Visual Basic.NET How to Program (PDF)

480 Graphical User Interface Concepts: Part 1 Chapter 12
Form Properties and
Events Description / Delegate and Event Arguments
Hide Hides form (does not destroy the form or release its resources).
Show Displays a hidden form.
Common Events (Delegate EventHandler, event arguments EventArgs)
Load Occurs before a form is displayed to the user. The handler for this
event is displayed in the editor when the form is double-clicked in the
Visual Studio .NET designer.
Fig. 12.4 Common Form properties, methods and events (part 2 of 2).
When we create controls and event handlers, Visual Studio .NET generates a large
amount of the GUI–related code. Constructing GUIs can be performed graphically, by
dragging and dropping components onto the form and setting properties via the Properties
window. In visual programming, the IDE generally maintains GUI-related code and
the programmer writes the necessary event handlers.
12.3 Event-Handling Model
GUIs are event driven—they generate events when a program’s user interacts with the GUI.
Typical interactions include moving the mouse, clicking the mouse, clicking a button, typing
in a textbox, selecting an item from a menu and closing a window. Event information
is passed to event handlers, which are methods that are called as a result of specific events.
For example, consider a form that changes color when a button is clicked. Clicking the button
generates an event and passes it to the button’s event handler, causing the event-handler
code to change the form’s color.
Events are based on the notion of delegates, which are objects that reference methods
(see Section 10.11). Event delegates are multicast (class MulticastDelegate), which
means that they represent a set of delegates with the same signature. Multicast delegates
enable event calls to be sent sequentially to all delegates contained within the multicast delegate.
To learn more about delegates, see Chapter 10, Object-Oriented Programming: Polymorphism.
In the event-handling model, delegates act as intermediaries between the objects
creating (raising) events and the methods handling the events (Fig. 12.5).
call calls
Object A generates event E Delegate for event E
Fig. 12.5 Event-handling model using delegates.
Handler 1 for event E
Handler 2 for event E
Handler 3 for event E