CS320n

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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.
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What We Will Do Today

• Work on more class level methods
• Learn about built in functions
• Learn to use expressions
• Review the asSeenBy option

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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

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Bad Example 1
A class level method should not call world-level
methods.
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Bad Example 2
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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?

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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
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Parameterized Version of skateAround

• Making the object to skate around a parameter now
  removes the tie to the penguin in this world

calling skateAround
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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?

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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

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Built-in Functions. (a.k.a. questions)

• The built-in function, distance to can be used to
  determine the distance the skater must move.

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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 -gt target
• call to move looks like this now

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Problem?

• What will happen when the skateAround method is
  called?

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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

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Expressions

• We dont 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

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Creating an Expression
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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

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Result of Expression
Stops before running over penguin
And skates around
What if alreadyless than 2 metersfrom an object?
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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.

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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

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Circling Gunfighters
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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.