Event driventheory
- 1. Unit 14 – Event Driven Programming What is event driven programming? 1
- 2. Key Features of EDP 1. Forms & Controls 2. Trigger functions 3. Event handlers 4. Event loops 2
- 3. Forms and Controls A GUI (graphical user interface) is what the user sees when the program runs and usually consists of one or several forms Each form contains various controls which allow the user to interact with the program in different ways e.g. Textboxes, Menus, Buttons Each type of control has its own events e.g. a Form events include: Load (open for first time, set default values) KeyDown (when the user presses a key) 3
- 4. Load Form 4
- 5. Trigger functions Event driven programs respond to events triggered by the user via the mouse or keyboard Common user events include clicking the mouse, pressing a key, changing text Different controls have different events (events for a text box not the same for a button) The programmer chooses which event(s) to respond to by selecting the appropriate trigger function. 5
- 6. Button Events Event driven programs respond to events triggered by the user Every object has a range of trigger functions – one for each possible event A Button has a Click event, a GotFocus event, a MouseHover event etc 6
- 7. Event Handlers An event triggers the appropriate event handler Event handlers contain the code that runs when an event occurs These event handlers are self-contained sections of code – also called Procedures or Subroutines VB.NET uses Sub /End Sub to start and end the code This allows the programmer to work on one event of one control at a time and makes testing easier 7
- 8. Event Loops Programmers not normally aware of these The Event loop keeps testing the user interface to see if anything has happened (e.g. clicking on a button or pressing a key) When detected, the event is passed to the trigger function which then finds the appropriate event handler to run the code written for this purpose Other (non user) activities can also cause an event to trigger – typically used in networking and when reading from or writing to files 8
- 9. Event Loop 1 User interacts with page 2 An ‘event’ is triggered EVENT! 4 The page’s appearance is 3 A code handler runs in response updated/modified as a result 9
- 10. Advantages Flexibility: Wide choice of controls and trigger functions Suitability for GUI: uses WIMP system (Windows, Icons, Menus, Pointers) Simplicity and Ease of development: Add and code one control at a time, allows a programmer to build up their program gradually with simple building blocks 10
- 11. Flexibility Wide choice of controls – can be dragged and dropped onto form Good choice of trigger functions – programmer can decide what will happen when Programmer can assist user with intuitive interface, restrict input to a limited range, provide visual feedback and context-sensitive help 11
- 12. Suitable for GUIs Are particularly suitable for event driven programming - uses WIMP system Windows, Icons, Menus, Pointers They have many objects which the programmer can use to provide the user with alternative ways to interact with the program (mouse or keyboard) Most controls are independent – so code can be naturally modular 12
- 13. Simplicity and Ease of Development Event-driven languages such as C#, VB.NET and Visual C++ allow a programmer to build up their program gradually with simple building blocks Programmers can add and code one object at a time, using simple constructs Pre-written code and drop-down lists showing choice of possible commands Errors are highlighted and step through debugging Code modules can be re-used or imported from libraries, built-in wizards can be used Online and local help files are readily available 13
- 14. Disadvantages Errors can be more difficult to spot than with simpler, procedural programs Programs with complex GUIs may be slower to load and run than simpler programs – particularly if RAM is insufficient Programs with too many forms can be very confusing and/or frustrating for the user 14
- 15. Which control would a programmer use to….? Allow a Yes/ No answer only? Allow a wide range of keyboard input? Limit user input to the numbers 1, 2, 3 & 4? Allow a user to subtly change the colour or brightness selected in a graphics application Allow a user to access commands without the mouse, but using keyboard navigation only? 15