241 lines
7.7 KiB
Markdown
241 lines
7.7 KiB
Markdown
# Storage
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### Storage Principles
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#### App-Specific Requirements
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- Store files that are meant for our use only
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- Store files that we intend to share with other apps
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- e.g. taking a photo
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- Store private, primitive data
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- Config data, passwords, cookies & keys, etc.
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- Store private, structured data
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- text messages
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#### Mobile Storage Principles
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- Efficiency and sharing vs privacy and security
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- Limited storage capacity
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- Highly personal information stored
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- Lots of different data types stored on phone
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- Media
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- Images, videos, music
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> Access to the mail should be protected since this is sensitive user data. However, if a reference to an image attachment is given to an image viewer, that image viewer will not have permission to open the attachment since it has no reason to hold a permission to access all e-mail.
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##### Storage Directories
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`/system` -> stores Android OS & libraries
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`/data` -> where user data is stored (every application is a user)
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`/mnt/sdcard` (symlinked as `sdcard`) -> *External* Storage
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### Basic Logical Data Storage on Android
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##### Different Volumes
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###### Internal
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- Limited space for app-specific data
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- Reliable (not *removable*)
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- Soldered RAM
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###### External
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- Lots of data
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- May (or may not) be physically removable
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##### Different kinds of data
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- Data that is only meaningful for our app should use app-specific storage
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- Shareable media content should use shared storage so that other apps can access the content
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- Also called scoped storage
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##### File-based abstractions
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- Shared preferences
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- File-based storage
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- SQLite database
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- Structured data, small binary blobs
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## Internal File Storage
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Internal data storage is private to the app
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- Other apps (and the user) cannot access it
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- Kernel-enforced user perms
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- Removed on uninstall
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- Data is stored in files
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- `/data/data/com.example.project/files/`
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- Full path not accessible; `files/` becomes the chroot
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- This ensures we cannot traverse the directory upwards and gain access to other applications' files
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##### Cache Files
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`/data/data/com.example.project/cache`
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`getCacheDir()` -> gets application’s file directory
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Programmer should manage the files
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- **May** be deleted when internal storage becomes full
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- **Will** be deleted when application is uninstalled
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- A *well-behaved* application will delete them when no longer in use
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- Recommended to use less than 1MB
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#### Shared Preferences
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- Stored on a per-application basis
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- e.g. saving config info (settings)
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- Should not be used for data transfer (use intents and binders instead)
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- Primitive data in key-value pairs
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- Can have multiple preference files per application
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```xml
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<?xml version='1.0' encoding='utf-8' standalone='yes' ?>
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<map>
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<string name="colourscheme">light</string>
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</map>
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```
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```java
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SharedPreferences settings = getSharedPreferences(CONFIG_STORAGE_NAME, 0);
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SharedPreferences.Editor editor = settings.editor;
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if (settings.getBoolean(MainActivity.CONFIG_THEME, false))
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setTheme(...);
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else
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...
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```
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#### External Storage
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*Legacy name*
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- Every Android device provides externally accessible storage (SD card)
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- Even phones without a physical SD card must have a **logical representation** of external storage
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- Most phones will have one storage device partitioned into `internal / external`
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- Phones must do this to conform to the Android API
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- *Private* Application files
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- *Internal* storage on the external partition
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- `getExternalFilesDir()`
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- `/sdcard/Android/data/com.example.project/`
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- *Public* general files
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- World reachable
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- Other apps can read and modify these files
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- Each *user* has their own *virtual* SD card
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- Can be mounted externally (and unmounted/disconnected)
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##### Getting Access to External Storage
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- Should check state with `Enviroment.getExternalStorageState()`
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- It is a separate file system, potentially removable
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- `Environment.MEDIA_MOUNTED`
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- `Environment.MEDIA_MOUNTED_READ_ONLY`
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- Use `getExternalStoragePublicDirectory (String type)` to obtain file for the directory
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- Media is stored by type
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- Music in the music dir, etc.
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- Pass a type to obtain the subdirectory for that type
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##### Getting Scoped storage access
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- To access files that other apps have created
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- We must have `READ_EXTERNAL_STORAGE` permission (in the manifest)
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- With this one permission, the app can do whatever (look at downloads, listen to music, look at pictures, etc.)
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- This is where scoped storage comes into play
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- The files must reside in the media collection (images/music/videos, etc.)
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- or request legacy storage `MANAGE_EXTERNAL_STORAGE` (before Android 11)
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- Apps only have access to
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- App-specific directory on external storage
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- Specific types of media that the app has created and contributed to well-defined collections
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- If the app takes a picture, it is allowed to store and look at that picture but not allowed to access pictures not taken by that app
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- You don't need to request perms to read/write images that we are writing to the MediaStorage
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# Android Databases
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When the data stored is logically structured, queries are needed to find it based on the structure.
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- Android provides local database support
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- Can run full SQL queries
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- Each app’s databases are local to it
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- `Database.db` stored in internal storage
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- Powered by SQLite
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#### Android and SQLite
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- Wrapped in two main classes
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- Database represented by `SQLiteDatabase` object
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- Allows us to run SQL queries on the database
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- `SQLiteOpenHelper`
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- Supports the application lifecycle
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- `SQLiteOpenHelper` -> `onCreate()`
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- Creates the database the **first time** it is called
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- This database will exist until the application is installed
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- `SQLiteOpenHelper` -> `onUpdate(int oldVer, int newVer)`
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- Changing the version number allows the database to be dropped and recreated
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- Create an instance of our `SQLiteOpenHelper` subclass
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- Get reference to `SQLiteDatabase` using
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- `getReadableDatabase()`
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- `getWriteableDatabase()`
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##### Querying a Database
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`void execSQL()` -> used to execute SQL queries that don't return anything
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`query()` and `rawQuery()`
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- These return a Cursor object pointing to the results
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`Cursor rawQuery (String sql, String[] selectionArgs)`
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###### Cursors
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- Provide random access to results of a query
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- Enable us to enumerate the rows returned by the query
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- `moveToFirst()`, `moveToNext()`
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- `getString(colIndex)`, `getInt(colIndex)`
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- Have a `close()` method to close the query when finished
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We can pass cursors to other applications
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###### CursorLoader
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- A query may last some time
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- Database may be large
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- May be in a different process
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- Don’t block the main thread
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- `CursorLoader`
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- Populates views asynchronously
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- Auto-updating
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(deprecated)
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#### Data-Driven Views
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- *Connect* a cursor to a `CursorAdapter` and a `ListView`
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- Think mapping rows of a database to which entry in the list
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- Each row must have an ID field
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- `RecyclerView`
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- An optimised, flexible version of the above
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- Only creates views for visible data
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- As the user scrolls, more data is added
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- We can define our own adaptor
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### Database Abstraction
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Abstraction of database architecture
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- Easier to update storage code
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- Expose column indices as static class variables
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- `c.getInt(0)` ✗
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- `c.getInt(DBHelper.NAME)` ✓
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- Helper methods keep database internals from *leaking* into other classes
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- Return a collection of results rather than the cursor
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- **Sanitise user inputs**
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- SQL injections still apply
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