Bruce Scharlau, University of Aberdeen, 2012

Data storage options for mobiles

Mobile Computing

Outline

• Storage in general

• Simple

• Databases

• Frameworks for ORM (object relational mapping)

Bruce Scharlau, University of Aberdeen, 2012

We can place data elsewhere on the network

Use a web service to store data elsewhere – save photos to flickr, files to some other app in the cloud.

Bruce Scharlau, University of Aberdeen, 2012

Can save files automatically, or at user discretion with time values, etc. (twitter, email apps, or photo capture)

Bruce Scharlau, University of Aberdeen, 2012

Mobiles + (Tunnels | Hills) = Lost connection

Need to keep data on the mobile

Could be preferences, scores, something… which isn’t already kept such as contacts, photos, or other data types provided by other apps.

Whatever, you need to determine the best way to store it on the handset

Bruce Scharlau, University of Aberdeen, 2012

Data storage raises issues• How often will the data be accessed?

• How quickly does the data need to appear?

• How often will the data be updated/edited?

• Will the data be added to over time?

• Will the data be deleted?

• How will the data need to be used?

Bruce Scharlau, University of Aberdeen, 2012

These answers will suggest solutions for you

Preference files are a light-weight option

Both Android and iOS use preferences as light-weight option

Bruce Scharlau, University of Aberdeen, 2012

In Android these are not sharable across applications, unless you expose them as a ‘content provider’.

In iOS you can tell NSUserDefaults which domains (apps) to search, so point to another app

We can write larger data to file

• You can either write to a new file, or to a pre-included file under

• Depending upon how much data you want to read/write you have different options.

• You can open socket to remote location and save/read data as stream

• You can build object in memory and then write out object as required and build method to restore too.

Bruce Scharlau, University of Aberdeen, 2012

You can only access files available to the application

Can also serialise objects too

• Store the object as array of bytes is also possible.

• In iOS this only works with specified objects: NSArray, NSDictionary, NSString, NSDate, NSNumber, and NSData (and some of their subclasses)

• In Android you can serialise objects as you would in Java: just need to remember order and do reverse to restore object

Bruce Scharlau, University of Aberdeen, 2012

Java uses RecordStore for objects

Java Mobile uses RecordStore, a byte array for storage

Bruce Scharlau, University of Aberdeen, 2012

Bruce Scharlau, University of Aberdeen, 2012

A RecordStore is NOT a database

A RecordStore is an array of bytes

with MIDP 2.0 a RecordStore can be shared across MIDlet suites

RecordStore names must be unique

Bruce Scharlau, University of Aberdeen, 2012

If multiple apps share an RMS you need to write the logic for this

RMS operations are thread-safe, but threads must still coordinate the reading and writing of data to the same record store.

This is especially important if your RecordStore is shared in a MIDlet suite.

Bruce Scharlau, University of Aberdeen, 2012

When serialising objects might need to consider sorting

• Need a means to determine whether objects are:– EQUIVALENT, FOLLOWS or PRECEDES if

order important, or– EQUIVALENT, BIGGER, SMALLER if size

attribute important

We can also persist data to a db

• Android and iOS can use SQLite db

• Each db is private to the application. In principle you could expose the data, if you expose the application as a content provider.

• In iOS all data can be shared between apps.

Bruce Scharlau, University of Aberdeen, 2012

Android Notepad tutorial uses database

Bruce Scharlau, University of Aberdeen, 2012

Useful db helper class for access and crud details

Android in Action db example covers more complex example

Bruce Scharlau, University of Aberdeen, 2012

Stores locations to database within application as objects

Android in Action app uses db helper classes with sql

Bruce Scharlau, University of Aberdeen, 2012

public static class Location {

public long id; public long lastalert; public int alertenabled; public String zip; // include city and region because geocode is expensive public String city; public String region;

public Location() { }

public Location(final long id, final long lastalert, final int alertenabled, final String zip, final String city, final String region) { this.id = id; this.lastalert = lastalert; this.alertenabled = alertenabled; this.zip = zip; this.city = city; this.region = region; }

Source: android in action, code

Part of DBHelper class showingLocation object

Class also holds crud details to map object to sql

Android in action app maps objects to sql for ease

Bruce Scharlau, University of Aberdeen, 2012

public void insert(final Location location) { ContentValues values = new ContentValues(); values.put("zip", location.zip); values.put("city", location.city); values.put("region", location.region); values.put("lastalert", location.lastalert); values.put("alertenabled", location.alertenabled); this.db.insert(DBHelper.DB_TABLE, null, values); }

public void update(final Location location) { ContentValues values = new ContentValues(); values.put("zip", location.zip); values.put("city", location.city); values.put("region", location.region); values.put("lastalert", location.lastalert); values.put("alertenabled", location.alertenabled); this.db.update(DBHelper.DB_TABLE, values, "_id=" + location.id, null); }

Source: android in action, code

Mapping makes coding easier

SQLite provides advanced db features

• There is transaction support

• You can use prepared statements based on java.sql and set items as have done before – faster and more secure

• You have a cursor to keep track of location within a resultset

Bruce Scharlau, University of Aberdeen, 2012

Can map objects to db

Enables off network use and can sync later when connected

Bruce Scharlau, University of Aberdeen, 2012

Might be pushing limits of device though with extra classes and memory usage

Can read items from network as xml and convert to objects, which map to db

Android storage Summary

• Can use preferences for each app

• Can write/read files as with Java

• Can persist/read items over network (when available)

• Can use SQLite one db per app

Bruce Scharlau, University of Aberdeen, 2012

Can also use ORM systems for more complex data storage

• Object relational mapping provides way to deal with objects and store them as such in a data base automatically

• Framework determines the mappings from objects to tables/rows, etc

Bruce Scharlau, University of Aberdeen, 2012

iOS uses Core Data for ORM

Based on model view control pattern Core Data creates suitable objects for data in the controller that marshals data

Similar to what happens in Ruby on Rails, or Hibernate in Java

Bruce Scharlau, University of Aberdeen, 2012

Android equivalents are available too

• http://ormlite.com/ provides ORM-lite based on Java and SQLite – assumes simple needs

• http://greendao-orm.com is more complex, and faster system that generated ORM per your project needs

Bruce Scharlau, University of Aberdeen, 2012

All depends upon data storage needs

• How often will the data be accessed?

• How quickly does the data need to appear?

• How often will the data be updated/edited?

• Will the data be added to over time?

• Will the data be deleted?

• How will the data need to be used?

Bruce Scharlau, University of Aberdeen, 2012

These answers will suggest solutions for you