1. Welcome to CIS 083 !
Events
CIS 068

2. Overview
Subjects:
Structured Programming vs. Event Driven Programming(EDP)
Events
Events in JAVA
GUIs as Example for EDP
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3. 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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4. An Example Show both circles Exit Program The circles
Buttons toggle circles
Menu
Reset
Exit
ON / OFF
ON / OFF
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5. Do you really want to do that ?
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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6. All commands must be executable at every time
An Example
Required:
All commands must be executable at every time
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7. 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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8. Sequential Approach
Of course this approach doesn’t fulfill the requirements !
(If button 1 is activated, button 2 and the menu are disabled)
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9. 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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10. 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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11. 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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12. 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 program’s flow ?
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13. Sequential vs. Event-driven Programming
Comparison between Sequential
And
Event Driven Programming
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14. Sequential Programming
In sequential programs, the program is under control
The user is required to synchronize with the program:
Program tells user it’s ready for more input
User enters more input and it is processed
While … if then… structures highly define the user’s 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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15. 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
Shouldn’t the program be required to synchronize
with the user?
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16. Sequential Programming
Advantages
Architecture is iterative (one step at a time)
Easy to model (flowcharts, state machines)
Easy to build
Limitations
Can’t 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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17. 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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18. 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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19. Event Driven Programming
There’s 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
Don’t have to deal with order of events
This is especially helpful, when the order is unknown !
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20. 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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21. Typical Applications Other Examples: Event driven I/O LEGO Mindstorms
Especially networking, where I/O is very slow
Events occur, when
Connection is made
When ready to send
When data arrives
LEGO Mindstorms
Sensors report changes in environment
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22. Event Driven Programming
How does it work or
where’s the ‘MAIN’ ?
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23. 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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24. Participants in an OO - Event Driven System
Source
(e.g. mouse)
Listener
Event
Public void mouseClicked(
MouseEvent e){ … }
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25. 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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26. Registering Listeners
Source
(eg. Button)
Listener 3
Listener 4
REGISTER (‘please inform me’)
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27. Registering Listeners: JAVA example
here !
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28. Where’s 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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29. The advantage: Where’s the ‘MAIN’ ?
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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30. Events in JAVA: Sources
Event Sources:
Mouse
Keyboard
GUI – Objects (AWT / Swing)
Button
ChoiceBox etc.
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31. 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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32. Events in JAVA: Listeners
Classes implementing the ‘Listener’ – interfaces (package java.awt.event)
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33. 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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34. JAVA-Events: GUI – Example
Implementation of the first example (without Menu and Dialog)
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35. JAVA-Events: GUI – Example
JFrame
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36. 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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37. 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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38. 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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39. JAVA-Events: GUI – Example
The Main - Window: JFrame
Layout
Add JPanels
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40. 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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41. GUI – Example Constructor This method is invoked by JButton !
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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42. Outlook
Next time:
GUI – Elements !
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