Research tells me that raising an event from the constructor itself is not feasible as the object may not be fully initialised... so where can I fire an event from as soon as the constructor has fired?
One thing you can do is add a method to handle additional post ctor tasks:
Friend Class FooBar
Public Sub New
' your code here
End Sub
Public Sub Create
' do anything you want
End Sub
End Class
Elsewhere:
Friend WithEvents Foo As Foobar
' ...
Foo = New FooBar ' Foo doesnt exist until ctor code executes and the
' code returns to here.
Foo.Create ' do whatever you like, as long as any other
' objects referenced have been created.
The reason calling a sub from the ctor to raise an event wont work with a class is this:
Private Sub SomeEvent(sender As Object, e As EventArgs) Handles Foo.SomeEvent
Console.Beep()
End Sub
the key is Handles Foo.SomeEvent
There is no Foo yet to handle the event. It doesnt crash and there event is raised, but there is no object for the listener to catch/handle the event. Enough of a form is created in InitializeComponents, that it does work with a form.
There might also be an Interface to implement something like this, I know of some for Components, but not classes.
You could use the Load or Show events from the Shown.
Private Sub myForm_Shown(sender As Object, e As EventArgs) Handles Me.Shown
End Sub
or
Private Sub myForm_Load(sender As Object, e As EventArgs) Handles Me.Load
End Sub
You can accomplish this by adding an Action(Of T) parameter to your constructor and invoke the delegate on the very last line.
Public Class Foo
Public Sub New(ByVal action As Action(Of Foo))
'...
'...
'...
If (Not action Is Nothing) Then action.Invoke(Me)
End Sub
End Class
Example
Public Class Form1
Private Sub Button1_Click(sender As Object, ev As EventArgs) Handles Button1.Click
Dim foo1 As New Foo("foo1", AddressOf Me.HandleFooCtor)
Dim foo2 As New Foo("foo2", Sub(f As Foo) MessageBox.Show(f.Name))
End Sub
Private Sub HandleFooCtor(f As Foo)
MessageBox.Show(f.Name)
End Sub
Public Class Foo
Public Sub New(name As String, Optional ByVal action As Action(Of Foo) = Nothing)
'...
'...
'...
Me.Name = name
If (Not action Is Nothing) Then action.Invoke(Me)
End Sub
Public ReadOnly Name As String
End Class
End Class
Related
I am in a project with multiple form.
I create a TicTacToe form here :
Private Sub MenuTicTacToe(ByVal sender As Object, ByVal e As System.EventArgs)
Dim page As Form = New TicTacToe
page.Show(Me)
End Sub
Here is a TicTacToe form:
Public Class TicTacToe
Public opponent as String
'Some code where user set opponent
Public Function Receive(S As String)
if string = opponent
'Some code
End Function
End Class
I would like to call my function Receive in my main form
If i do:
TicTactoe.Receive(S)
It call a instance of Receive where opponent does not exist.
I would like to find the oppened form of TicTacToe and call Receive
Thanks
Comments in line
Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
page.Receive("Joe")
End Sub
'A form level variable to hold a reference to the instance of TicTacToe
'Although vb.net can use default instances, you have created an explicit
'instance of TicTacToe so you need to keep a reference if you want to
'refer to this instance.
Private page As TicTacToe
Private Sub MenuTicTacToe(ByVal sender As Object, ByVal e As System.EventArgs)
page = New TicTacToe()
page.Show(Me)
End Sub
Partial Public Class TicTacToe
Inherits Form
Public opponent As String
'Functions must be declared as a Type
'If you do not need a return value use a Sub
Public Function Receive(S As String) As String
Dim someString As String = ""
If S = opponent Then
'Do something
End If
'There must be a return Value
Return someString
End Function
End Class
Use this to show the form
Dim page As TicTacToe
Private Sub MenuTicTacToe(ByVal sender As Object, ByVal e As System.EventArgs)
page = New TicTacToe
page.Show(Me)
End Sub
Then you can use
page.Receive(S)
Edit
To use multiple forms
For Each f As TicTacToe in Application.OpenForms().OfType(Of TicTacToe)
f.Receive (S)
Next
In C#, you'd need a new instance, but as you are in VB, the compiler already does that for you.
What you are currently doing, is creating a new instance of the TicTacToe form and showing it:
Private Sub MenuTicTacToe(ByVal sender As Object, ByVal e As System.EventArgs)
Dim page As Form = New TicTacToe
page.Show(Me)
End Sub
But you don't save that instance anywhere. Then, in your next piece of code, you are using a different instance, which is the static one created by the compiler:
TicTacToe.Receive(S) // TicTacToe is the static instance
Therefore, you end up calling two different instances, which explains why there is no opponent set.
To get around this problem, do not create a new instance. In your Private Sub MenuTicTacToe, just use the instance created by the compiler, and you won't have this problem, just like this:
Private Sub MenuTicTacToe(ByVal sender As Object, ByVal e As System.EventArgs)
TicTacToe.Show(Me)
End Sub
Hope this helps.
The next code works for my, but I d'ont know if it's the best way to do it.
Of this way I need to write: _Button1 = Button1 and _MyVar = MyVar
This way of doing it seems repetitive and long when the parameters
passed to the Class Constructor are many more.
Public Class Form1
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles Me.Load
Dim myVar As String = "My children"
Dim NewClass1 As New Class1(Button1, myVar)
'... do more ...
End Sub
End Class
Public Class Class1
Private WithEvents _Button1 As Button
Private _MyVar As String
Public Sub New(ByVal Button1 As Button, ByVal MyVar As String)
_Button1 = Button1
_MyVar = MyVar
'... do more ...
End Sub
Private Sub _Button1_Click(sender As Object, e As EventArgs) _
Handles _Button1.Click
MsgBox("Button1 clicked and I love: " & _MyVar)
End Sub
End Class
_Button1 = Button1 and _MyVar = MyVar This way of doing it seems repetitive and long when the parameters passed to the Class Constructor are many more.
This is actually dependency injection and is usually a good thing. If you end up with too many constructor parameters then it should be a clue that your class is doing too many things and is in violation of SOLID principles.
However, your Class1 should not have dependencies on Form1, it should be the other way around. Your code could much more simply be:
Public Class Class1
Friend Sub DoSomething(ByVal MyVar As String)
MsgBox(MyVar)
End Sub
End Class
Public Class Form1
Public Property class1() As Class1
Sub New()
InitializeComponent()
Me.class1 = New Class1()
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
class1.DoSomething("blah, blah")
End Sub
End Class
While I am not demonstrating dependency injection here (for simplicity), you can see Class1 only knows that it can do some work, it doesn't know about the Form at all.
You could subscribe to the button click event in Class1 instead of passing the button into it. It would be cleaner, but still bad design.
I am having an issue when trying to delete ListView Items from a second form.
For example, if I use the following command on Form1 it works:
Listview1.SelectedItems(0).Remove
However, if I attempt to remove from Form2 like so:
Form1.Listview1.SelectedItems(0).Remove
I get the following error:
"Invalid argument=value of '0' is not valid for 'index'. Parameter name: index"
I then tried to get a count of items from the listview on Form2 and it gives me a return of 0
Form1.Listview1.Items.Count
I'm not sure what my problem is.
Update
I have posted a brief example of my code (using your suggestion as I can understand it):
frmShowMessages
Private Sub ViewMessage()
Dim frm As New frmViewMailMessage
frm.Show()
End Sub
Public Sub DeleteItem(ByVal index As Integer)
lsvReceivedMessages.Items(index).Remove()
End Sub
frmViewMessage
Private instanceForm as frmShowMessages
Private Sub frmViewMailMessage_Load(sender As Object, e As EventArgs) Handles MyBase.Load
instanceForm = New frmShowMessages()
End Sub
Private Sub cmdDelete_Click(sender As Object, e As EventArgs) Handles cmdDelete.Click
instanceForm.DeleteItem(_index)
End Sub
Hopefully my code can help identify where my issue is.
In VB.net usually you get a default Form instance for each of your Form. Probably you are creating an instance of Form1 and then you are trying to access ListView1 of default instance.
E.g.
Sub ButtonClick()
Dim f As New Form1()
f.Show()
' at this point if you access f's ListView you will get correct count
f.ListView1.Items.Count
' however if you try to access default instance it will NOT have any item
Form1.ListView.Items.Count
End Sub
It means your instance f is NOT equal to default Form1 instance.
Solution can be, make the f variable as class level variable and use it everywhere. Or if Form1 will have only 1 instance, then you can use the default instance everywhere.
Personally I would NOT go with direct control accessing over forms. I would create a Public method which should return the data as list to the caller, in this case your Form2.
UPDATED-2:
As per your given scenario, I am simplifying things for you, and doing implementation using Event.
Public Class frmShowMessages
Private Sub btnOpenMessage_Click(sender As System.Object, e As System.EventArgs) Handles btnOpenMessage.Click
Dim frmView As New frmViewMessage(Me.ListView1.SelectedItems(0).Index)
AddHandler frmView.MessageDeleted, AddressOf DeleteMessageHandler
frmView.Show()
End Sub
Private Sub DeleteMessageHandler(sender As Object, e As frmViewMessage.MessageDeletedEventArgs)
Me.ListView1.Items.RemoveAt(e.MessageIndex)
End Sub
End Class
Public Class frmViewMessage
' a class which will be used for Event communication
Public Class MessageDeletedEventArgs
Inherits EventArgs
Public Property MessageIndex As Integer
Public Sub New(ByVal iIndex As Integer)
MyBase.New()
Me.MessageIndex = iIndex
End Sub
End Class
' main event which will alert the parent that a message deletion should be done
Public Event MessageDeleted As EventHandler(Of MessageDeletedEventArgs)
' private variable that will hold the MessageIndex
Private Property MessageIndex As Integer
' method that is responsible to raise event
Protected Overridable Sub OnMessageDeleted()
RaiseEvent MessageDeleted(Me, New MessageDeletedEventArgs(Me.MessageIndex))
End Sub
' we want to create this Form using the MessageIndex of ListView
Public Sub New(ByVal iMessageIndex As Integer)
Me.InitializeComponent()
Me.MessageIndex = iMessageIndex
End Sub
' the delete button will raise the event to indicate parent that
' a deletion of message should be done
Private Sub btnDelete_Click(sender As System.Object, e As System.EventArgs) Handles btnDelete.Click
Me.OnMessageDeleted()
End Sub
End Class
Public Event DocumentCompleted As WebBrowserDocumentCompletedEventHandler
Dim arg() As Object = {homeTeam, guestTeam}
AddHandler browser.DocumentCompleted, New
WebBrowserDocumentCompletedEventHandler(AddressOf DoStuff)
Private Sub DoStuff(ByVal sender As Object, ByVal e As WebBrowserDocumentCompletedEventArgs)
End Sub
How can I pass the homeTeam and guestTeam when firing the DocumentCompleted event.
I want to ge the above to values to inside the Dostuff method.
Please help.
First of all, you cannot have this hanging in the middle of nowhere:
Dim arg() As Object = {homeTeam, guestTeam}
AddHandler browser.DocumentCompleted,
New WebBrowserDocumentCompletedEventHandler(AddressOf DoStuff)
AddHandler probably needs to be in some Initialize method, which could be inside Sub New, after InitializeComponent, or inside Form_Load, or as soon as you expect it to be triggered (after a specific event). Notice here that you are using a default event of a native .NET component, with a default event type. In this case you cannot directly consume anything other than what it already provides, when triggered. See WebBrowser.DocumentCompleted Event on MSDN.
You can, however, override all relevant classes and have your own MyWebBrowser control and your own event, with would contain additional properties. See below example:
Public Class Form1
Sub New()
' This call is required by the designer.
InitializeComponent()
Dim browser As New MyWebBrowser
AddHandler browser.MyDocumentCompleted, AddressOf DoStuff
End Sub
Private Sub DoStuff(ByVal sender As Object, ByVal e As MyWebBrowserDocumentCompletedArgs)
Dim guestTeam As String = e.GuestTeam 'guest team
Dim homeTeam As String = e.HomeTeam 'and home team are both accessible
'so you can do some processing on them
End Sub
Public Class MyWebBrowserDocumentCompletedArgs : Inherits WebBrowserDocumentCompletedEventArgs
Dim _homeTeam As String
Dim _guestTeam As String
Public ReadOnly Property HomeTeam
Get
Return _homeTeam
End Get
End Property
Public ReadOnly Property GuestTeam
Get
Return _guestTeam
End Get
End Property
Sub New(url As Uri, homeTeam As String, guestTeam As String)
MyBase.New(url)
_homeTeam = homeTeam
_guestTeam = guestTeam
End Sub
End Class
Public Class MyWebBrowser : Inherits WebBrowser
Public Delegate Sub MyWebBrowserDocumentCompletedEventHandler(e As MyWebBrowserDocumentCompletedArgs)
Public Event MyDocumentCompleted As MyWebBrowserDocumentCompletedEventHandler
Protected Overrides Sub OnDocumentCompleted(e As System.Windows.Forms.WebBrowserDocumentCompletedEventArgs)
MyBase.OnDocumentCompleted(e)
'homeTeam and guestTeam need to be extracted from the current instance of MyWebBrowser, and passed further
RaiseEvent MyDocumentCompleted(New MyWebBrowserDocumentCompletedArgs(e.Url, "homeTeam", "guestTeam"))
End Sub
End Class
End Class
If your project is relatively small, you can indeed have those as global variables, as #Vlad suggested in the comments.
Please have a look at the code below:
Public Delegate Sub TestButtonClick(ByVal test As Integer)
Public Class Person
Private Name As String
Private ID As Integer
Public Event ButtonClick As TestButtonClick
Public Sub DelegateTest1(ByVal Test As Integer)
MsgBox(Test)
End Sub
Public Sub ChangeName()
RaiseEvent ButtonClick(1)
End Sub
Public Sub DelegateTest2()
MsgBox("Delegate Test 2")
End Sub
Public Sub DelegateTest3()
MsgBox("Delegate Test 3")
End Sub
End Class
Public Class Form1
Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
Dim p1 As Person = New Person
AddHandler p1.ButtonClick, AddressOf p1.DelegateTest1
AddHandler p1.ButtonClick, AddressOf p1.DelegateTest2
AddHandler p1.ButtonClick, AddressOf p1.DelegateTest3
p1.ChangeName()
End Sub
End Class
The output is:
1
DelegateTest2
DelegateTest3
I do not understand why this application compiles i.e. the delegate accepts an integer in its signature but Person.DelegateTest2 and Person.DelegateTest3 do not.
If I change Person.DelegateTest2() to the following then I do get an error as I would expect:
Public Sub DelegateTest2(ByVal Test As Integer, ByVal Test2 As Integer)
MsgBox("Delegate Test 2")
End Sub
Why does the Delegate allow you to pass zero arguments when it has arguments i.e. an integer in my case?
Don't forget that VB.NET inherits all the legacy baggage from the beloved VB. You could make it strict by putting the following to the top of your file so that it behaves as a real .NET programming language and not some hybrid crap:
Option Strict On
Also I would recommend you setting this to be the default option so that you don't find yourself in the wilderness.