CS 120 Lecture 10
Events (2)
(Alice In Action, Ch 6)
28 September 2012
Slides Credit: Joel Adams, Alice in Action
Objectives
• Create new events in Alice
• Create handler methods for Alice events
• Use events to build interactive stories
Alice in Action with Java
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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
Alice in Action with Java
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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
Alice in Action with Java
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Event Design
• Six keys for six types of helicopter movement
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‘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
Alice in Action with Java
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Helicopter Flight Simulation
• Plan the world for a helicopter simulation
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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
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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
Alice in Action with Java
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Helicopter Flight Simulation
• Making the helicopter ascend (continued)
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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
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Helicopter Flight Simulation
Alice in Action with Java
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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
Alice in Action with Java
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Helicopter Flight Simulation
Alice in Action with Java
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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
Alice in Action with Java
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Helicopter Flight Simulation
• Connect the left and right arrow keys with the method
– Depressing arrow key sends a Left or Right argument
Alice in Action with Java
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Helicopter Flight Simulation
• Define go()to handle forward or backward movement
– Logic is similar to logic for turnSlightly()method
Alice in Action with Java
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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
Alice in Action with Java
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Alice Tip: Using 3-D Text (continued)
Alice in Action with Java
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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
Alice in Action with Java
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Repositioning Text that is Off-Camera
(continued)
Alice in Action with Java
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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
Alice in Action with Java
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Adding a Background (continued)
Alice in Action with Java
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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
Alice in Action with Java
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Making Text Appear or Disappear
(continued)
Alice in Action with Java
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Making Text Appear or Disappear
(continued)
Alice in Action with Java
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Summary
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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
Alice in Action with Java
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• Readings:
Student To Do’s
– Alice in Action, Chapter 6
• Homework Due Monday @ Lab Session
• Midterm: Friday, Oct 12th on material through today
• We will start Java next week.
– Make sure you have the Java textbook.
• Alice Project
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Details on Monday in Lab
Can not be dropped
Groups of 1 to 3 people
In-Class Demos on Tuesday Oct 15th
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