VB.NET

Async Task Chain

See more Async Examples

Demonstrates how to combine a sequence of asynchronous tasks into a single task chain to be run in a background thread. VB.NET

Dim success As Boolean = False

' This example assumes the Chilkat API to have been previously unlocked.
' See Global Unlock Sample for sample code.

success = False

Dim mailman As New Chilkat.MailMan

' Set the POP3 server's hostname
mailman.MailHost = "pop.example.com"

' Set the POP3 login/password and any other requirements..
mailman.PopUsername = "myLogin"
mailman.PopPassword = "myPassword"
mailman.PopSsl = True
mailman.MailPort = 995

' Connect to the POP3 server:
success = mailman.Pop3BeginSession()
If (Not success) Then
    Debug.WriteLine(mailman.LastErrorText)
    Exit Sub
End If


' Get the number of messages in the mailbox.
Dim numMessages As Integer = mailman.GetMailboxCount()
If (numMessages = 0) Then
    Debug.WriteLine("No email messages in the POP3 mailbox.")
    Exit Sub
End If


Dim taskChain As New Chilkat.TaskChain

' Create async task objects to fetch each email message
' by its sequence number
Dim i As Integer
Dim task As Chilkat.Task

For i = 1 To numMessages

    task = mailman.FetchByMsgnumAsync(i)
    If (mailman.LastMethodSuccess = False) Then
        Debug.WriteLine(mailman.LastErrorText)
        Exit Sub
    End If


    ' Append each task to the task chain.
    success = taskChain.Append(task)
    If (Not success) Then
        Debug.WriteLine(taskChain.LastErrorText)
        Exit Sub
    End If




Next

' At this point, no tasks have actually started running.
' All we've done so far is to create tasks for the work that will be done
' when the task chain is run.

' Start the task chain running in a background thread.
' Each task is run one after the other (on the same background thread) until all tasks have completed.
' The task chain will stop at the first task that fails.
taskChain.StopOnFailedTask = True
success = taskChain.Run()
If (Not success) Then
    Debug.WriteLine(taskChain.LastErrorText)
    Exit Sub
End If


' The application is now free to do anything else
' while the emails are being downloaded

' For this example, we'll simply sleep and periodically
' check to see if the taskchain if finished. 
While taskChain.Finished <> True

    ' Sleep 100 ms.
    taskChain.SleepMs(100)

End While

' A finished task chain could be one that was canceled, aborted, or truly finished.  

' If the task chain "completed", then it ran to completion.  A "completed" task will
' have a StatusInt equal to 7.   If the task finished, but was not completed, then it must've
' been aborted or canceled:
If (taskChain.StatusInt <> 7) Then
    Debug.WriteLine("Task did not complete.")
    Debug.WriteLine("task chain status: " & taskChain.Status)
    Exit Sub
End If


Dim email As New Chilkat.Email

Dim numTasks As Integer = taskChain.NumTasks
Dim taskIdx As Integer = 0

While (taskIdx < numTasks)

    task = taskChain.GetTask(taskIdx)

    ' Load the email object with email downloaded by this task.
    success = email.LoadTaskResult(task)
    If (Not success) Then
        Debug.WriteLine("Failed to load email object for task.")
    Else
        Debug.WriteLine(email.From & "; " & email.Subject)
    End If




    taskIdx = taskIdx + 1
End While