Events (Alice In Ac.on, Ch 6) Slides Credit: Joel Adams, - - PDF document

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Events (Alice In Ac.on, Ch 6) Slides Credit: Joel Adams, - - PDF document

Feb 21, 2024 203 likes •396 views

CS 101 Lecture 26/27 Events (Alice In Ac.on, Ch 6) Slides Credit: Joel Adams, Alice in Action Objectives Programming to respond to events Create new events in Alice Create handler methods for Alice events

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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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Alice in Action with Java 3

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

Alice in Action with Java 4

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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Alice in Action with Java 5

Events (continued)

Alice in Action with Java 6

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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Alice in Action with Java 7

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

Alice in Action with Java 8

The Right Door (continued)

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Alice in Action with Java 9

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

Alice in Action with Java 10

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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Alice in Action with Java 11

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

Alice in Action with Java 12

The Right Door Revisited (continued)

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Alice in Action with Java 13

Event Handling is Simultaneous

  • Example: left door tells jokes while right door turns
  • Alice handles simultaneous events well

Alice in Action with Java 14

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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Alice in Action with Java 15

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

Alice in Action with Java 16

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

  • f inTheAir and respond accordingly
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Alice in Action with Java 17

Helicopter Flight Simulation

  • 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

Alice in Action with Java 18

Helicopter Flight Simulation

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Alice in Action with Java 19

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

Alice in Action with Java 20

Helicopter Flight Simulation

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Alice in Action with Java 21

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

Alice in Action with Java 22

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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Alice in Action with Java 23

Helicopter Flight Simulation

  • Define go()to handle forward or backward movement

– Logic is similar to logic for turnSlightly()method

Alice in Action with Java 24

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 in Action with Java 25

Alice Tip: Using 3-D Text (continued)

Alice in Action with Java 26

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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Alice in Action with Java 27

Repositioning Text that is Off-Camera (continued)

Alice in Action with Java 28

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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Alice in Action with Java 29

Adding a Background (continued)

Alice in Action with Java 30

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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Alice in Action with Java 31

Making Text Appear or Disappear (continued)

Alice in Action with Java 32

Making Text Appear or Disappear (continued)

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Alice in Action with Java 33

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