Alice in Action with Java Chapter 6 Events

Alice in Action with Java2 Objectives Create new events in Alice Create handler methods for Alice events Use events to build interactive stories

Alice in Action with Java3 Events An interactive program processes data input by a user Events are actions generated by a user or a program –Ex: When the world starts caused by Play click Two steps to making a program respond to an event –Choose (or define) a method to handle the event –Tell Alice to invoke the method when the event occurs An event handler is a method called in response to an event An event-driven program is based on events and handlers

Alice in Action with Java4 Types of Events

Alice in Action with Java5 Handling Mouse Clicks: The Magical Doors Goal: add events to world built in Section –Review: 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 First step: stop door from automatically telling jokes

Alice in Action with Java6 The Right Door How to handle a mouse event –Choose or define a method Define a method when responsive behavior is complex –Create an event that invokes the handler Enabling the right door to respond to a mouse event –Add When the mouse is clicked on something –Specify castle.door1 as the event source –Use a turn() message to handle the event

Alice in Action with Java7 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 Java8 The Right Door Revisited Two-state behavior –Describes one of two states that an object can occupy –The state is determined by the value of a property –An if statement controls switching between states Logic error: right door continues to turn with user click Right door should open if closed, and close if open 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 Java9 Event Handling is Simultaneous Example: left door tells jokes while right door turns Alice handles simultaneous events well 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 Java10 Categorizing Events Mouse event: triggered by mouse movement or click Keyboard event: triggered when user presses a key Program event: triggered when variable value changes

Alice in Action with Java11 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 Java12 Design Six keys for six types of 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 position and mnemonic values Choosing correct keys improves usability

Alice in Action with Java13 Programming in Alice Setting up 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 Making the helicopter’s propeller spin –Use heli blade() to handle the press of Play button Making the helicopter ascend –Define Boolean property inTheAir for helicopter –Define helicopter. ascend()

Alice in Action with Java14 Programming in Alice (continued) 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 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 Java15 Programming in Alice Let arrow keys move event –Not appropriate for controlling helicopter movement –helicopter would move while on the ground Define turnSlightly() to handle left or right turns –turnSlightly() takes a Left or Right argument –helicopter turns only if inTheAir is true –Arrow keys are associated with the method –Depressing arrow key sends a Left or Right argument Define go() to handle forward or backward movement –Logic is similar to logic for turnSlightly() method

Alice in Action with Java16 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 Java17 Repositioning Text that is Off-Camera Right-click instructions to access methods Three settings for positioning text in front of camera –Choose methods-> setPointOfView( camera –Choose methods->move(, ) ->FORWARD->10 meters –Choose methods->turn, ) ->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 Java18 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 Java19 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 Java20 Making Text Appear or Disappear (continued)

Alice in Action with Java21 Alice Tip: Scene Transitional Effects for the Camera Transition: bridge between two scenes (or shots) Some transition effects –Cut: instantaneous jump from one scene to another –Fade: darken existing scene, light another –Wipe: cover existing scene, uncover new scene Alice does not provide built-in transitional effects Alice does provide building blocks for transitions

Alice in Action with Java22 Setup for Special Effects Creating shutters for your camera –Add four square s outside of the viewport –Change names to indicate shutter positions –Change each square color to black –Change each square.vehicle to camera –Move the shutters outside of the viewing area Shutters will be manipulated to create effects

Alice in Action with Java23 Setup for Special Effects (continued)

Alice in Action with Java24 The Fade Effect A fade effect darkens and then lightens a screen Create effect using complementary methods Overview of implementing fadeToBlack() –Set the topShutter ’s opacity to zero percent –Move the topShutter to cover the camera viewport –Set topShutter opacity back to 100 percent Overview of implementing fadeFromBlack() –Reverse the actions of fadeToBlack() Use fadeToBlack() and fadeFromBlack() in pairs –The scene changes between method calls

Alice in Action with Java25 The Barndoor Edge Wipe Effect Edge wipe: edges cover one scene, reveal another Barndoor wipe: shutters slide like doors of a barn –Vertical: doors close and open from sides of screen –Horizontal: doors close and open from top and bottom Create barndoor wipes with complementary methods Implementing verticalBarndoorClose() –Move the left and right shutters into the viewing area Implementing verticalBarndoorOpen() –Move the left and right shutters out of the viewing area

Alice in Action with Java26 The Box Iris Wipe Effect Iris contains the open area of the screen End one scene by shrinking the iris As iris expands, the new scene is revealed Defining the box iris close effect –Simultaneously move all four shutters onto viewport –Sender specifies duration of the effect –Sender specifies percentage the iris is closed One parameter and two local variables are needed Box iris open effect is complementary to close effect

Alice in Action with Java27 Reusing Transition Effects Limitations of save object as… technique –Cannot be used to save properties as objects –Consequence: camera cannot be saved with shutters Solution: define transitions in a template world Contents of template (named TransitionEffects ) –camera, light, ground, and square s Using TransitionEffects template world –Open template as starting world for user story –Use File->Save world as… to save and rename

Alice in Action with Java28 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 Java29 Summary (continued) Add 3D Text dialog box: tool for inserting text square : convenient shape for creating a text background and transition effects Transition effect: bridge between two scenes Types of transition effects: cut, fade, and wipe A transition effects template world includes ground, light, camera, and effects objects