Classes, Objects, and World-level Methods Alice

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

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

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)

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.

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

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…

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

Method Editor Window Elements of Visual Programming Animation Programs

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

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

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

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

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

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

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

Corrected Do Together Elements of Visual Programming Animation Programs

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.

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)

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.

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

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

Demo A demonstration of writing the catchAttention method

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)

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

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"

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