1. Classes, Objects, and World-level Methods
Alice

2. Class / Object Class Object
A template describing the characteristics (properties, methods, and functions) of an object of a specific type
Object
An instance of a class; each object of one class has identical characteristics, but property details may vary
Programming in Alice
© 2006 Dr. Tim Margush

3. Method / Function Method Function
A programming unit that can be executed on demand by using its name; corresponds to a sequence of actions
Function
A programming unit that can be executed on demand by using its name; represents a value
Programming in Alice
© 2006 Dr. Tim Margush

4. Classes, Objects, & Methods
Object-oriented programming uses classes, objects, and methods as basic programming components.
These components help you to
design programs
think about an intricate program
test programs
find and remove errors (bugs)

5. In your programs, you have been using…
Classes
In Alice, classes are predefined as 3D models
Objects
An object is an instance of a class.

6. Capitalization is a matter of Style
Classes
Classes: Penguin (Uppercase name)
Objects
Objects: penguin, penguin1, penguin2, opus (lowercase names)

7. In your programs, you have also used…
built-in (predefined) methods
Examples: move, turn to face, say
World.my first method
Example:
In the snowpeople world (Chapter 2), we wrote program code where the snowman tried to get the attention of a snowwoman.
As a reminder, see next slide…

8. Adding Instructions to world.my first method
The method should be open in the method editor window of Alice. (bottom right)
if not select the world from the object tree, the method tab, and click the edit button next to my first method
Elements of Visual Programming
Animation Programs

9. Method Editor Window
Elements of Visual Programming
Animation Programs

10. Step 1 – Snowman turns
Select the object you want to perform the object
Select the method / action you want the object to perform
could use turn or turn to face
often many ways to accomplish the same task
Click and drag it to the method editor window
Elements of Visual Programming
Animation Programs

11. Snowman Turns
Can adjust aspects of how the snowman turns to face the snowwoman
click the more option
right now duration and style are the only things you should alter
Elements of Visual Programming
Animation Programs

12. Step 2 – Combined Action
We want the snowman to say “ahem” and blink at the same time
actions are normally sequential
to do actions together, at the same time, use a “Do together” block
Click and drag “Do together” block into the method
Elements of Visual Programming
Animation Programs

13. Affecting subparts The snowman does not have “blink eyes” method
Can accomplish a blink by affecting subparts
Select snowman object from object tree and expand subparts
expand the head
now we can give commands to individual parts, in this case the eyes
have eyes move up and down
specify direction and distance of move
Elements of Visual Programming
Animation Programs

14. Step 2 – First Attempt world.my first method looks like this
TEST the method
play the movie. Does it do what we want?
Elements of Visual Programming
Animation Programs

15. Logic Error The program works, but does not do what we intended.
This is an example of a logic error
very easy in Alice to see logic errors
the movie does not do what we wanted
What’s the problem?
Elements of Visual Programming
Animation Programs

16. Do together and Do in order
All commands in the Do together block are executed simultaneously
So what is the result if you move an eye up .1 meters and down .1 meters at the same time?
Apparently nothing
So while we want the eyes to move together and to say “ahem” we want the eyes to first move up and then down
Use a Do in order block inside the Do together block
Elements of Visual Programming
Animation Programs

17. Corrected Do Together Elements of Visual Programming
Animation Programs

19. Larger Programs
As you become more skilled in writing programs, you will find that your programs quickly begin to increase to many, many lines of code.
Games and other "real world" software applications can have thousands, even millions of lines of code.
In this session, we begin to look at organizing large programs into small manageable pieces.

20. Goals of OOP Organize a large program into small pieces
Design and think about an intricate program
Reusable code allows for well written piece of code to work in more than one program
Test find and remove errors (bugs)

21. Modifying the program
To make the snowpeople animation more realistic, we might want the snowman to be a little less shy. In our original program, the snowman tries to catch the snowwoman's attention only once. Perhaps he could try to get her attention more than once.
To make this modification, additional lines would be added to the code.
This would make the program code longer and more difficult to read and think about.

22. A Solution A solution to the problem is to
define our own method
name the new method catchAttention
Then, we can drag-and-drop the catchAttention method into the edit box just like the built-in methods

23. Demo: The solution First, to associate the new method with the World
select the World tile in the Object Tree
select the methods tab in the details area
click on the "create new method" button

24. Demo
A demonstration of writing the catchAttention method

25. World-level method catchAttention is a world-level method because it
is defined as a method for World
has instructions that involve more than one object (snowman, snowwoman, camera)

26. Using the catchAttention method
The catchAttention method is executed by calling (invoking) the method from my first method

27. Why? Why do we want to write our own methods?
saves time -- we can call the method again and again without reconstructing code
reduces code size – we call the method rather than writing the instructions again and again
allows us to "think at a higher level"
can think “catchAttention" instead of
“turn head to face the camera, then say ‘Ahem’ while moving eyes up and down"
the technical term for "think at a higher level" is "abstraction"

28. Assignment Read Tutorial04 Lab04 World-level Methods
How to create them
How to call them
When it is appropriate to use them
Lab04