(alice in action, ch 6) - computer science(alice in action, ch 6) slides credit: joel adams, alice...

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Events (Alice In Action, Ch 6)

Slides Credit: Joel Adams, Alice in Action

CS 101 Lecture 27

31 July 2013

Alice in Action with Java 2

Objectives

• Create new events in Alice

• Create handler methods for Alice events

• Use events to build interactive stories

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Events

• Event: action generated by a user or a program

– Ex: clicking the Play button will generate “When the

world starts” event

• Interactive programs, such as games, are event-driven

• Two steps to making a program respond to an event

– Choose or define a method to handle the event

• Such a method is called an event handler

• Define an event handler when responsive behavior is

complex

– Tell Alice to invoke the method when the event occurs


More on Events

• Conflicts can arise when coding parallel event logic

– Example: two handlers modify the same property

• How to avoid conflict

– Ensure that handlers modify a property in sequence

• Event categories

– Mouse event: triggered by mouse movement or click

– Keyboard event: triggered when user presses a key

– Program event: triggered when variable value

changes

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Events (continued)


Handling Mouse Clicks: The Magical Doors

• Castle door tells random knock-knock jokes

• Events that will be added to the original program

– Right door opens when the user clicks it

– Left door tells knock-knock joke when the user clicks it

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The Right Door

• First step: stop door from automatically telling jokes by

making world.my first Method do nothing

• Recall the two steps to handle an event

– Use a predefined method or define a new method

– Create an event that invokes the handler

• Enabling the right door to respond to a mouse event

– The door open/close motion feels simple let us

choose turn() to handle the event

– Add When the mouse is clicked on something

– Specify castle1.door as the event source


The Right Door (continued)

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The Left Door

• Enabling the left door to respond to a mouse event

– Specify castle.door2 as the source of this event

– Drag-and-drop world’s random joke method as handler

• Test the program by clicking each door


The Right Door Revisited

• Logic error: right door continues to turn with user click

• Right door should open if closed, and close if open

– i.e. exhibiting the two-state behavior

• General approach for dealing with multi-state behavior

– Add an object property variable to store the object state

– In object methods, use control flows such as the if

statement to generate appropriate actions and perhaps

also change the object state value afterwards

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The Right Door Revisited

• Fix: implement two-state behavior for right door

– Add Boolean property rightDoorClosed to castle

– Replacement handler: openOrCloseRightDoor()

– Build turn logic around the value of rightDoorClosed


The Right Door Revisited (continued)

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Event Handling is Simultaneous

• Example: left door tells jokes while right door turns

• Alice handles simultaneous events well


More on Events

• Conflicts can arise when coding parallel event logic

– Example: two handlers modify the same property

• How to avoid conflict

– Ensure that handlers modify a property in sequence

• Event categories

– Mouse event: triggered by mouse movement or click

– Keyboard event: triggered when user presses a key

– Program event: triggered when variable value

changes

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Handling Key Presses: A Helicopter

Flight Simulator

• The problem

– The mayor’s cat is lost and city government has halted

– You need to use your helicopter to find the cat

– Search begins at an airport outside the city


Event Design

• Six keys for six types of helicopter movement

– ‘a’ key: ascend

– ‘d’ key: descend

– Up arrow key: move forward

– Down arrow key: move backward

– Left arrow key: turn left

– Right arrow key: turn right

• Keys are chosen for two reasons

– Convenient positions and mnemonic values

• Choosing correct keys improves usability

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Helicopter Flight Simulation

• Plan the world for a helicopter simulation

– Add airport, helicopter, city terrain, buildings, and a cat

– Position the helicopter at the airport

– Position camera to be peering out in front of helicopter

– Set the camera’s vehicle property to be helicopter



• Event handling

– Observation: feasible helicopter motion

depends on its status in air

• E.g. it can only descend, turn L/R, or

move F/B, when it is in air

– Define Boolean property inTheAir for helicopter

– In event handler methods, check the value

of inTheAir and respond accordingly

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• Making the helicopter ascend (continued)

– Add a When a key is typed event

– Change event to While a key is typed event

– Associate the ‘A’ key with the event

– Enable the handler to perform while ‘A’ key is pressed



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Helicopter Flight Simulation • Making the helicopter descend

– Define descend()method for helicopter

• Method logic mirrors the logic of ascend()

• Note that the ground provides a floor for the descent

– Enable descend()to perform while ‘D’ key is pressed



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Helicopter Flight Simulation • Define turnSlightly()to handle left or right turns

– helicopter turns only if inTheAir is true

– turnSlightly()takes a Left or Right argument

– Arrow keys are associated with the method

– Depressing arrow key sends a Left or Right argument



• Connect the left and right arrow keys with the method

– Depressing arrow key sends a Left or Right argument

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• Define go()to handle forward or backward movement

– Logic is similar to logic for turnSlightly()method


Alice Tip: Using 3-D Text

• Helicopter simulator should have flight instructions

• Solution: add 3-D text that explains the interface

• Creating 3-D text for the simulator

– Return to the Add Objects screen

– Click Create 3D Text (at far end of Local Gallery)

– Add flight instructions in the text box

– Click OK

– Rename the text object, instructions

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Alice Tip: Using 3-D Text (continued)


Repositioning Text that is Off-Camera

• Right-click instructions to access methods

• Three settings for positioning text in front of camera

– Choose methods->

setPointOfView(<asSeenBy)->camera

– Choose methods->move(<direction>,<amount>)

->FORWARD->10 meters

– Choose methods->turn<direction>,<amount>)

->LEFT->1/2 revolution

• Linking text to camera and changing text color

– Set instructions.vehicle to camera

– Set instructions.color to yellow

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Repositioning Text that is Off-Camera

(continued)


Adding a Background

• Text for instructions is difficult to read

• Solution: provide a background for the text

• One way to add a background to instructions

– Insert square object (Square is in Shapes Folder)

– Resize and reposition square behind instructions

– Set square.color property to black

– Make light turn to face the instructions

– Set the square.vehicle property to instructions

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Adding a Background (continued)


Making Text Appear or Disappear

• Another example of two-state behavior

• Implementation strategy

– Add handler to switch value of isShowing property

• Logic of toggleInstructionVisibility()

– Negate the value of isShowing property

– Apply negation to both square and instructions

• Completing the implementation of the handler

– Add a When a key is typed event

– Associate the toggle method with the spacebar

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(continued)



(continued)

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Summary

• Event: action generated by a user or a program

• User events: keyboard events and mouse events

• Event handler: method called in response to an

event

• Event-driven program: program directed by events

and handlers

• Two-state behavior: pattern that switches the state of

an object

Student To Do’s • Readings:

– Alice in Action, Chapter 6

• Alice HW due Thursday

• Homework HTML Frames/CSS and Photoshop

– Due Monday

• Alice Project – Can not be dropped. 10% of class grade.

– Due Next Wednesday – DEMO IN CLASS! (No late submissions)

– Details on web (tonight)

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(alice in action, ch 6) - computer science(alice in action, ch 6) slides credit: joel adams, alice...

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