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Penn CIT 591 - Event loops

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Event loopsProgramming in prehistoric timesVery early interactive programsCommand-driven programs (20 years ago)Modern event-driven programsJava hides the event loopBuilding a GUIVocabulary IVocabulary IIThe EndJan 14, 2019Event loops2Programming in prehistoric timesEarliest programs were all “batch” processingThere was no interaction with the userInputOutputProcess3Very early interactive programsBASIC was an early interactive languageStill a central computer, with terminalsStyle of interaction was “filling out forms”Process inputAsk user for inputAsk user for inputProcess inputetc.4Command-driven programs(20 years ago)Allow the user to enter “commands” Much more flexibleStill only a single source of inputsNot good enough for modern programsAsk user for commandRead and parsecommandExecute commandquit5Modern event-driven programsMultiple sources of inputmouse clickskeyboardtimersexternal eventsA new program structure is requiredWait for eventDispatch eventQuit6Java hides the event loopThe event loop is built into Java GUIsGUI stands for Graphical User InterfaceInteracting with a GUI component (such as a button) causes an event to occurAn Event is an objectYou create Listeners for interesting eventsThe Listener gets the Event as a parameter7Building a GUITo build a GUI in Java,Create some ComponentsUse a layout manager to arrange the Components in a windowAdd Listeners, usually one per ComponentPut code in the Listeners to do whatever it is you want doneThat's it!Of course, there are a lot of details....8Vocabulary IEvent – an object representing an external happening that can be observed by the programevent-driven programming – A style of programming where the main thing the program does is respond to Eventsevent loop – a loop that waits for an Event to occur, then dispatches it to the appropriate codeGUI – a Graphical User Interface (user interacts with the program via things on the screen)9Vocabulary IIComponent – an interface element, such as a Button or a TextFieldLayout Manager – an object (provided by Java) that arranges your Components in a windowListener – an object you create to execute some code appropriate when an Event occurs10The


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