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06 - Android Data Persistance

Main methods for data persistance​

  • Shared preferences
  • Internal storage
  • External storage
  • SQLite Database

Saving data in between same Activity instances​

Data forwarding actually!

  • onSaveInstanceState
  • onRestoreInstanceState

Shared preferences​

SharedPreferences – save and retrieve key-value pairs

Any primitive data

  • Booleans
  • Floats
  • Ints
  • Longs
  • Strings

To get a SharedPreferences object

  • getSharedPreferences(name, mode) – if you need multiple pref files, identified by name (first parameter)
  • getPreferences(mode) – single pref file for activity, no name specified

Mode

  • MODE_PRIVATE (0) - default
  • MODE_WORLD_READABLE
  • MODE_WORLD_WRITEABLE

To WRITE values

  • Call edit() to get SharedPreferences.Editor
  • Add values with putString(), putBoolean, …
  • Commit values with commit()
override fun onStop() {
    super.onStop()

    val sharedPref = getPreferences(Context.MODE_PRIVATE) ?: return
    with (sharedPref.edit()) {
        putInt(getString(R.string.saved_high_score_key), newHighScore)
        commit()
    }
}

To READ values

Use methods such as getString, getBoolean

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)

    // ?: - Elvis operator
    val sharedPref = getPreferences(Context.MODE_PRIVATE) ?: return
    val defaultValue = resources.getInteger(R.integer.saved_high_score_default_key)
    val highScore = sharedPref.getInt(getString(R.string.saved_high_score_key), defaultValue)
}

Internal storage​

  • Private to application (default)
  • No access for user or other apps
  • When app gets uninstalled – files are deleted with it

Create/Write​

Create and Write private file

  • Call openFileOutput(), with filename and operating mode
  • Returns FileOutputStream
  • Write to the file with write()
  • Close the file with close()

Modes

  • MODE_PRIVATE – create (or replace)
  • MODE_APPEND
  • MODE_WORLD_READABLE
  • MODE_WORLD_WRITEABLE

Read​

Read from private file

  • Call openFileInput() with filename
  • Returns FileInputStream
  • Get bytes with read()
  • Close stream with close()

Static app files​

Save static file during compile time

  • Place in /res/raw
  • Open with OpenRawResource(), passing R.raw.<filename>
  • Returns inputStream
  • File is read-only!!!

File cache​

Caching data

  • Do not need data forever
  • getCacheDir()
  • When space is low, Android will delete cache
  • Stay within reasonable space limits (1mb?)
  • Deleted with uninstall
  • Manage cache files yourself

File methods​

Other useful methods

  • getFilesDir() - Gets the absolute path to the filesystem directory where your internal files are saved.
  • getDir() - Creates (or opens an existing) directory within your internal storage space.
  • deleteFile() - Deletes a file saved on the internal storage.
  • fileList() - Returns an array of files currently saved by your application.

External storage​

All Android devices support shared "external storage"

Can be removable storage media (sd-card)

Or internal, non-removable storage

Files are world-readable

Access​

Getting access – manifest

  • READ_EXTERNAL_STORAGE
  • WRITE_EXTERNAL_STORAGE
<manifest ...>
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    ...
</manifest>

Availability​

Check availability

  • Use getExternalStorageState()

Media might be mounted to a computer, missing, read-only, or in some other state.

// Checks if a volume containing external storage is available
// for read and write.
fun isExternalStorageWritable(): Boolean {
    return Environment.getExternalStorageState() == 
        Environment.MEDIA_MOUNTED
}

// Checks if a volume containing external storage is 
// available to at least read.
fun isExternalStorageReadable(): Boolean {
    return Environment.getExternalStorageState() in
            setOf(Environment.MEDIA_MOUNTED, 
                Environment.MEDIA_MOUNTED_READ_ONLY)
}

Public files​

  • Files acquired through your app should be saved to a "public" location where other apps can access them and the user can easily copy them from the device
  • Use one of the shared public directories (Music/, Pictures/, and Ringtones/)
  • Use Enviroment.getExternalStoragePublicDirectory()
    DIRECTORY_MUSIC, DIRECTORY_PICTURES, DIRECTORY_RINGTONES

Private​

  • Private files (textures, sounds for app, etc) (actually semi-private)
  • Use a private storage directory on the external storage
  • Use getExternalFilesDir()
  • Takes Type, use null when no type
  • From 4.4 onwards does not require permissions
  • Files are hidden from Media Scanner (but not from other apps with permissions)