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Understanding Visual Basic Events

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In the days before graphical environments such as Microsoft windowsWindows, applications were developed using a procedural approach. This meant that the path a user would take through an application was dictated by the programmer at the point the application was developed. For example. , the application might display a menu listing several options, and prompt the user to select an option. Depending on the user's selection, the application might display a data input screen in which data would need to be entered in the order in which the fields were displayed on the screen. Once the data was entered the user would then be able to press a key to save the data and return to the menu screen again.
The world of the graphical user interface is very different from the old days. Today a user is presented with windows containing multiple controls such as buttons, text fields, toggles and sliders. The mouse allows the user to interact with any of the visible controls in any order they choose. The programmer can, therefore, no longer dictate the path a user will take through an application. A new way of handling the interaction between a user and an application was needed. This approach is known as ''event handling''.
Instead of a procedural approach to programming, applications are now ''event driven''. In essence, the developer defines what the user interface is to look like, and then defines subroutines which will be called when particular events are triggered. For example, the Visual Basic programmer might create a Form containing a number of Buttons. For each Button, the programmer will define the Visual Basic code to be executed when a ''Click'' event is triggered on each button. When the application runs, an ''event loop'' sits in the background and waits for an event to be triggered in the application. Once an event is triggered (such as button click or a a keystroke in a text field) the event loop looks at the object from which the event was triggered to see if an event handler has been defined by the developer for that particular event type. If a handler has been defined, that subroutine is executed and the event loop continues to loop waiting for the next event to be triggered.
== Event Triggers ==
There are a number of ways events can be triggered:
* '''User Interaction ''' - One of the most common ways for events to be triggered is through user interaction with controls in a Form. Each control has a number of events which can be triggered. For example, a text field can trigger an event when a user types a character or when a user moves the mouse over the control.
* '''Object Event Triggering ''' - Objects can trigger their own events. For example, the ''Timer'' object can be configured to trigger its own Timer event after a specified amount of time has elapsed.
* '''Operating System ''' - The Windows operating system can trigger events which can be handled by the application. For example, Windows will trigger an event when part of a window is obscured by another window. In this situatio,n situation Visual Basic receives an event when the previously obscured window area is re-exposed so that it knows to repaint the area.
* '''Programmatically triggered events ''' - The event of any object may be triggered by the programmer from within the Visual Basic code. For example, a program may need to simulate the event triggered by a user clicking on a button.
== Wiring Up Events in Visual Studio ==
In this example we are going to create a Form containing a TextBox, a Label and a Button. We are then going to set up event handlers such that any text typed into the TextBox will appear in the Label control. We will also wire up an event handler on the Button to close the application when the button is pressed.
The first step is to design the Form. Using the Toolbox (see [[Designing Forms in Visual Basic]] for an overview of designing forms) add a TextBox, Label and Button to the Form so that the form appears as follows:
<google>ADSDAQBOX_FLOW</google>
We will begin by defining an event procedure for the TextBox. To access the ''textInput'' control event procedures simply double click on the TextBox control in the Form. Visual Studio will subsequently display the event procedures for the ''textInput'' control. By default Visual Studio picks the most common event, the ''TextChanged'' event, and creates a stub of the subroutine to be called when that event is triggered by the user.
In our example we want every keystroke performed by the user in the TextBox to be displayed by the Label control. In this case the ''TextChanged'' event is exactly the event we want. If, however, we had wanted to write the procedure for a different event on the TextBox we could simply click on the right-hand drop down menu above the code area to view and and select from a list of events available on this type of control:
[[Image:visual_studio_event_list.jpg]]
<tt>myLabel.Text</tt>
To modify the event procedure, therefore, the code this for this event needs to be modified as follows:
<pre>
[[Image:visual_studio_event_completion.jpg]]
The implementation of the TextChanged event handler for our ''inputText'' control is now complete. Now we need to implement the event procedure for the Button. The drop down menu to the left of the event type menu contains a list of all the objects in our application and may be used to switch to editing the event procedures for an object other than the currently selected TextBox control. Use this menu to select the ''myButton'' control. Once again Visual Studio selects the most likely event, in this case ''Click'' which, in this case is the event we wish to handle. Modify the event procedure to call ''Close()'' as follows:
<pre>
Now, when the button is pressed the application will exit.
Finally, build and run the application by pressing the '''F5''' key or clicking on the Debug button in the toolbar (the button displaying the green ''Play'' triangle). The application should compile and run. When you type text into the TextBox the characters should also appear in the label:
[[Image:vb_event_example.jpg]]

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