Review of Chapter 4Sections 1 and 2

World-level methodsinvolve two or more objects

break a large problem into smaller, logical units

follow a design process called stepwise refinement

make the program more organized, easier to edit and debug, and allow the re-using of code (by calling the method more than once)

Parameters

Allow methods to be “generic” i.e. reusable with different values

Types areNumber

Boolean

Object

Other (such as sound or color)

Parameters

The value to be used by the method is sent via an argument in the method call

The “calling” method sends the specific value

The method executes its code using that value

Class-level Methods and Inheritance

Chapter 4, Section 3

Class-level Methods Some actions are naturally associated with a specific class of objects.

Examples A person walking

A wheel rolling

We can write our own methods to define an action for a specific class of objects -- a class-level method.

An example (building technique)

How can we create a skate method for ice skater objects?

We need to:

(1) tell Alice to associate the new method (the one we are about to write) with an ice skater, and

(2) write a new method to animate the ice skater skating.

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 panel

•click on the create new method button

Storyboard for skate

The slide actions each require several motion instructions, so we will break down these two actions into smaller steps

Skate:Do together move skater forward 2 meters Do in order slide on left leg slide on right leg

Stepwise Refinement

Refinement of slideLeftDo 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 slideRightDo in order Lift left leg and turn upper body forward Lower left leg and return body upright

Demo

Ch04Lec3Skater

Concepts illustrated in this example world A method defined for a specific type of object defines an action for that object.

A method can call other methods.

In this example, the skate method calls slideRight and slideLeft.

Reuse

Writing methods to make an ice skater perform a skating motion is an intricate task. We would like to have the iceSkater skate in other worlds without having to write the methods again.The idea of being able to use previously written program code in another program is known as reuse.

A new class1) Rename iceSkater as cleverSkater.

2) Save out as a new class. Alice saves the new class as CleverSkater.a2c

Inheritance

The CleverSkater class inherits all the properties and methods from the original IceSkater class, and also

has the newly defined methods (skate, slideLeft, slideRight)

In other programming languages, the concept of creating a new class based on a previously defined class is called inheritance.

Importing CleverSkater

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

Guidelines

To avoid potential misuse of class-level methods, follow these guidelines:

Avoid references to other objects

Avoid calls to world-level methods

Play a sound only if the sound has been imported and saved out as part of the new class

If these guidelines are not followed and an instance of the new class is added to another world, Alice will open an Error dialog box to tell you something is wrong.

Bad Example

What if there is no penguin in the new world where a cleverSkater object is imported?

Problem

Suppose you really want to write a class-level method where another object is involved?

For example, a method to make the skater skate around another object-- in this scene, the penguin.

Parameter

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

Translation to Code

Most of the skateAround storyboard design is straightforward and easy to code.One step, however, requires some thought:

cleverSkater move to whichObject -- what distance should the cleverSkater move?

Calling a built-in function

The instruction to move the skater to whichObject (penguin, in this example) would look like this:

Unfortunately, the skater will collide with the penguin because the distance between two objects is measured center-to-center.

Expression

To avoid a collision, use a math operator to create an expression that adjusts the distance.

Math operators in Alice: addition + subtraction multiplication * division /

Example:

Demo

Ch04Lec3SkateAround

Concepts illustrated: A parameter acts as a placeholder for the object that will be passed in

A call to the distance to function returns a number value

A math expression can be created as part of an instruction

Tips & Techniques 4Visible and Invisible Objects

Opacity

Opacity is a measure of how "see-through" an image or an object is.

The less opaque an object is, the more see-through it is.

Opacity of 100%, cannot see through the object

Opacity of 0%, object is transparent (like clear glass)

In Alice, an object with an opacity of 0% is invisible.

Changing the Opacity

A change in opacity can be used to simulate real world conditions.

Example A fish swimming away from the camera should fade away because water blurs our vision of distant objects.

Demo

Ch04Lec4LilfishOpacity

Concepts illustrated in this example As opacity is gradually decreased, the fish becomes less visible.

At an opacity of 0%, the object is still in the world (can still see it listed in the object tree) but is effectively invisible.

isShowing

The isShowing property has a Boolean value, either true or false.

Setting an object's isShowing property to false makes the object invisible in the world.

Once again, the object is not removed from the world, it is simply not visible in the world.

Demo

Ch04Lec4ChickenIsShowing

Concepts illustrated in this example A billboard can be used to display a title screen for the animation.

A 2-second wait gives the user time to read the billboard.

Changing the isShowing property to false makes the title screen disappear.

Two properties

isShowing and opacity are two different properties.

isShowing works like an on/off switch

opacity works like a dimmer switch

Although each can be used to make an object invisible, changing one does not automatically change the value of the other!

Practical Uses of Invisible Objects

An invisible object is sometimes useful as a stationary marker that creates

a target for a move to instruction

an external reference point for object rotational motion

Demos

Ch04Lec4InvisibleTarget Invisible circle makes it possible to move the object to a landing target.

Ch04Lec4InvisibleReferencePoint External reference point acts as a pivot for the rotational movement of each object.

Assignment

Chapter 4, Section 3 Lab

For next time:

Read Chapter 5, Section 1

Create storyboard for Project 2