Events (Alice In Action, Ch 6) 25 September 2012 Slides Credit: - PDF document

CS 120 Lecture 09 Events (Alice In Action, Ch 6) 25 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 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 • 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 • 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 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 More on Events • 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 • 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 Alice in Action with Java 14 7

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 15 Student To Do’s • Readings: – Alice in Action, Chapter 6 • Homework posted on Web – Demo next Monday in Lab 16 8