Event Handling

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Event Handling

• Tonga Institute of Higher Education

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Event Driven Programs

• Event Driven Programs The program responds to a
  user event
• Event - An action or occurrence detected by a
  program
• User
• Clicking a mouse button
• Pressing a key
• System
• Running out of memory
• An error occurring
• For example, your program might work like this
• Show a form with a welcome message. The form
  will also contain a textbox, OK button and
  instructions to enter their name and click the Ok
  button.
• Wait for the user to respond
• When the user performs an action,
• If the user clicks the X in the top right corner,
  quit the program
• If the user typed in their name and pressed the
  OK button, show a message that you are happy to
  meet them
• Gives more freedom to the user. It reacts to the
  users actions
• Gives user the impression that they are in
  control

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Current Situation

• We learned how to display a graphical user
  interface
• However, it doesnt do anything
• Event Something that is triggered by the user
  of a GUI
• Handler Code that can detect and respond to
  events

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Creating Events

• VS.NET automatically creates events for you
• Double clicking on the control will automatically
  create code to handle the most common event
• Code in this place will be executed when the
  button is clicked
• This place is called a subroutine

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Demonstration

• Creating the most common event for a control

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Creating Uncommon Events - 1

• To create an uncommon event
• View the code
• Select the control you wish to create an event
  for
• Select the event you wish to create

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Creating Uncommon Events - 2

• To create an uncommon event
• View the code
• Select the control you wish to create an event
  for
• Select the event you wish to create

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Demonstration

• Creating an uncommon event for a control

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Creating Events

• When you using the techniques previously
  discussed, VS .NET automatically creates events
  in your code.
• If you want, you can create this code yourself

The Event handled
Name of the Control
Keyword indicates that this method handles an
event
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Demonstration

• Working with Event Code

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MessageBox

• An attention getter
• Displays information to the user
• Forces user to do something before continuing
• MessageBox.Show("Hi")

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Demonstration

• Displaying a MessageBox when a button is clicked

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Class Activity

• Double Click on the button
• Add the following code to the subroutine
• MessageBox.Show("Hello World!")
• Run the application

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Opening A Form With Code

• Sometimes, we need to open a form when an event
  occurs. (Example Open a student form when a
  button is clicked)

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Demonstration

• Opening a form with code

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Hiding a Form vs. Exiting an Application

• We can use events to hide and show forms
• frmCustomer.visible false
• This is different from exiting an application
• Application.Exit
• A program can hide all of its forms but still be
  running. In this case, the program is not usable
  and is needlessly using the computers resources
• If we are using VS.NET, the program will always
  stop running when we press the stop button
• However, when we are using the .exe file, we must
  use the task manager to shut down programs that
  are running without any forms showing

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Demonstration

• Hiding a Form vs. Exiting an Application

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Error when Quitting a Program Due to Network
Configuration

• Application.Exit does not work properly when a
  project is located on a network drive
• You will get this error
• It will work if the program is run locally
• We dont need to worry about it

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Demonstration

• Error Demonstration