Android ListView

ListView

• An ordered collection of selectable choices
• Key attributes in XML:
  • android:clickable to set the list to be clickable or not
  • android:id to assign a unique ID for use in Java code
  • android:entries to set the options to appear in the list, which must match an array in strings.xml

Static Lists

• Content is fixed and known before the app runs
• Declare the list elements in the strings.xml resource file
• Example: Android, iPhone, Max OS X

Dynamic Lists

• Content is read or generated as the program runs
• Comes from a data file, internet, etc.
• Must be set in Java code
• Example: reading a file oses.txt to create a list

List Adapters

• Helps turn list data into list view items
• Common adapters: ArrayAdapter, CursorAdapter
• Syntax for creating an adapter: ArrayAdapter name = new ArrayAdapter(activity, layout, array);
• Attach the adapter to the list by calling the setAdapter method of the ListView object in Java code

Waiting for a Result

• If the calling activity wants to wait for a result from the called activity:
  • Call startActivityForResult instead of startActivity
  • Pass a unique ID number to represent the action being performed
  • Write an onActivityResult method that will be called when the second activity is done
  • Check for the unique ID as was passed to startActivityForResult
  • Ask the intent for any extra data
  • Modify the called activity to send a result back using setResult and finish methods

Files and Storage

• Android can read/write files from two locations:
  • Internal storage
  • External storage

Internal Storage

• Built into the device
• Guaranteed to be present
• Typically smaller (~1-4 GB)
• Can't be expanded or removed
• Specific and private to each app
• Wiped out when the app is uninstalled

File and Streams

• java.io.File represents a file or directory
• Methods: canRead, canWrite, create, delete, exists, getName, getParent, getPath, isFile, isDirectory, lastModified, length, listFiles, mkdir, mkdirs, renameTo
• java.io.InputStream, OutputStream represent flows of data bytes from/to a source or destination
• Stream objects are often passed as a parameter to other objects like java.util.Scanner, java.io.BufferedReader, java.io.PrintStream to do the actual reading/writing

Using Internal Storage

• An activity has methods to read/write files:
  • getFilesDir() returns internal directory for the app
  • getCacheDir() returns a "temp" directory for scrap files
  • getResources().openRawResource(R.raw.id) reads an input file from res/raw/
  • openFileInput("name", mode) opens a file for reading
  • openFileOutput("name", mode) opens a file for writing
• Can use these methods to read/write files on the device
• Many methods return standard java.io.File objects
• Some return java.io.InputStream or OutputStream objects, which can be used with standard classes like Scanner, BufferedReader, and PrintStream to read/write files

Internal Storage Examples

• Example 1: read a file and put its contents into a TextView
• Example 2: write a short text file to the internal storage and read it back

External Storage

• External storage: Card that is inserted into the device (such as a MicroSD card)
• Can be much larger than internal storage (~8-32 GB)
• Can be removed or transferred to another device if needed
• May not be present, depending on the device
• Read/writable by other apps and users; not private to the app
• Not wiped when the app is uninstalled, except in certain cases

External Storage Permission

• If the app needs to read/write the device's external storage, it must explicitly request permission to do so in the AndroidManifest.xml file
• On install, the user will be prompted to confirm the app's permissions

Checking if Storage is Available

• isExternalStorageWritable() method to check if external storage is writable
• isExternalStorageReadable() method to check if external storage is readable

Accessing Web Data

• To read data from the web, request the INTERNET permission in the AndroidManifest.xml file
• Use the java.net.URL class to connect to a file or page at a given URL and read its data

Playing Sound Effects

• Find sound files such as .WAV, .MP3
• Put sound files in the project folder app/src/main/res/raw
• Refer to audio files in Java code as R.raw.filename
• Load and play clips using Android's MediaPlayer class
• Methods: create, start, stop, pause, isLooping, isPlaying, getCurrentPosition, release, seekTo, setDataSource, setLooping

Multiple Activities

• Many apps have multiple activities
• Example: address book app with main activity for listing contacts and another activity for viewing details
• An activity can launch another activity in response to an event
• The activity can pass data to the other activity
• The second activity can send data back to the first activity when it is done