Methods (part 2) Alice In Ac5on, Ch 2

Transcription

1 CS101 Lecture 18 Methods (part 2) Alice In Ac5on, Ch 2 30 October 2012 Slides Credit: Joel Adams, Alice in Action With thanks to John Magee for his guidance about these materials Objec5ves Build world- level methods to help organize a story into scenes and shots Use dummies to reposi5on the camera for different points of views within a scene Understand how an object s posi%on, orienta%on, and point of view are described, changed and determined Documen5ng your code with comments. Understand Flow of Control with methods. 2 1

2 Methods Methods behavior- producing messages (from the sender s view) behaviors/ac5ons in response to requests, messages (from the recipient s view) E.g. in world.my_first_method: whiterabbit.pointat(camera) Conven5on 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. 3 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 addi5onal behaviors (last week) 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 solu5ons of smaller problems into a solu5on for the original, big problem Hiding complex details with abstrac5on. Alice in Action with Java 4 2

3 World Methods for Scenes and Shots User stories can be divided into scenes and shots Scene: segment of a story, usually set in one loca5on Shot: part of a scene, normally from one fixed camera view Use mul5ple scenes and shots to create a program that reflects the user story and has a modular design Two shots of one scene 5 Methods for Scenes Example: develop a user story with three scenes Crea5ng 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 6 3

4 Methods for Scenes (continued) Alice in Action with Java 7 Methods for Scenes (continued) Alice in Action with Java 8 4

5 Methods for Scenes (con5nued) 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 Methods for Scenes (continued) Alice in Action with Java 10 5

6 Methods for Scenes (continued) 11 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

7 Methods for Shots (con5nued) Implemen5ng 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 organiza5on of user story Scene and shot messages are stored in the world Alice in Action with Java 13 Methods for Shots (continued) Alice in Action with Java 14 7

8 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 mul5ple parts) examples: flapwings()for dragon, hop() for a rabbit Alice in Action with Java 15 Program Documenta5on 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 8

9 Alice Tip: Using Dummies Review Scenes comprise shots Shots are filmed with the camera in a given posi5on Alice places a camera object in every world Two techniques for shi_ing posi5on of camera Use set of mo5on- related messages, such as move () Use an invisible marker called a dummy 17 Dummies Dummy: invisible marker with a point of view Dummies are used to change a camera s posi5on Descrip5on of a scene that will use dummies Wizard intervenes to prevent trolls from taking a castle Camera changes posi5on for each of three shots Story conforms to structure in Figure 2-11 (less Shot 4) Seang up the first shot of Scene 2 Add castle, wizard, and trolls to build the scene Click more controls bubon and then drop a dummy Go to object tree and rename dummy scene2shot1 18 9

10 Dummies (continued) 19 Dummies (continued) 20 10

11 Dummies (con5nued) Seang up the second shot of Scene 2 Using camera controls, zoom in on the wizard Press the drop dummy at camera bubon Rename the second dummy, scene2shot2 Seang up the third shot of Scene 3 First dummy will be reused for this shot A_er dummies are inserted they will be programmed 21 Dummies (continued) Alice in Action with Java 22 11

12 Using setpointofview() to Control the Camera obj.setpointofview(obj2) Changes the posi5on of obj to obj2 Example: camera.setpointofview(adummy) Adding code to the first shot of Scene 2 Drag a doinorder statement to the edi5ng area Click on camera object in the object tree Drag setpointofview()to the edi5ng 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 Using setpointofview() to Control the Camera (continued) Alice in Action with Java 24 12

13 Using setpointofview() to Control the Camera (continued) Alice in Action with Java 25 Using setpointofview() to Control the Camera (con5nued) Adding code to the second shot of Scene 2 Set the opacity of the wizard to 0 in proper5es pane Drag wizard s opacity property to edi5ng 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 abribute to true 26 13

14 Using setpointofview() to Control the Camera (continued) 27 Using setpointofview() to Control the Camera (continued) 28 14

15 Summary Divide and conquer approach: decomposing a user story into scenes and shots Define methods to support modular design and provide advanced opera5ons World methods: messages sent to the world Comments: remarks that explain program statements Flow of Control: How methods modify sequen5al execu5on. Dummy: invisible marker with posi5on and orienta5on (a point of view) 29 15