Unit 20: Event Driven Programming Unit Descriptor

Aims Discuss what is an event driven program Define what is an event Look into event handlers and triggers Discuss event loops Look at example applications and languages Discuss key characteristics of an event driven program Discuss advantages and disadvantages of event driven programming Castle College

What are event driven programs? Event driven programs are typically used with GUI operating systems What events are triggered by a user using an OS? Clicks Movements Keys Timer What other software can you think of that is event driven? Spreadsheets, Databases, the lists go on..... Castle College

Define what an event is? An event is anything that happens to an object when the program is running. What is an object? Controls are objects, what controls can you think of? Form Buttons Lists Pictures etc...... Castle College

Page setup (Microsoft) Castle College

Define what an event handler is? Most objects have a large variety of possible events, i.e. Click, double click, mouse move etc... Event handler is the subroutine (procedure) that holds the code that runs when an event has occurred. Private Sub btnDisplayMessage_Click() MsgBox (“Hello World”) End Sub Event triggers selects the appropriate event handler that determines what code is executed. What part of the code shows the trigger? Castle College

Event Challenge Castle College Private Sub txtEnterText_Change() MsgBox ("Hello Life") End Sub Private Sub txtEnterText_Click() MsgBox ("Rock on") Private Sub btnClose_Click() MsgBox ("Hello World") End Sub Private Sub btnClose_MouseUp() MsgBox ("The King of Rock") Private Sub btnMessage_Click() Private Sub btnMessage_MouseMove() MsgBox ("Hello People") Castle College

Event loops Event driven programming languages need to have event loops. These loops are needed to keep testing the user interface to detect whether anything has happened, i.e. A mouse click. Even the programmer is not normally aware of this kind of loop. An event loop is part of an event driven program’s make up. There are other kinds of loops that the programmer will use, this is the same principle, however it is not programmed into an application by the programmer – it comes as part of the environment. Castle College

What are the key characteristics? These are the key characteristics of event driven programming languages. Event handlers Trigger functions Event loops Forms (which contain controls) Castle College

Programming languages A programming language is used by a programmer to develop event driven applications. Example event driven languages VB.net C# VBA VB6 Etc. Castle College

Some advantages of event driven programming Flexibility Programmer has control of where to place code and how to start it. Suitability for GUI An event driven language makes use of GUI controls to trigger Events. Simplicity of programming Visual programming makes application layout much easier to achieve. Code can simply be attached to various controls. Effective testing tools Castle College

A disadvantage of event driven programming Can be slow Loads of processing power is taken due to the need of the event loop. More processing is required due to the trigger functions as they match the type of event with the event handler. Castle College

Conclusion Discussed what is an event driven program Defined what is an event Looked into event handlers and triggers Discussed event loops Looked at example applications and languages Discussed key characteristics of an event driven program Discussed advantages and disadvantages of event driven programming Castle College