[main]: Add mdp to lecture notes

2026-10-08 13:07:18 +01:00
1 parent 283096a43d
commit 4f85b781ea
@@ -0,0 +1,164 @@
+# 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
+
+![image-20220107180058110](img/af.png)
+
+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 - **C**reate **R**ead **U**pdate **D**elete
+
+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
+
+![image-20220107180837589](img/ag.png)
+
+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 &#128197;
+
+###### 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 &#9993;
+
+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)