Events (2) (Alice In Action, Ch 6)

1. CS 120 Lecture 10 Events (2)(Alice In Action, Ch 6) 28 September 2012 Slides Credit: Joel Adams, Alice in Action

2. Objectives • Create new events in Alice • Create handler methods for Alice events • Use events to build interactive stories 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. 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

5. 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 Alice in Action with Java

6. 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 vehicleproperty to be helicopter Alice in Action with Java

7. 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

8. Helicopter Flight Simulation • Making the helicopter ascend (continued) • Add a When a key is typed event • Change event to While a keyis typed event • Associate the ‘A’ key with the event • Enable the handler to perform while ‘A’ key is pressed Alice in Action with Java

9. Helicopter Flight Simulation Alice in Action with Java

10. 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

11. Helicopter Flight Simulation Alice in Action with Java

12. Helicopter Flight Simulation • Define turnSlightly()to handle left or right turns • helicopterturns only ifinTheAiris 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

13. 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

14. Helicopter Flight Simulation • Define go()to handle forward or backward movement • Logic is similar to logic for turnSlightly()method Alice in Action with Java

15. 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

16. Alice Tip: Using 3-D Text (continued) Alice in Action with Java

17. 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

18. Repositioning Text that is Off-Camera (continued) Alice in Action with Java

19. 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 squareobject (Square is in Shapes Folder) • Resize and reposition squarebehind instructions • Set square.colorproperty to black • Make light turn to face the instructions • Set the square.vehicleproperty to instructions Alice in Action with Java

20. Adding a Background (continued) Alice in Action with Java

21. 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 squareand instructions • Completing the implementation of the handler • Add a When a key is typedevent • Associate the toggle method with the spacebar Alice in Action with Java

22. Making Text Appear or Disappear (continued) Alice in Action with Java

23. Making Text Appear or Disappear (continued) Alice in Action with Java

24. 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 Alice in Action with Java

25. Student To Do’s • Readings: • 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 • Details on Monday in Lab • Can not be dropped • Groups of 1 to 3 people • In-Class Demos on Tuesday Oct 15th