The Creation, Use, and Consumption of Events with Microsoft Visual Basic .NET Zach Kramer Support Pr

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The Creation, Use, and Consumption of Events with
Microsoft Visual Basic .NETZach KramerSupport
ProfessionalDeveloper Support PSSMicrosoft
Corporation
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Objectives

• Examine the purpose of events
• Understand the handling of events
• Explore the creation of events
• Review the underlying pieces that are used to
  construct an event
• Explore some advanced topics about events

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Agenda

• Purpose of Events
• Consuming Events
• Creating and Raising Events
• Review of Events from ILDASM
• Advanced Event Concepts
• Event Example
• Summary

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Purpose of Events

• Programming paradigm
• A way of controlling the flow of application
• Occur the way life does
• Something occurs and the application reacts
• Programs choose to handle or ignore events
• Allows alerts without polling
• Avoid linear programming
• Communication of the status during a long-running
  operation

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Consuming EventsHandlers

• Objects that raise events must have those events
  connected to handlers.
• Handlers are methods that are called when events
  occur.
• Multiple handlers can be connected for one event.
• Handler methods can be connected to multiple
  events.

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Consuming Events (2)Declare an Instance of an
Event Source

• To be an event source, you must declare the
  object WithEventsFriend WithEvents Button1 As
  New Button
• The object must expose at least one event to be
  declared WithEvents.
• WithEvents enables an object to be an event
  source.

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Consuming Events (3) Creating a Handler Function

• Determine the correct method signature
• Using the object browser

• Create a method with that signature

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Consuming Events (4) Handles Keyword

• Handles keyword connects events to the handler
  method
• Event must have the same signature as the handler
  method
• Handles has the following conventionltmethod
  siggt Handles ltobjectgt.lteventgt

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Consuming Events (5) AddHandler and RemoveHandler

• First parameter is the event
• Second parameter is the AddressOf of the handler
• AddHandler
• Used to dynamically connect handlers to events
• Can be called multiple times for the same
  event-handler combination
• Handlers executed in the order that they were
  added
• RemoveHandler
• Used to dynamically remove handlers for events
• Must be called for each event-handler combination
  that was added to fully disconnect an object

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Consuming Events (6) AddHandler and
RemoveHandler (continued)

• Events and garbage collection
• Handlers will keep objects alive
• A handled event is a pointer to a function in the
  object handling the event
• Event source keeps objects alive while it is
  alive
• Two Classes handle events from same object
• Both classes have a reference to event source
• Set class2 nothing without removing handlers
• class2 will not be garbage collected
• Event source has a pointer to functions in class2
• Remove handlers or destroy the event source

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Creating and Raising EventsPublic Event
Simplest Case

• Just declare the event with a signaturePublic
  Event myEvent(ByVal str as String)
• Exposes the event with the above signature
• Takes care of all the add and remove handlers
• Creates the delegates
• Also creates private delegate instance

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Creating and Raising Events (2) Creating
Delegates

• Delegates are outlines for function pointers
• Instances of delegates are function pointers
• Events are multicast delegates
• Delegate declarationPublic Delegate Sub
  FileEvent(ByVal FullFileName As String)
• Event declaration with delegatePublic Event
  FileCompleted As FileEvent
• Allows multiple events to be declared with the
  same signature (delegate as template)

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Creating and Raising Events (3) Raise Event

• To raise an event, just use RaiseEventRaiseEven
  t myEvent(strVal)
• RaiseEvent method
• Checks to see if any handlers have been connected
  to the delegate instance
• Calls the private delegate instance
• The delegate instance then calls each handler
  that has been added
• In C you must do all this in code

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Creating and Raising Events (4) Convention for
Calling Events

• It is a convention to create an ON method that is
  called to actually raise the eventPrivate Sub
  OnMyEvent(strVal as String) RaiseEvent
  myEvent(strVal)End Sub
• Allows custom code before or after the event is
  raised each time
• ON method is called anytime that the event occurs
  in code

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Creating and Raising Events (5) Functionality
Added by Compiler

• Event and delegate
• Event that does not use a delegate (simple
  instance) will have a delegate created by the
  compiler
• Add and remove handler
• Methods that are called when a handler is added
  or removed from an event are also created
• Private delegate instance
• Private instance actually holds the function
  pointers that are added to the public events and
  are called to raise the event

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Review of Events from ILDASM Overview
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Review of Events from ILDASM (2) Delegate
Definitions

• The delegate definitions that are used by our
  events
• Multicast delegates
• Types of delegate instance that hold the function
  pointers to the event handlers
• DirectoryCompletedEventHandler was created by the
  compiler

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Review of Events from ILDASM (3) Delegate
Definitions (continued)
Public Event DirectoryCompleted(ByVal
directoryName As String) Public Event
DirectoryStarted(ByVal directoryName As
String) Public Delegate Sub FileEvent(ByVal
FullFileName As String) Public Delegate Sub
FileFoundDelegate(ByVal FullFileName As String,
ByRef continue As Boolean)
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Review of Events from ILDASM (4) Public Events

• Public definitions of events that will be exposed
• Instances of delegate definitions
• Shell delegates do not actually hold any
  references to handler methods
• Contain an add/remove method for connecting
  different handlers to these public events

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Review of Events from ILDASM (5) Public Events
(continued)
Public Event DirectoryCompleted(ByVal
directoryName As String) Public Event
DirectoryStarted(ByVal directoryName As
String) Public Event FileCompleted As
FileEvent Public Event FileFound As
FileFoundDelegate Public Event IterationStarted
As EventHandler
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Review of Events from ILDASM (6) Private
Delegate Instances

• Private definitions of the events that will be
  exposed
• Instances of the delegate definitions
• The add/remove methods of the public events add
  and remove handlers to these multicast delegate
  instances
• Notice the word event is appended to the name of
  each public event

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Review of Events from ILDASM (7) Add and Remove
Functions for Handlers

• Hidden add/remove events
• Called by runtime when adding and removing
  handlers
• Add function pointers to private delegate
  instances
• Created by the compiler

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Review of Events from ILDASM (8) Overview
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Advanced Event ConceptsThreads

• OLE/COM handled a lot of the marshalling of
  events to the correct thread in legacy
  architecture
• Microsoft Visual Basic .NET is free threaded
• Events are raised on the currently executing
  thread
• No event queues except in the user interface
• Events are executed in a linear fashion unless
  threading is explicitly used

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Advanced Event Concepts (2) User Interface

• UI updates must be carried out on the same thread
  as the UI message pump (Win32 convention)
• No guarantee that events will be raised on the
  same thread if handled in a UI environment
• UI can manage this using Invoke
• Can be dealt with in the event source
• Use a handle to a UI element and calling Invoke
• Setting up data return through polling
• WinForms timer

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Advanced Event Concepts (3) Invoke and
InvokeRequired

• Invoke
• Marshall a delegate instance back to a calling
  thread
• Uses the UI message pump of the control Invoke is
  called on
• InvokeRequired
• Tests if Invoke needs to be called based on the
  current and UI thread context values

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Advanced Event Concepts (4) Invoke and
InvokeRequired (continued)

• Verify Invoke needs to be called
• If Label1.InvokeRequired Then
• Declare an array of event arguments
• Dim myArgs() As Object o, e
• Declare a delegate instance with the address of
  the function to execute on the UI thread
• Dim del As New Iterate_StartDelegate(AddressOf
  Iterate_Start)
• Call invoke on the control whose controlling
  thread the delegate should be executed on
• Label1.Invoke(del, myArgs)

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Advanced Event Concepts (5) Accessing Compiler
Created Variables

• Compiler creates many event components
• The event class can access these private events
  but they do not show up in IntelliSense or the
  Object BrowserIf Not IsNothing(FileFoundEvent)
  Then FileFoundEvent(fileInfo.FullName,
  con)End If
• Accessing these private events allows an event
  call similar to C

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Event ExamplePieces of the Event

• Class IterateFileSystem
• Declares the events
• Raises the events
• Main form
• Declares instance of IterateFileSystem
• Starts file system iteration on a new thread and
  shows the Status form
• Handles two IterateFileSystem events
• Status form
• Handles remaining IterateFileSystem Events
• Shows status of the file system iteration

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Event Example (2)Class IterateFileSystem
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Event Example (3) Class IterateFileSystem

• Raises initial events
• Exits when iteration is complete

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Event Example (4) Class IterateFileSystem
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Event Example (5) Main Form

• The text box takes a folder to be searched
• The Iteration Status label is changed when events
  are raised from IterateFileSystem
• Declares the initial instance of the event source
  (IterateFileSystem) and status form
• Passes reference to event source to status form

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Event Example (6)Main Form
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Event Example (7) Main Form
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Event Example (8) Main Form
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Event Example (9) Main Form
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Event Example (10) Status Form

• File event fires when a file is encountered and
  updates main label and progress bar
• Directory events update title bars and Cancel
  button
• Events are all connected with handles
• This form gets an instance of the event source
  from the creating program

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Event Example (11) Status Form
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Event Example (12) Status Form
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Event Example (13) Status Form

• Must RemoveHandlers because the Event Source is
  not destroyed
• Handlers are pointers to the Form object and will
  keep it alive
• Removing handlers added with Handles keyword

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Summary

• Notification that something has occurred
• Event Source must create and raise events
• Consumer must create Event Source WithEvents
• Consumer must handle at least some events

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Additional Resources

• Q319823 HOW TO Handle Events in Visual Basic
  .NET
• MSDN Event Handlers Visual Basic and
  Chttp//msdn.microsoft.com/library/default.asp?
  url/library/en-us/vbcon/html/vboriEventHandlers.a
  sp
• MSDN Event Handling in Windows
  Formshttp//msdn.microsoft.com/library/default.a
  sp?url/library/en-us/vbcon/html/vbconEventHandlin
  g.asp

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