Saving Data (Persistent Storage) Summarized from By Dr.

Transcription

1 Saving Data (Persistent Storage) Summarized from By Dr. Oren Mishali

2 Most Android Apps Need To Save Data Save information about the app state during onpause() Most non-trivial apps need to save user settings Some apps must manage large amounts of information in files and databases A Reminder: We ve already learned about saving state in a Bundle during onpause(). What is the difference? => Here we talk about a persistent storage We are going to learn about 3 different mechanisms for persistent storage in Android: Shared Preferences Database storage Files

3 Shared Preferences

4 The SharedPreferences API A SharedPreferences object points to a file containing key-value pairs provides simple read/write methods Use it when you need to save a relatively small collection of key-values Each SharedPreferences file can be private (to the app) or shared A Related API The Preference API (note the difference) helps you build a user interface for your app settings. It uses SharedPreferences to save the app settings. See the Settings API guide for more details.

5 Get a Handle to a SharedPreferences (1/2) getsharedpreferences() Use this method if you need multiple shared preference files identified by name specified with the first parameter You can call this from any Context in your app

6 Get a Handle to a SharedPreferences (2/2) getpreferences() Use this method if you need to use only one shared preference file private for the activity Use this from an Activity You don't need to supply a name

7 Creating a Global SharedPreferences Use this option if you want to allow other apps to access your data For that, create a shared preferences file with MODE_WORLD_READABLE or MODE_WORLD_WRITEABLE

8 Write to Shared Preferences Create a SharedPreferences.Editor by calling edit() on your SharedPreferences Pass the keys and values you want to write, then call commit()

9 Read from Shared Preferences To retrieve values from a shared preferences file, call methods such as getint() and getstring() Providing the key for the value you want, and optionally a default value to return if the key isn't present

10 SQL Database Storage

11 Saving Data in SQL Databases Android provides API s for saving data in a SQLite database Saving data to a database is ideal for repeating or structured data The APIs are available in the android.database.sqlite package

12 Using a Database Handler We will create a database handler class that handles all database s CRUD operations Create, Read, Update, Delete The class will hold the tables schemas, including operations on the tables The handler class will extend a provided helper class called SQLiteOpenHelper public class MyDbHandler extends SQLiteOpenHelper { }

13 public abstract class android.database.sqlite.sqliteopenhelper A helper class to manage database creation and version management You create a subclass implementing oncreate(), and onupgrade() This class takes care of opening the database if it exists creating it if it does not and upgrading it as necessary

14 public SQLiteOpenHelper (Context, String name, CursorFactory factory, int version) This method always returns very quickly since the database is not yet created It is only created when getwritabledatabase() or getreadabledatabase() is called Database names must be unique within an application Use null for the default cursor factory The version of the database should start from 1

15 getwritabledatabase(), getreadabledatabase () Both return SQLiteDatabase for R/W or for reading These methods can be long-running Since database may be created or upgraded Therefore, make sure you call them in a background thread, e.g., using AsyncTask

16 public abstract void oncreate (SQLiteDatabase db) Called when the database is created for the first time This is where the creation of tables and the initial population of the tables should happen

17 public abstract void onupgrade (SQLiteDatabase db, int oldversion, int newversion) Called when the database needs to be upgraded The implementation should use this method to drop tables, add tables, or do anything else it needs to upgrade to the new schema version The SQLite ALTER TABLE command may be used See documentation here Note: onupgrade() is triggered when the database version provided to the constructor of SQLiteOpenHelper is updated

18 A Running Example We ve learned the basic principles Now we ll go over an example in the popular site The example shows how to store user contacts in SQLite database Note: in the method addcontact(), note that the method db.insert(..) automatically sets an id for the inserted row

19 Saving Data in Files

20 Saving Data in Files A file system is suited to reading or writing large amounts of data in start-tofinish order For example, it's good for image files

21 Android s Internal and External Storage All Android devices have two file storage areas: internal and external storage External storage may live in a removable storage medium such as a micro SD card In any case, the API distinguishes between external and internal storage Apps are installed onto the internal storage by default. This can be changed using the android:installlocation attribute in the manifest. Users may appreciate this option when the APK size is very large. This link summarizes the differences between the two types of storage. Internal storage is best when you want to be sure that neither the user nor other apps can access your files. External storage is the best place for files that don't require access restrictions and for files that you want to share with other apps or allow the user to access with a computer.

22 Obtaining R/W Permissions To read/write to the external storage, you must request permissions in the manifest Internal storage doesn t require permissions Note: permissions may be needed for accessing many other resources such as camera, internet access, etc. See this guide for more details

23 Save a File on Internal Storage This is one alternative Other alternatives may use getfilesdir() and getcachedir() see examples here Technically, other apps can access your file if it is set as readable, and if they have your package and file names

24 Save a File on External Storage (1) You should first check that the external storage is available, e.g., that the user has not removed the SD card

25 Save a File on External Storage (2) Public files - should be available to other apps and to the user. Upon app s uninstall the files should remain available to the user Private files - belong to the app and should be deleted when the user uninstalls your app They are still accessible by the user and other apps Use the method getexternalstoragepublicdirectory() to access the directory of the public files Use the method getexternalfilesdir() to access the directory of the private files See examples here