Creating a Wait For Event

[Cheetah]

Hi there,

I have created a class, and I want to create something so that it "waits" for an event to be raised and then continues.

Is it possible to do this?

Thanks.

 

[JohnH]

Subscribe to the event and use a waithandle (Manual/AutoResetEvent) to signal the waiting method which would call WaitOne on this. Caller probably have to run it asynchronous (different thread). Alternative is a thread sleeping loop that waits to see if for example the event handler sets a Boolean flag.

 

[Cheetah]

Choose the first method as I felt the second method wouldn't work for my situation.

First method working as I thought.

For anyone interested - this explains it well: http://www.developerfusion.co.uk/show/5184/3/

Thank you, once again.

 

[Cheetah]

I am trying to create a web browser on a new thread and I get this - is there a way to solve this?

System.Threading.ThreadStateException was unhandled
  Message="ActiveX control '8856f961-340a-11d0-a96b-00c04fd705a2' cannot be instantiated because the current thread is not in a single-threaded apartment."

 

[JohnH]

SetApartmentState(STA) on the thread before you start it.

 

[Cheetah]

The events don't see to be raised for the webbrowser when i put it on a new thread.

Imports System.Threading

Public Class Form1

    Dim WaitEvent As New AutoResetEvent(False)

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        Dim thr As New Thread(AddressOf StartBrow)
        thr.SetApartmentState(ApartmentState.STA)
        thr.Start()

        WaitEvent.WaitOne()

    End Sub

    Private Sub StartBrow()

        Dim webBrowser As New WebBrowser
        webBrowser.Navigate("Http://www.google.com")

        AddHandler webBrowser.DocumentCompleted, AddressOf Web_Document_Completed


    End Sub

    Private Sub Web_Document_Completed(ByVal sender As Object, ByVal e As System.Windows.Forms.WebBrowserDocumentCompletedEventArgs)

        MessageBox.Show(e.Url.AbsolutePath)

        WaitEvent.Set()

    End Sub

End Class

EDIT:

What I am trying to achieve is for it to wait for the document completed event to be raised before it continues.

 

[JohnH]

You are probably also preventing the DocumentCompleted event for the browser when blocking the UI thread. Add this code after you start the thread and you will see what happens:

thr.Join()
MsgBox("joined")

I would in thread instead loop for ReadyState=Complete after the Navigate call instead of using the event, you also need Application.DoEvents() in this loop for the browser control to update. But why don't you just create the control in UI thread and handle the event there?

A general side note: add the event handler before you call the method that cause it to be raised, else you could very well miss it.

 

[Cheetah]

You are probably also preventing the DocumentCompleted event for the browser when blocking the UI thread. Add this code after you start the thread and you will see what happens:

thr.Join()
MsgBox("joined")

I would in thread instead loop for ReadyState=Complete after the Navigate call instead of using the event, you also need Application.DoEvents() in this loop for the browser control to update. But why don't you just create the control in UI thread and handle the event there?

A general side note: add the event handler before you call the method that cause it to be raised, else you could very well miss it.

The thread.join didn't seem to work.

I wasn't aware of the Application.DoEvents method, I am going to try it that way now. I will report back soon.

Thanks.


EDIT:

What you said works - thanks very much.

I simply have this on the same thread now:

        Dim web As New WebBrowser
        web.Navigate("Http://www.google.com")

        Do Until web.ReadyState = WebBrowserReadyState.Complete

            Application.DoEvents()

        Loop

        MessageBox.Show(web.DocumentTitle)

 

[JohnH]

No, thread.join doesn't work, like I said you add that code to get the understanding of what happened with you previous code and why it didn't work. I you didn't see it; the other thread ended and the browser instance ceased to exist before the document was loaded, then UI thread was blocking on waithandle that was never set.

 

[Cheetah]

No, thread.join doesn't work, like I said you add that code to get the understanding of what happened with you previous code and why it didn't work. I you didn't see it; the other thread ended and the browser instance ceased to exist before the document was loaded, then UI thread was blocking on waithandle that was never set.

Ah right, so the thread closes when the code has finished executing.

So in my case the thread (thr) would have closed after I had added the handler.