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COIT20270 Application Development for Mobile Platforms Week 5: Displaying Pictures and Menus, Data Persistence

Dr. R. Balsys, CQU, 2016.

Source: Beginning Android Programming with Android Studio, J.F. DiMarzio, 2016

Week 5 – Displaying Pictures and Menus

Objectives, to :

use image views to display pictures

Use menu’s with views

Examine some additional views

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CQU - COIT20270 Application Development for Mobile Platforms

Using Image Views to Display Pictures

You display images with ImageView, Gallery, ImageSwitcher and GridView views

An ImageView is used to display an individual image

The Gallery view displays images as a center-locked horizontally scrolling list of images

The ImageSwitcher view is used when you wish to apply animation or transitions when moving from one image to another

The GridView shows images as a two dimensional array of rows and columns of images

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CQU - COIT20270 Application Development for Mobile Platforms

Using Menu’s With Views

There are two main types of menus in Android

Options menu – activated using the keyboard Menu button and it displays menus for the current activity

Context menu – displays information about the current view in an activity when the user taps and holds on the view

The way menu’s are created and handled in Android 6 and greater has changed and is different to that given in the text

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The Menu XML file

An XML defining the menu items is added as an XML file to the projects res/menu folder

The menu items should be contained within

<menu xmlns:android="http://schemas.android.com/apk/res/android">

 

</menu>

All menu items are defined within <item> </item> tags

As a minimum the item need the following options

android:id - identifier of the item

android:orderInCategory – the position of the item in the menu, higher goes first

android:title – the text of the menu item

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The Menu Inflater

The onCreateOptionsMenu() is called when the Menu is drawn

Call getMenuInflater().inflate(R.menu.menu_main, menu);

and then return true;

You should replace menu_main with the name of the xml file you created your menu resource in

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The onOptionsItemSelected method

The onCreateOptionsMenu() is called when the Menu is drawn

Call getMenuInflater().inflate(R.menu.menu_main, menu);

and then return true;

You should replace menu_main with the name of the xml file you created your menu resource in

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Context Menu

To associate a context menu with a view you call the setOnCreateContextMenuListener() on the view

The item to associate the context menu with uses the setOnCreateContextMenuListener() to listen for any tap and holds on the view

You override the onCreateContextMenu() method and call your CreateMenu() method within it

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CQU - COIT20270 Application Development for Mobile Platforms

Get student to Google epistemological

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Data Persistence

Objectives, to :

Save and load data with user preferences

Persist data to files

Create and use databases

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CQU - COIT20270 Application Development for Mobile Platforms

Saving and Loading User Preferences

To save data objects you can

Write the objects to a file

Save the object to a database

Use a SharedPreferences object

To use a SharedPreferences object you save a name/value pair to SharedPreferences and this is then saved into an XML file for you

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CQU - COIT20270 Application Development for Mobile Platforms

Accessing Preferences in an Activity

You 1st need to create a myapppreferences.xml file to use for saving your data

Within this xml file you need to create key:value pairs for the data you wish to save

Create an activity that extends the PreferenceActivity class and call the addPreferencesFromResource() method to load the xml file containing the preferences

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CQU - COIT20270 Application Development for Mobile Platforms

Retrieving and Modifying Preference Values

To make use of preferences you use the SharedPreferences class

The getSharedPreferences() method is used to get an instance of the SharedPreferences class using the name of the XML preferences file

You then create a SharedPreferences.Editor() object and use a putString() method to change the value of a string preference

To save the change to the XML file you use the commit() method

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CQU - COIT20270 Application Development for Mobile Platforms

Demonstrate the files working and discuss how they work

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Changing the Default Name of a Preferences File

Using the PreferenceManager class you can call a getPreferenceManager() method and then set the instance of this to use a new name by calling the setSharedPreferenceName() and passing in the new name

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CQU - COIT20270 Application Development for Mobile Platforms

Demonstrate the file working and discuss how it works

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Persisting Data to Files

To store data to files use the java.io package

You can write data to the devices internal store by

Writing text to a file using the FileOutputStream class and the openFileOutput() method with MODE_WORLD_READABLE (public read access), MODE_PRIVATE (private access), MODE_APPEND or MODE_WORLD_WRITEABLE (public write access)

To convert a character stream to a byte stream create a OutputStreamWriter instance using OutputStreamWriter() and then use the write() method to write to the file. Remember to flush() and close() the file

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CQU - COIT20270 Application Development for Mobile Platforms

Demonstrate the file working and discuss how it works

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…Persisting Data to Files

You can read data from the devices internal store by

using the FileInputStream class and the openFileInput() passing the file name as an input parameter. Then use the instance of the FileInputStream to create a new InputStreamReader()

The input file is read in blocks of READ_BLOCK_SIZE bytes into a character array buffer you create. The read() method is used for this. read() returns -1 when the end of file is reached

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CQU - COIT20270 Application Development for Mobile Platforms

Demonstrate the file working and discuss how it works

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Saving to External (SD card) Storage

Use the getExternalStorageDirectory() method to get a File object that points to your SD card’s path

To write to the SD card you then create a new directory using the previously found path using mkdirs()

You can then use the previously discussed FileOutputStream() method to write objects to the file

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CQU - COIT20270 Application Development for Mobile Platforms

Demonstrate the file working and discuss how it works

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…Saving to External (SD card) Storage

To load data from the SD card use the path and filename approach used to write to the file and use a FileInputStream object instance along with an InputStream instance to read from the file

In order to write to a file you need to set the WRITE_EXTERNAL_STORAGE permission in your Android.Manifest file

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CQU - COIT20270 Application Development for Mobile Platforms

Demonstrate the file working and discuss how it works

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Choosing a Storage Option

If you have data that can be represented using name:value pairs then use the SharedPreferences approach to store your data

If your data is not in easily put into name:value pairs and is not going to be shared with other apps then use internal storage to save your data

If your data is to be shared with other apps or users then use the SD card storage option

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CQU - COIT20270 Application Development for Mobile Platforms

Demonstrate the file working and discuss how it works

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Using Static Resources

You can add files to your projects res/raw folder. These files are then usable in your app

Use the getResources() method to return a Resources object to get access to the file in res/raw

The resource ID is used to identify the required resource. It is the name of the file in res/raw without its extension

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CQU - COIT20270 Application Development for Mobile Platforms

Creating and Using Databases

Using a relational database to store tables of data is efficient as database queries can be used to retrieve the data

Android uses the SQLite database system to provide access to a database, only to the app that created it

The created database is stored in the /data/data/<package_name>/databases folder

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CQU - COIT20270 Application Development for Mobile Platforms

Demonstrate the files working and discuss how they work

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Creating a DBAdapter Helper Class

It is a good idea to create a helper class to create, open, close and use a SQLite database

A database has a name, tables and columns

You need to create constants to name the fields of your database

The DATABASE_CREATE constant contains the SQL statements for creating the table of the database

Within your DBAdapter class create a DatabaseHelper class that extends SQLiteOpenHelper

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CQU - COIT20270 Application Development for Mobile Platforms

Demonstrate the file working and discuss how it works

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…Creating a DBAdapter Helper Class

Override the onCreate() method to create the new database

Override the onUpgrade() method for use when the database is upgraded

You then define methods to use for opening, closing and adding, deleting or updating rows in the table

Android uses the Cursor class to return values from queries

ContentValues object is used to store name:value pairs using its put() method

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CQU - COIT20270 Application Development for Mobile Platforms

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CRUD Operations on the Database

As an example we will look at adding contacts to the contacts database

Use the DBAdapter class to create a new instance of the DBAdapter class

To retrieve contacts in the contacts table use the getAllContacts() method. This returns a Cursor object.

You can use the moveToFirst() and moveToNext() methods to examine each contact in turn. The contact details are found by calling DisplayContact() on the cursor object

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CQU - COIT20270 Application Development for Mobile Platforms

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…CRUD Operations on the Database

To retrieve a single contact use its ID and call the getContact() method to get the details which is returned in a Cursor object

To update a contact call the updateContact() method passing the ID as the 1st parameter

To delete a contact use the deleteContact() method with the contact ID as the parameter

To upgrade a new database to a new one change the DATABASE_VERSION field in the DBAdapter class to a higher value than the old one. This deletes the old table, ready for the new table to be created from an existing one or otherwise

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CQU - COIT20270 Application Development for Mobile Platforms

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Pre-creating a Database

Often the database is pre-created at design time and used at run-time

Tools are used to create the initial database, such as that at sourceforge.net/projects/sqlitebrowser. This tool allows you to visually create the fields for the database and populate the fields with values

The database is copied into the assets folder of your project. When the application is created you copy the database in the assets folder into the /data/data/<project_name>/databases folder if it does not already exist. You can then use it as before

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CQU - COIT20270 Application Development for Mobile Platforms

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