1. CS320n –Visual Programming
Class-level Methods and Inheritance – Part 2
Mike Scott
(Slides 4-3-2)
Thanks to Wanda Dann, Steve Cooper, and Susan Rodger for slides.

2. Class Level Methods - Part 2
What We Will Do Today
Work on more class level methods
Learn about built in functions
Learn to use expressions
Review the asSeenBy option
Visual Programming
Class Level Methods - Part 2

3. Guidelines for Class-Level Methods
Should a method be a class-level method or a world-level method?
For class level methods
avoid hard coded references to other objects
avoid calls to world level methods
Play a sound only if has been imported and saved as part of the new class. (sounds can be imported to world or individual objects)
If these guidelines are not followed and an instance of a class is added to another world, an error message is generated
Visual Programming
Class Level Methods - Part 2

4. Class Level Methods - Part 2
Bad Example 1
A class level method should not call world-level methods.
Visual Programming
Class Level Methods - Part 2

5. Class Level Methods - Part 2
Bad Example 2
Visual Programming
Class Level Methods - Part 2

6. But I want to skate around things
Is it reasonable to want a class-level method for the advanced skater where she skates around things?
In this particular case, a penguin?
Visual Programming
Class Level Methods - Part 2

7. Class Level Methods - Part 2
Solution
A solution is to write a class-level method with an object parameter that allows you to pass in the specific object.
cleverSkater.skateAround
Parameter: whichObject
Do in order
Do together
cleverSkater turn to face whichObject
cleverSkater lift right leg
cleverSkater move to whichObject
cleverSkater turn around whichObject
Visual Programming
Class Level Methods - Part 2

8. Parameterized Version of skateAround
Making the object to skate around a parameter now removes the tie to the penguin in this world
calling skateAround
Visual Programming
Class Level Methods - Part 2

9. Translating design into code
Most of the skateAround storyboard design is straightforward and easy to code.
The last two steps, however, require some thought:
advancedSkater move to target --
What distance should the cleverSkater move? advancedSkater turn around target --
how do we tell cleverSkater to turn (in a circle)
around another object?
Visual Programming
Class Level Methods - Part 2

10. Class Level Methods - Part 2
The distance to move
We can work out how far to move the skater based on initial position of objects and a little experimentation
Pick a distance, play the movie, more or less, refine until it looks good
Is this a very general solution though?
only works for one initial set up
look to make it work for many set ups
Visual Programming
Class Level Methods - Part 2

11. Built-in Functions. (a.k.a. questions)
The built-in function, distance to can be used to determine the distance the skater must move.
Visual Programming
Class Level Methods - Part 2

12. Class Level Methods - Part 2
Calling the Function
drag and drop the function advancedSkater distance to from the function menu to the distance portion of the move method and pick expressions -> target
call to move looks like this now
Visual Programming
Class Level Methods - Part 2

13. Class Level Methods - Part 2
Problem?
What will happen when the skateAround method is called?
Visual Programming
Class Level Methods - Part 2

14. Class Level Methods - Part 2
Too Much
Skater intersects penguin
told to move forward the distance to the target
moves skater all the way so centers of objects coincide
distance measured center to center
Visual Programming
Class Level Methods - Part 2

15. Class Level Methods - Part 2
Expressions
We don’t want the skater to move all the way to the target
to avoid collision use math operator and create an expression that moves to within 2 meters
Math operators is Alice: +, -, *, /
Example:
Visual Programming
Class Level Methods - Part 2

16. Creating an Expression1 3 2 4
Visual Programming
Class Level Methods - Part 2

17. Class Level Methods - Part 2
More on Expressions
Creating expressions with the pop up menus can be a pain
In this case, text based programming languages may be easier to use
Visual Programming
Class Level Methods - Part 2

18. Class Level Methods - Part 2
Result of Expression
Stops before running over penguin
And skates around
What if already less than 2 meters from an object?
Visual Programming
Class Level Methods - Part 2

19. Class Level Methods - Part 2
asSeenBy
To have the skater skate around another object, pass whichObject as an argument to the asSeenBy parameter in a turn instruction.
yes, passing a parameter to another method as a parameter
For more details on asSeenBy, review Tips & Techniques 2.
Visual Programming
Class Level Methods - Part 2

20. Class Level Methods - Part 2
More on AsSeenBy
Use invisible objects or dummy objects (isShowing property set to false) to have objects fly around in circles
Example: Have gunfighters circle each other
Visual Programming
Class Level Methods - Part 2

21. Class Level Methods - Part 2
Circling Gunfighters
Visual Programming
Class Level Methods - Part 2

22. Class Level Methods - Part 2
Testing
Each time you create a new class, test it:
Add an instance of the new class to a new world.
Write a short testing program to test each new method.
Testing increases your confidence in the ability to reuse your code in other worlds.
Visual Programming
Class Level Methods - Part 2