Download Android Programming Lecture 17: File and Network I/O Menus

Android Programming
Lecture 17: File and Network I/O
Menus
11/9/2011
File and Network I/O
Direct File I/O
• Android API provides hook into the Android phone’s file system • These methods to open files for input and output
– Methods don’t allow folder specification
– Typically employ app‐specific permissions • Can employ with general Java I/O classes and methods Android & Java I/O: Text Output
PrintWriter: use print() and println() method to write arbitrary strings of text, will automatically call toString() on objects
BufferedWriter: handles buffering of data before writing to disk (writes in one big burst instead of each characters – important for efficiency)
OutputStreamWriter: handles String to byte conversion
FileOutputStream: writes bytes to a file on disk
Android & Java I/O: Example Text Output Code
Java I/O code requires try/catch blocks
Try: Attempt to do file processing
Catch: If something goes wrong, let me know
Java I/O: Text Input
Common wrapping (using Android openFileInput, which gives back a FileInputStream)
Scanner scanner = new Scanner(
new BufferedInputStream(
openFileInput(filenameGoesHere)));
Android & Java I/O: Text Input
Scanner allows to read text by line, but also to parse out integers and doubles if they exist:
hasNextLine() – are there more lines
nextLine() – give me the next line
hasNextDouble() – is the next thing a double
nextDouble() – give me the next double
…. BufferedInputStream supports buffering of reads from file (efficiency)
FileInputStream supports reading of data from files
Android & Java I/O: Text Input
Continuation of program from a few slides back
Accessing General Web Data
• We have seen in previous slides how to access “web services” – Essentially, calling a function/method on another machine (details of how this works are hidden from programmer)
• What if we just want to access HTML data?
– Maybe we could grab Aramark menu data and have a PitMenu app?
Accessing General Web Data
• Direct HTTP access is needed commonly enough that the Android code API already supports it
• One approach: URLConnection
– Specify a URL
– Request connection to that URL
– Treat open connection as an input stream • We already know what to do at this point (from files)
– Close connection
• Because network based, requires try/catch to deal with potential errors
URLConnection
Specify a URL
URL dataURL = new URL(“address_here”);
Request connection to that URL
URLConnection = dataURL.openConnection();
Treat open connection as an input stream
BufferedReader br = new BufferedReader(new InputStreamReader(urlConnection.getInputStream());
Process data Above approach will allow line at a time reading
via BufferedReader
Close connection
urlConnection.getInputStream().close();
URLConnection: Example
Goal: Have app “scrape” web addresses out of an HTML web page – This technique is often used by “web‐crawlers” (such as Google)
URLConnection: Example
HTTPURLConnection
One can cast a URLConnection made to a HTTP address as a HttpURLConnection and take advantage of a few HTTP specific methods
One important example:
int getResponseCode() , String getResponseMessage()
Useful in handling protocol errors (404: page not found)
URLConnection: Example
URLConnection: OutputStream (POST)
It is possible to use a URLConnection stream as an output stream as well
Supports POST component of HTTP protocol, where data is sent to a server, such as when completing web forms
urlConnection.getOutputStream();
Menus
Menus
• There is a hardware menu button on Android phones and tablets
– Exposes Activity‐specific menus – These menus can hold up to six menu items, in a tiled fashion, or five items and a More button which displays an arbitrarily long list of additional menu options.
• This is the “Expanded Menu”, and it is hooked to the bottom of the screen
– Menus can trigger submenus, which are floating dialogs (they look much like drop down lists)
Menus: RideTheWake Example
Normal Menu
• In the RideTheWake
shuttle tracking app, the menu allows for:
– Choosing a bus
– Readjusting center if strayed away from bus
– Checking for temporary schedule changes
– Changing app settings (polling time)
Expanded Menu
Generating Menus: Meta Ideas
• A menu consists of one or more MenuItems
– Menu items in the primary (6‐option‐max) menu can be specified with an icon and text.
– The ExpandedMenu and Submenus do not support icons, but do support Checkboxes and RadioButtons
– All MenuItems can be associated with keyboard shortcuts and condensed titles
Handling Selection Events: Meta Ideas
• Activities are already designed to listen to menu button presses
– Just have to write functions
• Primary response mechanism: onOptionsItemSelected handler function – One function to handle any menu press
• Can also associate with each menu item a function to support handling menu selection:
– Use android:onClick=functionName attribute in XML Generating Menus: XML
• Define an xml file in res/menu
– Make one for each menu (activity)
– May have to make this directory
• Start with outer <menu> tag • Menu items are indicated by <item>
– Associate with id, icon, title, checkable, radiobox, shortcuts (numeric and alphabetic) attributes
– Submenus are defined as a menu within an item
Full details for <menu> & <item> syntax is here: http://developer.android.com/guide/topics/resources/menu‐resource.html
Generating Menus: XML Examples
Simple three item menu: Bus menu: Three item menu with submenu:
Icons are drawable resources
Icons show up for first five, rest are on
ExpandedMenu
Triggering Menus
When the menu hardware button is pressed, a listener function is called, which should inflate the menu XML:
public boolean onCreateOptionsMenu(Menu menu) { MenuInflater menuInflater= getMenuInflater();
inflater.inflate(R.menu.menu_name_here, menu);
return true;
}
Alternatively, you could build the menu directly in Java here [calling MenuItem constructor, setting attributes], but we’ll stick with XML – see page 125 of book for Java approach
Handling Selection Events
For Menus
A selected menu item is passed to the listener function:
public boolean onOptionsItemSelected(MenuItem item)
{
// call super function of this function to support any upstream processing
// check id, title, etc. to help decide how to act
// return true if handled completely
}
Simple Menu Example