(alice’in’ac.on,’ch6) - bu computer science• two steps to making a program respond to an...

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Events (Alice In Ac.on, Ch 6)

Slides Credit: Joel Adams, Alice in Action

CS 101 Lecture 26/27

Alice in Action with Java 2

Objectives

•  Programming to respond to events •  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

•  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 •  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

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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


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


Helicopter Flight Simulation

•  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


Helicopter Flight Simulation •  Connect the left and right arrow keys with the method

–  Depressing arrow key sends a Left or Right argument

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Helicopter Flight Simulation •  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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Making Text Appear or Disappear (continued)


Making Text Appear or Disappear (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

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