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 Use events to build interactive stories Alice in Action with Java 2 1
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 3 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 Alice in Action with Java 4 2
Events (continued) Alice in Action with Java 5 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 Alice in Action with Java 6 3
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 7 The Right Door (continued) Alice in Action with Java 8 4
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 9 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 Alice in Action with Java 10 5
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 11 The Right Door Revisited (continued) Alice in Action with Java 12 6
Event Handling is Simultaneous Example: left door tells jokes while right door turns Alice handles simultaneous events well Alice in Action with Java 13 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 14 7
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 15 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 16 8
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 17 Helicopter Flight Simulation Alice in Action with Java 18 9
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 19 Helicopter Flight Simulation Alice in Action with Java 20 10
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 21 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 22 11
Helicopter Flight Simulation Define go()to handle forward or backward movement Logic is similar to logic for turnslightly()method Alice in Action with Java 23 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 24 12
Alice Tip: Using 3-D Text (continued) Alice in Action with Java 25 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 26 13
Repositioning Text that is Off-Camera (continued) Alice in Action with Java 27 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 28 14
Adding a Background (continued) Alice in Action with Java 29 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 30 15
Making Text Appear or Disappear (continued) Alice in Action with Java 31 Making Text Appear or Disappear (continued) Alice in Action with Java 32 16
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 33 17