Event loops Jan 14 2019 Programming in prehistoric times Earliest programs were all batch processing There was no interaction with the user Input Process Output 2 Very early interactive programs BASIC was an early interactive language Still a central computer with terminals Style of interaction was filling out forms Ask user for input Process input Ask user for input Process input etc 3 Command 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 programs Ask user for command Read and parse command Execute command quit 4 Modern event driven programs Multiple sources of input mouse clicks keyboard timers external events A new program structure is required Wait for event Dispatch event Quit 5 Java 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 parameter 6 Building 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 7 Vocabulary 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 8 Vocabulary 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 occurs 9 The End 10
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