contacts**: Contact data, contacts for other apps, e.g. WhatsApp
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**Media**: Media metadata database
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**User Dictionary**: database for predictive spelling
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###### Storage Access Framework
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Is an abstraction of *documents*
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- Restricted access to external storage
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- Enumerated by the system content object picker
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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.
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###### How to use it?
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- Create our own by sub-classing `ContentProvider`
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- Must add it in the manifest
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- Or add/query data via an existing content provider
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### Data Model
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Very similar to a relational database table
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- A collection of records
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- Support for read/write
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- Support for typical database operations
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- CRUD - **C**reate **R**ead **U**pdate **D**elete
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Records are stored in rows, with each column providing different data fields
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- Each record has a numeric id (in the field ID) that uniquely identifies it
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- Tables exposed via URI
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- This is a further abstraction
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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)
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### Querying a Content Provider
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Content Providers **identify data** sets through **URIs**
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- `content://authority/path/id`
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- **content**: Data managed by a content provider
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- **authority**: ID for the Content provider
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- A fully qualified class name, e.g. `com.example.project`
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- **path**: 0 or more segments indicating the subset of data to be requested
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- e.g. table name
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- A RESTful resource philosophy
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- **id**: specific record (row) being accessed
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`/email/2021/Nov/17` -> would retrieve all emails received on Nov 17th 📅
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###### Content Resolver
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- Manages and supports content provider access
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- How to service a request for content
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- Similar to `ServiceManager`
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- Enables Content Providers to be used across multiple applications
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- Can observe a content provider to be informed of real-time modifications
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- e.g. `contentResolver` monitors the path and can notify if a new email pops up ✉
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Full URI: `ContactsContract.Contact.CONTENT_URI = "content://com.android.contacts/contacts/"`
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`ContentResolver.query(...)`
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- Returns a cursor instance for accessing results
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- Cursor is a pointer to a `cursorWindow`
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- A read-only reference to shared memory
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- Allocated by ashmem
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- Retrieved via Binder
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- Max `CursorWindow` size is 2 Mb
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###### Example of Querying Contacts
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- To access / modify requires a permission
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- `android.permission.READ_CONTACTS`
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- `android.permission.WRITE_CONTACTS`
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- Contacts has 3 components
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- Data
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- Rows (mime-typed) that can hold personal info
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- Raw Contacts
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- A contact for a given person from a given system
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- Gmail contact, Facebook contact, etc.
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- Associated with data entries
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- Contacts
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- Aggregated raw contacts
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- Single view of a *person*
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##### Modifying a Content Provider
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`Uri insert (Uri uri, ContentValues values)`
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Returns the URI of the newly inserted item
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`int update(Uri uri, ContentValues values, String where, String[] selectionArgs)`
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###### Parameters
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**uri**: the *table* we want to modify
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**content values**: values for the new row or key/value pairs where the key is the column name
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#### Creating a content Provider
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- Implement a storage system for the data
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- Structured data, can be SQLite or Room
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- Media & blobs
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- Implement a content provider
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- query, add, update, insert, etc.
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- onCreate
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- When content provider is instantiated, needs to instantiate the internal storage (create the table)
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- getType
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- single items, multiple items, mime type
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- Tell Android we are a provider
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- Declare in the manifest
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#### Contract
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- Defines metadata pertaining to the provider
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- Constant definitions that are exposed to developers via a compiled .jar file
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- Authority
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- URI
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- Meta-data
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- Column names
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#### URI Matching
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All of these methods take a URI as the first parameter
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- The object will need to parse it to know what to return
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`android.content.UriMatcher`
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- Provides mapping between abstraction of contract class to concrete db implementation
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- Does the calling app want all the data from the table or just a row?
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- `content://authority/tablename/`
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- whole table
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- `content://authority/tablename/1`
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- row
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- Or a virtual table (joins across various underlying resources)
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