In the first FAQ on Multithreading (FAQ796-5929) we went over a basic method of launching a thread. That method was fire and forget, and we couldn't pass any values. Delegates give us a lot more power in that we can pass in parameters and use call back methods. A Call Back is a sub that is called from when the new thread ends, it is run on the calling thread which means that we can access the GUI safely from it if the thread was launched from the GUI.
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click 'first, disable the button1 object 'so the user can't click it again Button1.Enabled = False del = New ProcessDelegate(AddressOf Me.Process) Dim cb As New AsyncCallback(AddressOf Me.ProcessComplete) del.BeginInvoke(cb, del) End Sub
'The Delegate definition Protected Delegate Sub ProcessDelegate()
'The delegate that will launch the 'process Protected del As ProcessDelegate
'The Callback method the will launch 'in the original thread when the thread 'completes Protected Sub ProcessComplete(ByVal ar As System.IAsyncResult) me.button1.enabled = true end sub
'The long process Protected Sub Process() For a As Integer = 1 To 10 For b As Integer = 1 To 100000000 'do something really 'important here Next 'simulate a progress bar '(This stuff should be handled in a 'thread safe manor, but this will 'work pre-2k5) Label2.Text = a.ToString Label2.Refresh() Next End Sub