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Methods (part 1) Alice In Action, Ch 2

Slides Credit: Joel Adams, Alice in Action

CS101 Lecture 08

10 July 2013

Objectives

• Build world-level methods to help organize a story into scenes and shots

• Use dummies to reposition the camera for different points of views within a scene

• Understand how an object’s position, orientation, and point of view are described, changed and determined

• Documenting your code with comments.

• Understand Flow of Control with methods.

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Methods • Methods

– behavior-producing messages (from the sender’s view) – behaviors/actions in response to requests, messages

(from the recipient’s view) – E.g. in world.my_first_method: whiteRabbit.pointat(camera)

• Convention for naming methods – Name should be a verb or verb phrase – Name should describe what the method does

• A method is a way to name a block of code.

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Methods • Objects have predefined methods for common tasks • Methods may also be created by Alice developers

– Two main reasons for building your own methods • To provide an object with additional behaviors (Friday) • To organize your story and program into more manageable

pieces (Today)

• Divide and conquer methodology – Break a big problem into smaller problems – Solve each of the smaller problems – Combine the solutions of smaller problems into a

solution for the original, big problem

• Hiding complex details with abstraction.

Alice in Action with Java 4

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World Methods for Scenes and Shots

• User stories can be divided into scenes and shots – Scene: segment of a story, usually set in one location – Shot: part of a scene, normally from one fixed camera view

• Use multiple scenes and shots to create a program that

reflects the user story and has a modular design •

5 Two shots of one scene

Methods for Scenes

• Example: develop a user story with three scenes

• Creating the first new method – Select the world object

– Click the create new method in the details area

– Enter playScene1 in the New Method dialog box

• Check new method by sending say() to ground

– First test fails because my_first_method() is empty

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Alice in Action with Java 7

Methods for Scenes (continued)

Alice in Action with Java 8

Methods for Scenes (continued)

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Methods for Scenes (continued)

• How to fix the first bug – Click on the tab for my_first_method – Drag a doInOrder control to the top of the pane – Click on world in the object tree – Drag playScene1() into the doInOrder

statement

• Extend technique used to build playScene1() – Add two methods: playScene2(), playScene3()

– New method sends a say() message to the ground – New Methods are called in my_first_method()

Alice in Action with Java 9

Alice in Action with Java 10

Methods for Scenes (continued)

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Methods for Scenes (continued)

Methods for Shots

• Example of a scheme using scenes and shots – Level 1: my_first_method()

– Level 2: three methods for three scenes

– Level 3: four methods for four shots in Scene 2

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Methods for Shots (continued)

• Implementing the scheme – Test each shot in Scene 2 using a say() method

– Call the four shot methods from playScene2()

– Call three scene methods from my_first_method()

• Structure diagram reflects organization of user story

• Scene and shot messages are stored in the world

Alice in Action with Java 13

Alice in Action with Java 14

Methods for Shots (continued)

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World and Object Methods

• World method: affects behavior of all objects in a world

• Object method: defines behavior for a single object (that may have multiple parts)

– examples: flapWings()for dragon, hop() for a rabbit…

Alice in Action with Java 15

Program Documentation

• Standalone readme, manual…

• Comments: explanatory remark ignored by Alice – an integral part of code

– Used to describe what code does at various levels • the overall program, individual methods, blocks of

statements….

– Useful for collaborators and developers themselves

– Important part of programming • Also a component evaluated for your program grades

Alice in Action with Java 16

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Alice Tip: Using Dummies

• Review – Scenes comprise shots

– Shots are filmed with the camera in a given position

– Alice places a camera object in every world

• Two techniques for shifting position of camera – Use set of motion-related messages, such as move()

– Use an invisible marker called a dummy

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Dummies

• Dummy: invisible marker with a point of view

• Dummies are used to change a camera’s position

• Description of a scene that will use dummies – Wizard intervenes to prevent trolls from taking a castle

– Camera changes position for each of three shots

– Story conforms to structure in Figure 2-11 (less Shot 4)

• Setting up the first shot of Scene 2 – Add castle, wizard, and trolls to build the scene

– Click more controls button and then drop a dummy

– Go to object tree and rename dummy scene2Shot1

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Dummies (continued)

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Dummies (continued)

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Dummies (continued)

• Setting up the second shot of Scene 2

– Using camera controls, zoom in on the wizard

– Press the drop dummy at camera button

– Rename the second dummy, scene2Shot2

• Setting up the third shot of Scene 3

– First dummy will be reused for this shot

• After dummies are inserted they will be programmed

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Alice in Action with Java 22

Dummies (continued)

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Using setPointOfView() to Control the Camera

• obj.setPointOfView(obj2)

– Changes the position of obj to obj2

– Example: camera.setPointOfView(aDummy)

• Adding code to the first shot of Scene 2 – Drag a doInOrder statement to the editing area

– Click on camera object in the object tree

– Drag setPointOfView()to the editing area • Select scene2Shot1 dummy as target and 0 duration

– Add say() statements for each of the trolls

– Add a comment to explain the purpose of the method

Alice in Action with Java 23

Alice in Action with Java 24

Using setPointOfView() to Control

the Camera (continued)

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Alice in Action with Java 25

Using setPointOfView() to Control

the Camera (continued)

Using setPointOfView() to Control the Camera (continued)

• Adding code to the second shot of Scene 2 – Set the opacity of the wizard to 0 in properties

pane – Drag wizard’s opacity property to editing area

• Set the opacity to 1 in the set() method

– Set the camera’s point of view to scene2Shot2 – Add a say() statement for the wizard

• Adding code to the third shot of Scene 2 – Reset camera’s point of view to scene2Shot1 – Point the three trolls at the wizard

• Set message’s onlyAffectYaw attribute to true

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Using setPointOfView() to Control

the Camera (continued)

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Using setPointOfView() to Control

the Camera (continued)

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Summary • Divide and conquer approach: decomposing a user

story into scenes and shots • Define methods to support modular design and

provide advanced operations • World methods: messages sent to the world • Comments: remarks that explain program

statements • Flow of Control: How methods modify sequential

execution. • Dummy: invisible marker with position and

orientation (a point of view)

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