Mobile and Ubiquitous Computing: Android Programming (part 4)

Transcription

1 Mobile and Ubiquitous Computing: Android Programming (part 4) Master studies, Winter 2015/2016 Dr Veljko Pejović Examples from: Mobile and Ubiquitous Computing Jo Vermeulen, University of Birmingham, UK

2 Data Storage SharedPreferences Internal storage External storage SQLite database

3 SharedPreference Preserve a small amount of primitive type (int, float, String, Boolean) data on a device Data are saved as key-value pairs Should be read/written by your app only Stored for as long as the app is installed on a device Common use: User preferences username, customisations, such as preferred WiFi AP, preferred theme, etc. Variables for conditional app execution When the app is launched for the first time set launched to True; next time, check if launched was set or not

4 Accessing SharedPreferences Reading SharedPreferences settings = getapplicationcontext().getsharedpreferences( preferences, MODE_PRIVATE); boolean waslaunched = settings.getboolean( launched, false); Need to know the type of data: getboolean() getstring() getall() returns a Map of key-value pairs Always use MODE_PRIVATE

5 Accessing SharedPreferences Writing SharedPreferences settings = getapplicationcontext().getsharedpreferences( preferences, MODE_PRIVATE); SharedPreferences.Editor editor = settings.edit(); editor.putboolean( launched, true); editor.commit(); Different put methods for different data types Don t forget to commit!

6 PreferenceFragment A special Fragment that connects XML preference hierarchy with SharedPreferences <PreferenceScreen xmlns:android=" > <PreferenceCategory android:title="@string/inline_preferences"> <CheckBoxPreference android:key="checkbox_preference android:title="@string/title_checkbox_preference /> <EditTextPreference android:key="edittext_preference" android:title="@string/title_edittext_preference /> </PreferenceCategory> <PreferenceScreen/> public class PrefsFragment extends PreferenceFragment public void oncreate(bundle savedinstancestate{ super.oncreate(savedinstancestate); addpreferencesfromresource(r.xml.preferences); } }

7 File Storage Android uses the common Java File API Files saved as internal or external files INTERNAL Always available. Files saved here are accessible by only your app by default. When the user uninstalls your app, the system removes all your app's files from internal storage. EXTERNAL Not always available a user can mount the external storage as USB storage World-readable, so files saved here may be read outside of your control. When the user uninstalls your app, the system removes your app's files from here only if you save them in the directory from getexternalfilesdir().

8 Internal Files Find where to save files getfilesdir() an internal directory for your app getcachedir() an internal directory for your app's temporary cache files If the system begins running low on storage, it may delete these cache files Read/write methods FileOutputStream openfileoutput (String name, int mode) opens file for writing (creates it if needed) FileInputStream openfileinput (String name) opens a file for reading

9 External Files Check if the storage is mounted public boolean isexternalstoragewritable() { String state = Environment.getExternalStorageState(); if (Environment.MEDIA_MOUNTED.equals(state)) { return true; } return false; } Write to private external storage: getexternalfilesdir() Write to public external storage: getexternalstoragepublicdirectory()

10 External Files If written to public external storage your files remain on the device even after your app is uninstalled Use Environment class constants for storing files in appropriate directories File file = new File(context.getExternalFilesDir( Environment.DIRECTORY_PICTURES), albumname);

11 SQLite A relational database implementation for embedded systems A library added to an app, rather than a standalone client-served DB engine The whole database is kept in a single file, and a write operation gets an exclusive lock over the file (no concurrent writing) SQL-92 language standard implemented (mostly) Supports ACID transactions (Atomic, Consistent, Isolated and Durable)

12 SQLite Use Android s SQLite database for structured data, especially if SQL queries can be used for improved data presentation Key classes: SQLiteOpenHelper To create, open, modify the tables in the database SQLiteDatabase To query the database

13 SQLiteOpenHelper Subclass SQLiteOpenHelper for your database Define your schema - formal declaration of how the database is organized, and the SQL statements that you use to create your database Call super() from the subclass constructor to initialize the underlying database Execute database creation in oncreate() oncreate() of the helper is called when a component tries to access a not-yet-existing database SQLiteOpenHelper abstracts the costly DB creation operations, and does not run them when not needed

14 SQLiteOpenHelper Access the database via SQLiteOpenHelper: getreadabledatabase() getwritabledatabase() These operations may take some time, thus, run them on a background thread using AsyncTask or IntentService

15 SQLiteOpenHelper Example Use _id for the primary key

16 SQLiteOpenHelper Example Use in Activity:

17 SQLiteDatabase Insert into database by calling insert() with ContentValues key-value pairs Query the database with query() and rawquery()

18 SQLiteDatabase ListViews populated with Adapters connected to the database are often the most convenient way of displaying DB content

19 Examining DB Content Databases stored in /data/data/<package name>/databases/ Can examine database with sqlite3 # adb -s emulator-5554 shell # sqlite3 /data/data/ si.uni_lj.fri.lrss.databaseexample/databases/ contacts_db

20 Examining DB Content

21 Background Processing

22 Concurrency in Android Java Threads The most general method Service Runs work in the background, however, by default on the same thread as the UI IntentService Background work on a separate thread, to contact the UI use local broadcast AsyncTask Background work on a separate thread, but with a tight integration with the UI

23 IntentService Suitable for long running one-off tasks when we don t want to affect the UI responsiveness IntentService survives Activity lifecycle changes No need to cancel the service when Activity is rebuilt Called using explicit Intent Starts on demand handles each Intent in turn; stops itself when it runs out of work

24 IntentService Define in AndroidManifest.XML Extend the class in your Java code

25 Invoking IntentService Create an explicit Intent for your IntentService Use startservice() to start the IntentService Add additional data if needed with the extra field

26 Handling Results (1) ResultReceiver in your Activity Subclass ResultReceiver, implement onreceiveresult Pass ResultReceiver through Intent

27 Handling Results (1) ResultReceiver IntentService sends results to ResultReceiver in a Bundle with send() method Example Display location address h"p://developer.android.com/training/loca4on/display- address.html

28 Handling Results (2) Broadcast A different approach Subclass BroadcastReceiver in your Activity Broadcast from IntentService

29 AsyncTask For short, more interactive tasks Runs on a separate worker thread, but keeps a link with the main UI thread via: onpreexecute onprogressupdate onpostexecute Define what we want to do in the background in: doinbackground Start with YourTask().execute() AsyncTask <?,?,?>? param types for input, progress, output

30 h"ps://github.com/googlesamples/android- play- loca4on/tree/master/ac4vityrecogni4on AsyncTask Example Just before the task starts This is done in the background, and the status is communicated via publishprogress()

31 AsyncTask Example Connects with the UI thread Immediately after the task is finished