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CS320n –Visual Programming

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. What We Will Do Today. Work on more class level methods Learn about built in functions Learn to use expressions

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CS320n –Visual Programming

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

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

  5. Bad Example 2 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? Class Level Methods - Part 2

  7. 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 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 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? Class Level Methods - Part 2

  10. 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 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. Class Level Methods - Part 2

  12. 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 Class Level Methods - Part 2

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

  14. 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 Class Level Methods - Part 2

  15. 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: Class Level Methods - Part 2

  16. Creating an Expression 1 3 2 4 Class Level Methods - Part 2

  17. 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 Class Level Methods - Part 2

  18. Result of Expression Stops before running over penguin And skates around What if alreadyless than 2 metersfrom an object? Class Level Methods - Part 2

  19. 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. Class Level Methods - Part 2

  20. 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 Class Level Methods - Part 2

  21. Circling Gunfighters Class Level Methods - Part 2

  22. 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. Class Level Methods - Part 2

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