1. Class-level Methods
Alice

2. World / Class Method World method
A general method that may refer to multiple objects; not closely associated with any particular class
Class method
A method belonging to a class, designed to operate on an object of that class

3. Animated Actions
Some actions are more naturally associated with a specific class of objects rather than with the overall world.
Examples
A person walking
A wheel rolling
A fish swimming

4. Class Level Methods
These methods apply to a particular class of objects
They are included as part of the class template
Some classes have such methods (beyond the primitive methods expected of all objects)
Every instance of the class has such a method
Create class level methods (functions) to extend the functionality of the class

5. Class-level Methods
We can write our own methods to add abilities/functions to a specific class of objects
This will result in a class-level rather than a world-level method
The next few slides will demonstrate how to build class-level methods

6. Primitive / User-defined Methods
Primitive methods
those directly provided by the classes that are part of the Alice system
move, turn, distanceTo, …
User-defined methods (and functions)
those added to a world or class as part of a program
These consist of collections of primitive and other user-defined methods

7. A Predefined Class Method
Mana (Local Gallery>People) has two predefined methods
These are different from built-in methods because you can edit them
Every instance of the Mana class has these methods
Predefined methods Note Edit button

8. Designing Class Level Methods
Be sure it will apply to all objects of the specific class
World level methods implement general functionality that can be applied to many different types of objects
Class level methods will apply to all objects of the class
Each object created from the class will include the new methods

9. Argument / Parameter Argument Parameter
A piece of information passed to a method or function that will be used in its computations
World.jump(who = Rabbit) // Rabbit is the Argument
Parameter
A placeholder for information passed to a method (or function) by the caller; provides a handle (name) by which the information is referred to in the method
In method definition:
World.jump(who) // who is the parameter
who move up 1 meter

10. An example (building technique)
How can we create a skate method for ice skater objects?
We need to:
tell Alice to associate the new method (the one we are about to write) with an ice skater, and
write a new method to animate the ice skater skating.

11. Demo: The solution
First, to associate the animation with the ice skater
select the iceSkater tile in the Object Tree
select the methods tab in the details area
click on the "create new method" button

12. Storyboard for skate
Skate:
Do together
move skater forward 2 meters
Do in order
slide on left leg
slide on right leg
The slide actions each require several motion instructions, so we will break these two actions down into smaller pieces… this technique is called stepwise refinement

13. Refined storyboard for skate
Refinement of slideLeft
Do in order
Lift right leg and turn upper body forward
Lower right leg and return body upright
Skate:
Do together
1) move forward 2 meters
2) Do in order
slideLeft
slideRight
Refinement of slideRight
Do in order
Lift left leg and turn upper body forward
Lower left leg and return body upright

14. Writing the code
The next step is to translate the design into program code.
For the slideLeft method, a possible translation is:
Design step Instruction
Lift the right leg turn the right thigh forward
Turn upper body forward turn the abs forward
Lower the right leg turn the right thigh backward
Return the body upright turn the abs backward

15. Demo
Demonstration of
slideLeft
slideRight
skate

16. Correspondence of design to code
Skate:
Do together
move skater forward 2 meters
Do in order
slide on left leg
slide on right leg

17. Question
Writing the methods to make an ice skater perform a skating motion is an intricate task.
Most likely, you would like to reuse these new methods in other worlds.
How can you make the skate method available for an ice skater in a different world?

18. Answer: Save out as a new class
1) Rename iceSkater as cleverSkater.
2) Save out as a new class. Alice saves the new class as CleverSkater.a2c

19. Creating a Class in Alice
Choose a class as a base class
Your new class will inherit everything from the base class
Create an object of the base class
Add properties, methods, and functions to the object's details
Test!
Save the object as a new 3D class model

20. Using CleverSkater
An instance of the CleverSkater class can be added to a new world – use File|Import.

21. Benefits of Creating a Class
New Classes supports
the reuse of program code -- allows the programmer to write code once and use it again later in different programs.
sharing code with others in a team project

22. Class Methods No-No's
Class-level methods should be closely related to an object of the class
If not, consider making the method a world- level one, or place it in another class
Programming in Alice
© 2006 Dr. Tim Margush

23. Class Methods No-No's Do not make methods too large
Instead, use step-wise refinement
Create smaller methods to accomplish the subtasks needed for more complex tasks
Programming in Alice
© 2006 Dr. Tim Margush

24. Class Methods No-No's
Do not refer to other objects in your class- level methods
Those objects may not exist in another world
Camera and world objects are a possible exception to this rule
Instead, create a parameter (of type object) and refer to the object in a more general sense
Programming in Alice
© 2006 Dr. Tim Margush

25. Assignment Read Chapter 4-3 Read Tips & Techniques 4
Class-level methods and Inheritance
Read Tips & Techniques 4
Visible and Invisible Objects