Visual FoxPro

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 FoxPro

LOCAL lnSuccess
LOCAL loDkim

lnSuccess = 0

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

loDkim = CreateObject('Chilkat.Dkim')

*  Fire an AbortCheck event callback every 100 ms during method calls.  In programming languages
*  where Chilkat supports event callbacks, the callback can request cancellation.
loDkim.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.
loDkim.AbortCurrent = 0

*  PrefetchPublicKey performs a synchronous DNS lookup, which HeartbeatMs monitors and
*  AbortCurrent can cancel.
lnSuccess = loDkim.PrefetchPublicKey("myselector","example.com")
IF (lnSuccess = 0) THEN
    ? loDkim.LastErrorText
    RELEASE loDkim
    CANCEL
ENDIF

? "Public key prefetched."

RELEASE loDkim