Welcome to CIS 068 !

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Welcome to CIS 068 !
Events
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Overview

• Subjects
• Structured Programming vs. Event Driven
  Programming(EDP)
• Events
• Events in JAVA
• GUIs as Example for EDP

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An Example

• The task write a program,showing
• a menu
• 2 circles
• 2 buttons

Menu
Reset Exit
ON / OFF
ON / OFF
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An Example
Show both circles Exit Program The
circles Buttons toggle circles
Menu
Reset Exit
ON / OFF
ON / OFF
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An Example
Menu
Reset Exit
Do you really want to do that ?
YES
NO
ON / OFF
ON / OFF
Confirm command by dialog
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An Example
Required All commands must be executable at
every time
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Sequential Approach
A first approach could be
Check Menu
Show Window
Check Button 1
Check Button 2
Check Button Y
Check Button N
Exit or Loop
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Sequential Approach
Of course this approach doesnt fulfill the
requirements ! (If button 1 is activated, button
2 and the menu are disabled)
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Sequential Approach 2
Show Dialog A
Check Menu
Set dialogA true
Check Button 1
Check Button 2
Show Dialog B
If dialogA
Set dialogB true
Check Button Y / N

If dialogB
Check Button Y / N

Loop
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Sequential Approach 2
Does this approach meet the requirements ? What
does the program, i.e. the processor, do most of
the time ? What about more complicated
structures (nested command structures,) ?
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Event Driven Approach
What about this approach
Process Menu
Menu activated
Process Button 1
Button 2 activated
Button 1 activated
Process Button 2
Button Y activated
Process Button Y
Button N activated
Process Button N
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Example Discussion

• And again
• Does this approach meet the requirements ?
• What does the program, i.e. the processor, do
  most of the time ?
• What about more complicated structures (nested
  command structures,) ?
• Who is in control of the programs flow ?

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Sequential vs. Event-driven Programming

• Comparison between Sequential
• And
• Event Driven Programming

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Sequential Programming

• In sequential programs, the program is under
  control
• The user is required to synchronize with the
  program
• Program tells user its ready for more input
• User enters more input and it is processed
• While if then structures highly define the
  users path through the program
• Program is predefined with respect to time
• Handling of sequentially independent commands
  requires enormous amount of work

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Sequential Programming

• Flow of a typical sequential program
• Prompt the user
• Read input from the keyboard
• Parse the input (determine user action)
• Evaluate the result
• Generate output
• Repeat

Shouldnt the program be required to synchronize
with the user?
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Sequential Programming

• Advantages
• Architecture is iterative (one step at a time)
• Easy to model (flowcharts, state machines)
• Easy to build
• Limitations
• Cant implement complex interactions
• Only a small number of features possible
• Interaction must proceed according to a
  pre-defined sequence
• To the rescue Event-driven programming

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Event Driven Programming

• Instead of a user synchronizing with the program,
  the program synchronizes with, or reacts to, the
  user
• All communication from user to computer occurs
  via EVENTS and the code that handles the events
• An event is an action that happens in the system
• A mouse button pressed or released
• A keyboard key is hit
• A window is moved, resized, closed, etc.

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Event Driven Programming

• Typically two different classes of events
• User-initiated events
• Events that result directly from a user action
• e.g., mouse click, move mouse, button press
• System-initiated events
• Events created by the system, as it responds to a
  user action
• e.g., scrolling text, re-drawing a window

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Event Driven Programming

• Theres no top-down flow of control, i.e. no
  Main-program defining the sequential flow
• Code fragments are associated with events and
  invoked when events occur
• Order of execution is decoupled
• Dont have to deal with order of events
• This is especially helpful, when the order is
  unknown !

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Typical Applications

• GUIs
• Modern GUIs are event driven. Event occurs when
  the user does something
• Move the mouse
• Press a button
• Activate a menu
• Resize Window

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Typical Applications

• Other Examples
• Event driven I/O
• Especially networking, where I/O is very slow
• Events occur, when
• Connection is made
• When ready to send
• When data arrives
• LEGO Mindstorms
• Events occur, when
• Sensors report changes in environment

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Event Driven Programming
How does it work or wheres the MAIN ?
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Participants in an OO - Event Driven System

• Event Source
• objects that generate events, e.g. buttons
• Event
• represent occurences, changes of state or
  requests which a program might need to handle
• store all event-specific information (e.g.
  mouseEvent x/y coordinates)
• Handler (Listener)
• objects that respond to events
• Provide the code needed to handle the event

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Participants in an OO - Event Driven System
Event Source (e.g. mouse)
Listener
Event
Public void mouseClicked( MouseEvent e)
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Registering Listeners

• The listener must be known to the event source,
  such that the event can be sent
• Listener is REGISTERED
• Listeners specify what kind of events they are
  interested in, i.e. specify the event sources
• Typically done by addXXXListener(Listener)

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Registering Listeners
Listener 1
Listener 2
Source (eg. Button)
Listener 3
Listener 4
REGISTER (please inform me)
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Registering Listeners JAVA example
here !
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Wheres the MAIN ?

• The Operating System (or JAVA) manages an
  event-queue
• The event-queue contains information about
  event-sources and their registered listeners
• As events occur, they are placed in the queue to
  be dispatched by the event - loop
• The OS (or JAVA) loops through the event-queue,
  passing the command to the specified listeners
• The MAIN control is passed to the OS (or
  JAVA), the program is in idle state until
  activated by events
• The MAIN is replaced by the event-loop

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Wheres the MAIN ?

• The advantage
• The OS (or JAVA) can optimize the processing
  time for managing the loop very efficiently
• The OS provides the management, the programmer
  only writes (and registers) listeners

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Events in JAVA Sources

• Event Sources
• Mouse
• Keyboard
• GUI Objects (AWT / Swing)
• Button
• ChoiceBox etc.

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Events in JAVA Events
Note this diagram is not complete. It just shows
the most common event classes
EventObject
AWTEvent
ActionEvent
ComponentEvent
InputEvent
WindowEvent
MouseEvent
KeyEvent
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Events in JAVA Listeners
Classes implementing the Listener interfaces
(package java.awt.event)
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Events in JAVA the example revisited
ButtonDemo is able to receive ActionEvents
Creates the event source
Adds button to frame but does NOT register
ButtonDemo !
Registers THIS instance of ButtonDemo to theButton
If theButton is pressed, an ActionEvent is sent
to the Method actionPerformed(..), i.e. the
method is invoked with the event as argument.
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JAVA-Events GUI Example
Implementation of the first example (without Menu
and Dialog)
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JAVA-Events GUI Example
JFrame
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JAVA-Events GUI Example
AREA 1
AREA 2
JFrame layout GridLayout(1,2) Provides 1 x 2
Grid. Each area of grid can contain a new GUI
component.
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JAVA-Events GUI Example
CENTER
SOUTH

• Each area contains a JPanel.
• JPanel layout BorderLayout()
• Used are only the CENTER and SOUTH areas.

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JAVA-Events GUI Example

• The CENTER contains a JComponent
• JComponent can be used for custom drawing
  using the paint() method
• The SOUTH area contains a JButton

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JAVA-Events GUI Example
The Main - Window JFrame
Layout
Add JPanels
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JAVA-Events GUI Example
The JPanel, containing the circle and the button
Layout
Create JComponent And Button
Register JComponent to Button !
Add JComponent and Button
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GUI Example
The JComponent, responsible for drawing the
circle
Constructor
This method is invoked by JButton !
Paint green background and, if required,
red circle
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Outlook
Next time GUI Elements !