Form1.vb
Imports System.Threading
Public Class Form1
Dim demoThread As Thread
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim Start As New Class1
Me.demoThread = New Thread( _
New ThreadStart(AddressOf Start.ThreadProcSafe))
Me.demoThread.Start()
End Sub
Delegate Sub SetTextCallback([text] As String)
Public Sub SetText(ByVal [text] As String)
' InvokeRequired required compares the thread ID of the
' calling thread to the thread ID of the creating thread.
' If these threads are different, it returns true.
If Me.textBox1.InvokeRequired Then
Dim d As New SetTextCallback(AddressOf SetText)
Me.Invoke(d, New Object() {[text]})
Else
Me.textBox1.Text = [text]
End If
End Sub
End Class
Class1.vb
Public Class Class1
Public Sub ThreadProcSafe()
Form1.SetText("This text was set safely.")
End Sub
End Class
Can someone tell me why this doesn't update the textbox?
It works when ThreadProcSafe is called when its inside Form1(and is still started by a thread) but when it's moved outside of the class into another, no warnings or errors but doesn't update.
The reason is that you are referring to the default instance in your second code snippet. Default instances are thread-specific so that second code snippet will create a new instance of the Form1 type rather then use the existing instance. Your Class1 needs a reference to the original instance of Form1.
If that's not practical then the solution is to not do the delegation in the form but rather do it in the class accessing the form, using the SynchronizationContext class.
Related
listView is owned by the class Form1. The subroutine anotherThread in a separate class transmission is started in a thread by a subroutine in Form1. Form1 owns another public subroutine addItemsListView, which uses Invoke.
When transmission.anotherThread calls addItemsListView, the subroutine runs, but listView remains blank.
Have tried delegates, invokes etc. inside each class but the same problem.
Class Form1
Property myTransmission = New transmission
Private Sub aSubRoutine() Handles MyBase.Load
Dim t As New Threading.Thread(
Sub()
myTransmission.anotherThread()
End Sub
)
t.Start()
End Sub
Public Sub addItemsListView(ByVal items As String())
If listView.InvokeRequired Then
listView.Invoke(Sub() addItemsListView(items))
Else
For each item In Items
listView.Items.Add(item)
Next
End If
End Sub
End Class
Class transmission
Public Sub anotherThread()
Form1.addItemsListView(New String() {"abc", "def"})
End Sub
End Class
So I expect "abc" and "def" to be in the listView but it remains completely blank. If I step through the code, however, everything seems to be running smoothly.
You aren't talking to your existing form. Pass it as a reference:
Public Sub anotherThread(inForm As Form1)
inForm.addItemsListView(New String() {"abc", "def"})
End Sub
then include your form when you call it:
myTransmission.anotherThread(Me)
I would like to be able to change an element of the main form inside a thread declared in a separated class (In this case I want to change a label text).
I tried the following code:
Form1:
Imports System.Threading
Public Class Form1
Public counter As Integer = 0
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim SecondClassObject As New SecondClass()
End Sub
End Class
SecondClass:
Imports System.Threading
Public Class SecondClass
Public Thread As New Thread(AddressOf Increment)
Public counter As Integer = 0
Sub New()
Thread.Start()
End Sub
Sub Increment()
While True
Form1.Label1.Text = counter
counter += 1
End While
End Sub
End Class
If I do the same thing using a thread but in the form code itself than the label text will change:
Imports System.Threading
Public Class Form1
Public counter As Integer = 0
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim thread As New Thread(AddressOf Increment)
thread.Start()
End Sub
Sub Increment()
While True
Label1.Text = counter
counter += 1
End While
End Sub
End Class
How should I do in order to archieve the same result using a thread in a separated class?
First, I do want to point out that you need to use an Invoke/Callback to safely set the label's text from the secondary thread. I don't know if you're doing that in your actual code base, but wanted to specify anyways.
Now, focused on the actual question, I believe that the easiest way to do as requested is to pass a reference to the original instance of Form1 to your SecondClass. Having a reference to the parent, means that you would be able to manipulate the parent's publicly exposed elements as needed.
Consider the below:
Public Class Form1
Public counter As Integer = 0
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim secondClass As New SecondClassObject(me)
End Sub
Delegate Sub SetTextCallback (value as String)
Public Sub SetText (value as string)
if me.Label1.InvokeRequired Then
dim d as New SetTextCallback(addressOf SetText)
Me.Invoke(d, New Object() {value})
Else
me.label1.text = value
End If
End Sub
End Class
Public Class SecondClassObject
private _parent as Form1
private myThread As New Thread(AddressOf Increment)
Public Sub New (byref p as Form1)
me._parent = p
myThread.Start()
End Sub
Sub Increment()
While True
Me._parent.SetText(counter)
counter += 1
End While
End Sub
End Class
What is happening is that the a reference to the parent is passed into the second class as a constructor, doing so allows us to interact with the parent from the second class.
Now, that is one way, but other options do exist. Things such as specialized events/handlers or wiring up databinding between the Form1.Label1 and a property exposed from the SecondClassObject. Even a singleton pattern, where the value to be incremented is shared between all instances, so when the SecondClassObject increments it, Form1 would be aware and know to update Label1.
Also, please note that the above code is for example purposes, and is missing things such as a defined declaration for Label1.
On windows it's not possible to change the UI from a non UI thread.
It looks like that you have to use Control.Invoke or better Control.BeginInvoke.
The problem with using Control.Invoke is that it's executed on the UI thread and the calling thread waits for completion. Which would be bad when your background worker continusly does some computations.
Hoping to get some best-practice advise with regards to capturing a returned message from an instantiated class on my form.
In my form (form1.vb), I have a label which reflects what is being done, with the code below.
Code in form1.vb to display message:
Public Sub DisplayMessage(ByVal Msg as String, ByVal Show as Boolean)
Application.DoEvents()
If Show Then
lblShow.Text = Msg
lblShow.Refresh()
End If
End Sub
I have came across three methods so far:
Direct Form Call. In this scenario the class directly calls the form's message routine:
form1.DisplayMessage("Show This Message", True)
RaiseEvent within class. In this scenario form1 is Friends WithEvents of the class sending the message, and the class raises the event to the form.
**Declared in Form1.vb**
Friend WithEvents Class1 as New Class1
**Declared in Class1.vb**
Public Event SetMessage(ByVal Msg As String, ByVal Show As Boolean)
**Used in Class1.vb**
RaiseEvent SetMessage("Show This Message", True)
Have an EventArgs class handle the event. In this scenario we have an EventArg.vb class which is instantiated whenever we raise the event.
**Declared in Form1.vb**
Friend WithEvents Class1 as New Class1
Private Sub class1_DisplayMessage(ByVal Msg As String, ByVal showAs Boolean, ByRef e As ProgressMessageEventArgs) Handles Class1.SetMessage
DisplayMessage(Msg, Show)
End Sub
**Declared in Class1.vb**
Public Event SetMessage(ByVal msg As String, ByVal Show As Boolean, ByRef e As ProgressMessageEventArgs)
Protected Sub CaptureMessage(ByVal msg As String, ByVal Show As Boolean)
RaiseEvent SetMessage(message, ShowList, New ProgressMessageEventArgs(message))
End Sub
**Used in Class1.vb**
RaiseEvent CaptureMessage("Show This Message", True)
**EventArg.vb created to handle ProgressMessageEventArgs class**
Public NotInheritable Class ProgressMessageEventArgs
Inherits System.EventArgs
Public txt As String
Public Sub New(ByVal txt As String)
MyBase.New()
Me.Text = txt
End Sub
End Class
Scenario 1 is seemingly the simplest, though I was advised against this and asked to raise an event instead. Over time I came across scenario 3 which involves an additional class vs scenario 2.
Therefore, the question is...
Between these three methods, which would be the "proper" way of returning a message from a class to the form? Is the additional EventArg class as per scenario 3 necessary since scenario 2 works fine as well?
Many thanks in advance.
My answer is none of the above. Consider this example
Public Class Form1
Private WithEvents myClass1 As New Class1()
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
myClass1.CountTo1000()
End Sub
Private Sub MyClass1_Updated(number As Integer) Handles myClass1.Updated
Me.Label1.Text = number.ToString()
End Sub
End Class
Public Class Class1
Public Event Updated(number As Integer)
Public Sub CountTo1000()
For i = 1 To 1000
System.Threading.Thread.Sleep(1)
RaiseEvent Updated(i)
Next
End Sub
End Class
You have a form and a class, and the form has a reference to the class (the class doesn't even know the form exists). Your business logic is performed in the class, and the form is used to input and display information. CountTo1000() is being called directly from the form, which is bad because basically the UI thread is being put to sleep 1000 times, while the class is trying to update the UI by raising the event after each sleep. But the UI never has time to allow the events to happen, i.e. to be updated. Placing an Application.DoEvents() after Me.Label1.Text = number.ToString() will allow the UI to update. But this is a symptom of bad design. Don't do that.
Here is another example with multi-threading
Public Class Form1
Private WithEvents myClass1 As New Class1()
' this handler runs on UI thread
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
' make a new thread which executes CountTo1000
Dim t As New System.Threading.Thread(AddressOf myClass1.CountTo1000)
' thread goes off to do its own thing while the UI thread continues
t.Start()
End Sub
' handle the event
Private Sub MyClass1_Updated(number As Integer) Handles myClass1.Updated
updateLabel(number.ToString())
End Sub
' invoke on UI thread if required
Private Sub updateLabel(message As String)
If Me.Label1.InvokeRequired Then
Me.Label1.Invoke(New Action(Of String)(AddressOf updateLabel), message)
Else
Me.Label1.Text = message
End If
End Sub
End Class
Public Class Class1
Public Event Updated(number As Integer)
Public Sub CountTo1000()
For i = 1 To 1000
System.Threading.Thread.Sleep(1)
RaiseEvent Updated(i)
Next
End Sub
End Class
This simple example shows how a thread can be created and run some code off the UI. When doing this, any method call from the non-UI thread must be invoked on the UI if it must access a UI control (Label1). The program runs smoothly since the Thread.Sleep is done on a different thread than the UI thread, with no need for Application.DoEvents, because the UI thread is otherwise doing nothing, and can handle the events being raised by the other thread.
I focused more on threading, but in both examples the design has a form with a class, and the form knows about the class, but the class doesn't know about the form. More about that can be seen here.
See also:
Why we need to check for InvokeRequired, then invoke: Control.InvokeRequired
A better option than Thread nowadays: BackgroundWorker
An even cooler option, if you can wrap your head around it: Async/Await
In order to keep responsiveness in the UI, I use a separate thread to execute various process, for example some FTP download.
Private Sub Button11_Click(sender As Object, e As EventArgs) Handles Button11.Click
Dim ThreadResync As System.Threading.Thread
ThreadResync = New System.Threading.Thread(AddressOf Bodacc_ResyncFTP)
ThreadResync.Start()
End Sub
Sub Bodacc_ResyncFTP()
Dim MyBodacc As bodacc_data = New bodacc_data
MyBodacc.Label_Status = Form1.Label1
MyBodacc.ResyncFTP()
End Sub
A way to update the UI with threading is the Delegate thingy, so in the bodacc_data I had to
Public Class bodacc_data
......
Delegate Sub UpdateLabelDelg(text As String, ThisLabel As Label)
Public Delegate_label As UpdateLabelDelg = New UpdateLabelDelg(AddressOf set_label)
Public Label_Status = Label
......
Sub set_label(stext As String, ThisLabel As Label)
ThisLabel.Text = stext
End Sub
.....
Sub ResyncFTP()
//says hello
If Label_Status.InvokeRequired = True Then
Label_Status.Invoke(Delegate_label, New Object() {"Working...", Label_Status})
Else
Label_Status.Text = "Working..."
End If
//do stuff
End Sub
End Class
It works like a charm. But I have many class doing more or less the same (disk update, database update, FTP update) and having to copy/past all the delegate / external label declaration / mini sub / invoke sound silly.
So I created a class to handle those UI update / delegate in order to have a quick access
Public Class Form_UI
Delegate Sub UpdateLabelDelg(text As String, ThisLabel As Label)
Public Delegate_label As UpdateLabelDelg = New UpdateLabelDelg(AddressOf set_label)
Private Labels(2) As Label
Sub New()
Labels(0) = Form1.Label1
Labels(1) = Form1.Label2
Labels(2) = Form1.Label3
End Sub
Sub set_label(stext As String, ThisLabel As Label)
ThisLabel.Text = stext
End Sub
Public Sub ChangeLabel(ByVal LabelNum As Integer, nText As String)
LabelNum = LabelNum - 1
If Labels(LabelNum).InvokeRequired Then
Labels(LabelNum).Invoke(Delegate_label, New Object() {nText, Labels(LabelNum)})
Else
Labels(LabelNum).Text = nText
Labels(LabelNum).Update()
End If
End Sub
End Class
So, now in the revamped bodacc_data and all others processing class I have only :
Public Class bodacc_data
......
Private MyUI as Form_UI
.....
Sub New()
MyUI = New Form_UI()
End Sub
Sub ResyncFTP()
//says hello
MyUI.ChangeLabel(1, "Working...")
//do stuff
End Sub
End Class
Question Why is MyUI.ChangeLabel not updating when the ResyncFTP is called in a thread, but works if called in the main thread (As in the code sample below)
Private Sub Button11_Click(sender As Object, e As EventArgs) Handles Button11.Click
Dim MyBodacc As bodacc_data = New bodacc_data
MyBodacc.ResyncFTP()
End Sub
Note that there is no error thrown. The notable weirdness is that <Form_UI>.ChangeLabel() never goes the .Invoke route but the normal update route. I strongly suspect a scope issue or insight issue.
When you create a windows forms app you set up a UI thread that is meant to be the owner of all the UI. The UI thread contains the message pump that is used to update all of the UI.
But what you're doing in Button11_Click is creating a new thread that goes and calls Dim MyBodacc As bodacc_data = New bodacc_data which, in turn, calls MyUI = New Form_UI().
You are creating a form on a non-UI thread. There is no message pump and therefore the UI doesn't update.
I know how to use BackgroundWorker (gui object in WinForms designer), and to manually instantiate Threads that elevate the custom event to the UI, however, I am having some trouble figuring out how to use the ThreadPool object (simplest form) to handle elevating an event to the form for "safe" UI manipulation.
Example is as follows :
Form1.vb
Public Class Form1
WithEvents t As Tools = New Tools
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
t.Unzip("file 1", "foo")
t.Unzip("file 2", "foo")
t.Unzip("file 3", "foo")
t.Unzip("file 4", "foo")
t.Unzip("file 5", "foo")
t.Unzip("file 6", "foo")
t.Unzip("file 7", "foo")
t.Unzip("file 8", "foo")
t.Unzip("file 9", "foo")
End Sub
Private Sub t_UnzipComplete(ZipInfo As Tools.ZipInfo) Handles t.UnzipComplete
TextBox1.Text = TextBox1.Text & ZipInfo.ZipFile & vbCr
End Sub
End Class
( add a multiline textbox, and a button to this form for the demo )
Tools.vb
Imports System
Imports System.Threading
Imports System.IO.Compression
Public Class Tools
#Region "Zip"
Private _zip As System.IO.Compression.ZipFile
Public Shared Event UnzipComplete(ByVal ZipInfo As ZipInfo)
Public Shared Event ZipComplete(ByVal ZipInfo As ZipInfo)
Public Class ZipInfo
Public Property ZipFile As String
Public Property Path As String
End Class
Public Sub Unzip(ByVal ZipFile As String, ByVal Destination As String)
Dim _ZipInfo As New Tools.ZipInfo
_ZipInfo.ZipFile = ZipFile
_ZipInfo.Path = Destination
ThreadPool.QueueUserWorkItem(AddressOf ThreadUnzip, _ZipInfo)
End Sub
Public Sub Zip(ByVal Folder As String, ByVal ZipFile As String)
Dim _ZipInfo As New Tools.ZipInfo
_ZipInfo.ZipFile = ZipFile
_ZipInfo.Path = Folder
ThreadPool.QueueUserWorkItem(AddressOf ThreadUnzip, _ZipInfo)
End Sub
Shared Sub ThreadUnzip(ZipInfo As Object)
RaiseEvent UnzipComplete(ZipInfo)
End Sub
Shared Sub ThreadZip(ZipInfo As Object)
RaiseEvent ZipComplete(ZipInfo)
End Sub
#End Region
End Class
What this code should do, is as follows :
On Button1_Click, add 9 items to the ThreadPool
On each thread completion (order is irrelevant), raise an event that elevates to Form1
The event being raised on Form1 should be UI safe, so I can use the information being passed to the ZipCompleted / UnzipCompleted events in the Textbox. This should be generic, meaning the function that raises the event should be reusable and does not make calls to the form directly. (aka, I do not want a "custom" sub or function in Tools.vb that calls specific elements on Form1.vb . This should be generic and reusable by adding the class to my project and then entering any "custom" form code under the event being raised (like when Button1_Click is raised, even though it's threaded, the other form interactions are not part of the Button1 object/class -- they are written by the coder to the event that is raised when a user clicks.
If you want to ensure that an object that has no direct knowledge of your UI raises its events on the UI thread then use the SynchronizationContext class, e.g.
Public Class SomeClass
Private threadingContext As SynchronizationContext = SynchronizationContext.Current
Public Event SomethingHappened As EventHandler
Protected Overridable Sub OnSomethingHappened(e As EventArgs)
RaiseEvent SomethingHappened(Me, e)
End Sub
Private Sub RaiseSomethingHappened()
If Me.threadingContext IsNot Nothing Then
Me.threadingContext.Post(Sub(e) Me.OnSomethingHappened(DirectCast(e, EventArgs)), EventArgs.Empty)
Else
Me.OnSomethingHappened(EventArgs.Empty)
End If
End Sub
End Class
As long as you create your instance of that class on the UI thread, its SomethingHappened event will be raised on the UI thread. If there is no UI thread then the event will simply be raised on the current thread.
Here's a more complete example, which includes a simpler method for using a Lambda Expression:
Imports System.Threading
Public Class Form1
Private WithEvents thing As New SomeClass
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Me.thing.DoSomethingAsync()
End Sub
Private Sub thing_DoSomethingCompleted(sender As Object, e As IntegerEventArgs) Handles thing.DoSomethingCompleted
MessageBox.Show(String.Format("The number is {0}.", e.Number))
End Sub
End Class
''' <summary>
''' Raises events on the UI thread after asynchronous tasks, assuming the instance was created on a UI thread.
''' </summary>
Public Class SomeClass
Private ReadOnly threadingContext As SynchronizationContext = SynchronizationContext.Current
Public Event DoSomethingCompleted As EventHandler(Of IntegerEventArgs)
''' <summary>
''' Begin an asynchronous task.
''' </summary>
Public Sub DoSomethingAsync()
Dim t As New Thread(AddressOf DoSomething)
t.Start()
End Sub
Protected Overridable Sub OnDoSomethingCompleted(e As IntegerEventArgs)
RaiseEvent DoSomethingCompleted(Me, e)
End Sub
Private Sub DoSomething()
Dim rng As New Random
Dim number = rng.Next(5000, 10000)
'Do some work.
Thread.Sleep(number)
Dim e As New IntegerEventArgs With {.Number = number}
'Raise the DoSomethingCompleted event on the UI thread.
Me.threadingContext.Post(Sub() OnDoSomethingCompleted(e), Nothing)
End Sub
End Class
Public Class IntegerEventArgs
Inherits EventArgs
Public Property Number() As Integer
End Class
You should register from the Form to events of the Tools class (you already have these events defined), of course the actual event will be fired under a non-UI thread, so the code it executes during the callback will only be able to update the UI via an Invoke()
You want to simply raise the event in the Tools class, the Invoke needs to be done because you want to update the UI, the Tools class should be concerned about that.
Change your event handling like so:
Private Sub t_UnzipComplete(ZipInfo As Tools.ZipInfo) Handles t.UnzipComplete
TextBox1.Invoke(Sub () t_UnzipComplete(ZipInfo))
End Sub
To register to the event from the view: (this would go in the Button1_Click event
AddHandler t.UnzipComplete, AddressOf t_UnzipComplete
Make sure you only register to the event one time
Does this solve your issue?
Private Sub t_UnzipComplete(ZipInfo As Tools.ZipInfo) Handles t.UnzipComplete
If TextBox1.InvokeRequired Then
TextBox1.Invoke(Sub () t_UnzipComplete(ZipInfo))
Else
TextBox1.Text = TextBox1.Text & ZipInfo.ZipFile & vbCr
End If
End Sub
You could create a callback to do the invoking in a safer way. Something like this:
Public Sub Unzip(ByVal ZipFile As String, ByVal Destination As String, _
ByVal SafeCallback As Action(Of ZipInfo))
And then the calling code does this:
t.Unzip("file 1", "foo", Sub (zi) TextBox1.Invoke(Sub () t_UnzipComplete(zi)))
Personally I think it is better - and more conventional - to invoke on the event handler, but you could do it this way.
Okay, so here is what I came up with using a combination of the information from everyone contributing to this question -- all excellent and VERY helpful answers, which helped lead me to the final solution. Ideally, I would like this as a straight "class", but I can accept a UserControl for this purpose. If someone can take this and do exactly the same thing with a class, that would definitely win my vote. Right now, I will really have to consider which one to vote for.
Here is the updated Tools.vb
Imports System
Imports System.Threading
Imports System.Windows.Forms
Imports System.IO.Compression
Public Class Tools
Inherits UserControl
#Region "Zip"
Private _zip As System.IO.Compression.ZipFile
Private threadingContext As SynchronizationContext = SynchronizationContext.Current
Private Delegate Sub EventArgsDelegate(ByVal e As ZipInfo)
Public Shared Event UnzipComplete(ByVal ZipInfo As ZipInfo)
Public Shared Event ZipComplete(ByVal ZipInfo As ZipInfo)
Public Class ZipInfo
Public Property ZipFile As String
Public Property Path As String
End Class
Public Sub Unzip(ByVal ZipFile As String, ByVal Destination As String)
Dim _ZipInfo As New Tools.ZipInfo
_ZipInfo.ZipFile = ZipFile
_ZipInfo.Path = Destination
ThreadPool.QueueUserWorkItem(AddressOf ThreadUnzip, _ZipInfo)
End Sub
Public Sub Zip(ByVal Folder As String, ByVal ZipFile As String)
Dim _ZipInfo As New Tools.ZipInfo
_ZipInfo.ZipFile = ZipFile
_ZipInfo.Path = Folder
ThreadPool.QueueUserWorkItem(AddressOf ThreadUnzip, _ZipInfo)
End Sub
Private Sub ThreadUnzip(ZipInfo As Object)
If Me.InvokeRequired Then
Me.Invoke(New EventArgsDelegate(AddressOf ThreadUnzip), ZipInfo)
Else
RaiseEvent UnzipComplete(ZipInfo)
End If
End Sub
Private Sub ThreadZip(ZipInfo As Object)
If Me.InvokeRequired Then
Me.Invoke(New EventArgsDelegate(AddressOf ThreadZip), ZipInfo)
Else
RaiseEvent ZipComplete(ZipInfo)
End If
End Sub
#End Region
End Class
If you drop this on Form1.vb, and select/activate the UnzipComplete/ZipComplete events, you will find that they will interact with the UI thread without having to pass a Sub, or Invoke, etc, from the Form. It is also generic, meaning it is unaware of what form elements you will be interacting with so explicit invoking such as TexBox1.Invoke() or other element specific calls are not required.