[RESOLVED] Threading, Invoking, Delegating and crazyness?

[richsmith]

Hello,
right, i think i need to use invoking and delegating and stuff, but i really cant get my head around it, i have searched and seen other posts and tutorials, but cant figure out how to do it in my project.

ok, i have class, which has a thread, and in this class in part of the code i want to get the webbrowser on form1 to refresh.

so would I put all the code in the class inside
Private Delegate Sub dont know what goes here

and something like:
form1.invoke (webbrowser1.refresh()) ??


hmm im confused

can anyone help?

thanks very much

[jmcilhinney]

You would create a method in your form that does what you want and then a delegate with the same signature. You then call Invoke on your form from another thread and pass a new instance of your delegate:

Private Delegate Sub DoSomethingDelegate()
 
    Private Sub DoSomething()
        'Do something here.
    End Sub
 
    Private Sub CallDoSomethingViaDelegate()
        Me.Invoke(New DoSomethingDelegate(AddressOf DoSomething))
    End Sub

If the method you want to invoke has no arguments or return value then you don't need to declare your own delegate:

Private Sub DoSomething()
        'Do something here.
    End Sub
 
    Private Sub CallDoSomethingViaMethodInvoker()
        Me.Invoke(New MethodInvoker(AddressOf DoSomething))
    End Sub

If the method you want to invoke does have arguments or a return value then you must declare your own delegate with the same signature:

Private Delegate Function DoSomethingDelegate(ByVal arg As String) As Integer
 
    Private Function DoSomething(ByVal arg As String) As Integer
        'Do something here.
    End Function
 
    Private Sub CallDoSomethingViaDelegate()
        Dim result As Integer = CInt(Me.Invoke(New DoSomethingDelegate(AddressOf DoSomething), New Object() {"string argument"}))
    End Sub

In your case, that means you need to define a procedure that calls WebBrowser1.Refresh() and then invoke it via a MethodInvoker instance, which is the easiest of all.

[richsmith]

i tried the middle bit of code, but i says that invoke is not a member of windowsapplication1.clsserver, do i need to delcare something or something?

and in the dosomething() sub is the code form1.webbrowser1.navigate("address") correct?

[jmcilhinney]

Invoke is a member of the Control class, so you need to call it on a control of some sort, like the form. I was assuming that the code would be called from the same form, which is why I used "Me", but it can be called from anywhere that you can see a reference to that from. The DoSomething method, or its equivalent in your code, would normally be in the same form as the WebBrowser control, so Me.WebBrowser1.Navigate would be appropriate.

[richsmith]

ok, i have moved to code to the form and call the CallDoSomethingViaMethodInvoker from my class, but now i have this error:
Invoke or BeginInvoke cannot be called on a control until the window handle has been created.

[jmcilhinney]

Can you post the code that starts the worker thread and the code that calls Invoke? Please show which class each section is in. Also, is your form visible at the time you call Invoke?

[richsmith]

yes form is visible.
in form1:

Private Sub DoRefresh()
        WebBrowser1.Navigate("http://www.delirious.co.uk")
    End Sub
 
    Public Sub CallDoRefreshViaMethodInvoker()
        Me.Invoke(New MethodInvoker(AddressOf DoRefresh))
    End Sub

in clsServer class:

Private Sub AcceptClient()
        Dim pendingClient As TcpClient
        Dim nameOfClient As String
        Dim IPofClient As String
        Dim refreshbrow As String
        pendingClient = Listener.AcceptTcpClient()
        reader = New System.IO.StreamReader(pendingClient.GetStream)
        nameOfClient = reader.ReadLine()
        IPofClient = reader.ReadLine()
        refreshbrow = reader.ReadLine()
 
        Clients.Add(pendingClient, nameOfClient)
        My.Computer.FileSystem.WriteAllText("C:\displog.txt", _
        Date.Now + " ID:" + nameOfClient + " IP:" + IPofClient + " - Connected" + vbCrLf, True)
        Clients.Clear()
 
        Form1.CallDoRefreshViaMethodInvoker()
 
    End Sub

thanks

[jmcilhinney]

The whole idea is that you cannot access a member of a control other than Invoke and the like across thread boundaries, and you are doing just that by calling CallDoRefreshViaMethodInvoker. You have to call the Invoke method from the second thread, so if the AcceptClient procedure is being executed in the second thread then you need call Invoke from there on an instance of the form.

[richsmith]

oh, right ok, its very confusing this, i think im being a bit thick, but how do i do an instance of the form?

[jmcilhinney]

The class that AcceptClient belongs to needs a reference to the form. How you get it one is up to you. Does this object get created by the form? If so, you can pass a reference to "Me" in the constructor. If not then you'll need to get a bit more creative. You may need to use a global variable but that should only be used if there is no other way to achieve it elegantly.

[r0ach]

Sorry, I haven't read too much of the post, but I have a question regarding threading, too.

I have my form that starts a class in a new thread. This new class starts, and keeps track of several other classes in their own threads. This class needs to notify it's "child-classes" that it's about to kill the their threads, so that they can perform some cleanup. How can I notify these classes? I had a property on the class that gets set to true by the calling thread, but it doesn't seem like they're receiving this. Any advice?

[jmcilhinney]

Sorry, I haven't read too much of the post, but I have a question regarding threading, too.

I have my form that starts a class in a new thread. This new class starts, and keeps track of several other classes in their own threads. This class needs to notify it's "child-classes" that it's about to kill the their threads, so that they can perform some cleanup. How can I notify these classes? I had a property on the class that gets set to true by the calling thread, but it doesn't seem like they're receiving this. Any advice?

This question is unrelated to the original question and should have been posted in a new thread.

If you want to terminate a thread from the outside you need to call Abort on it and then within the thread you handle the ThreadAbortException that this raises and do your cleanup then.

[r0ach]

Sorry, jmcilhinney. Just thought that since we are on threads, I'd pop it in there. Won't happen again. Thanks for the help, though.

[richsmith]

im still stuck on this whole referencing form1, i have tried:

Form1.Invoke(New MethodInvoker(AddressOf Form1.DoRefresh))

thats the only way i know how to reference, i think, im still kinda a beginner.

sorry for being stupid

[r0ach]

No, not quite like that.

Let's say you are starting a class in a new thread. You'd have the class's New() method looking like this:

Private m_frm As Form1
Public Sub New(ByVal frm As Form1)
  m_frm = frm
Next

Now, in the form, where you'd be starting the threads, you do this:

Dim MyObject As MyClass '// MyClass has the constructor above
MyObject = New MyClass(Me) '// instantiate the class, passing this form (Form1)
'// prepare thread
Me.Invoke(New MethodInvoker(AddressOf MyObject.ThreadProcedure))
'// ThreadProcedure is the Method in the class (MyClass) that will be running in it's own thread.

[jmcilhinney]

Your class needs a variable that refers to the form. You can't just use the name of the class, if that is what that code does. That's like having your name assigned to a String variable and trying to access by calling a member of the String class rather than the variable itself. Does the form create this clsServer object? If so, simpy rewrite the constructor to accept an argument of type Form1, which it can then save to a Private variable. The form would then pass "Me" as this argument and voila, the object now has a reference to the form that it can use to Invoke methods.

Edit:
Yeah, like that.

[richsmith]

i have attached my project, its gone a bit crazy now, couldnt get the code to work, just erroring, have tried putting it in other places.

[jmcilhinney]

I don't download projects unless I have a VERY compelling reason.

[r0ach]

Your first mistake was that you copied my sample JUST LIKE THAT into your project. Remove all my stuff from the form's code

At the top of your form change it like this:

Dim server As New clsServer(Me)

Now, in clsServer, you can access the webbrowser using the m_frm reference to Form1.

[richsmith]

right thanks roach, that made it more understandable now, and after a bit of fidling it works, thanks very much, thanks jm for explaing threading and invoking better then msdn explained it, sorry if i seemed a bit stupid.
thanks

[jmcilhinney]

Multi-threading is a complex topic and always takes some time to get your head around, even for the experts.