Visual Basic 6.0

Monitor and Cancel a Running DKIM Operation

Demonstrates the Chilkat Dkim.HeartbeatMs and Dkim.AbortCurrent properties, which monitor and cancel a long-running operation such as a DNS public-key lookup. HeartbeatMs sets how often an AbortCheck event fires, and AbortCurrent requests cancellation.

Background: Most Dkim work is instant, but anything touching DNS can stall on a slow or unresponsive server. HeartbeatMs makes the object emit a periodic AbortCheck callback during such calls so an application stays responsive and can show progress, and AbortCurrent — set from that callback or from another thread — breaks a blocked synchronous call out early. Together they turn an otherwise uninterruptible operation into a cancellable one.

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Visual Basic 6.0

Dim success As Long
success = 0

'  Demonstrates the Dkim properties HeartbeatMs and AbortCurrent, which monitor and cancel a
'  long-running operation such as a DNS public-key lookup.

Dim dkim As New ChilkatDkim

'  Fire an AbortCheck event callback every 100 ms during method calls.  In programming languages
'  where Chilkat supports event callbacks, the callback can request cancellation.
dkim.HeartbeatMs = 100

'  AbortCurrent may be set to 1 to request cancellation of a running operation.  It is
'  typically set from another thread, or from within an AbortCheck callback.  Setting it 0
'  here ensures the operation is not pre-emptively aborted.
dkim.AbortCurrent = 0

'  PrefetchPublicKey performs a synchronous DNS lookup, which HeartbeatMs monitors and
'  AbortCurrent can cancel.
success = dkim.PrefetchPublicKey("myselector","example.com")
If (success = 0) Then
    Debug.Print dkim.LastErrorText
    Exit Sub
End If

Debug.Print "Public key prefetched."