Friday, January 12, 2007

Callback functionality for COM/ActiveX objects in LotusScript

Unfortunately there is no such callback functionality in Lotus Notes. The only type of callback which developers can use are for Notes native objects.

The only solution I found is to periodically poll a variable from COM object and when the variable's value is changed from the previous poll value, it means something happened and you can take an appropriate action in LotusScript.

For example, you have an ActiveX with function which returns statuses "Busy", "Idle", "In progress", "Finished" for a telephony application. You can not make callback when status changes to immediately notify LotusScript, but you can poll status every 1 second using LotusScript:

Dim status as String
Set MyPhone=CreateObject("PhoneApp")
Call MyPhone.MakeCall("+1818222333444")
status=MyPhone.GetStatus()
While status<>"Finished" And status<>"Idle"
status=MyPhone.GetStatus()
if status="In Progress" then
Print "Call is in progress, continue looping."
end if
if status="Busy" then
Print "Phone is busy, status will change to Finished automatically soon."
end if
Sleep 1
DoEvents
Wend
Call LogCall(StartTime, EndTime, PhoneNumber)


This solution works pretty well in most cases. Make sure you have Sleep and Doevents in your code, otherwise Notes client will hang until the loop is finished.
To avoid locking Notes client while code is running, set agent's property "Run in background client thread" and user will be able to continue working with Notes client even if the agent is still running.

A similar solution was described in my article in The View magazine for working with Skype API.

To make a shorter delay time than 1 second, you can use Sleep 0.5

Another work-around is to use Visual Basic as a wrapper to the COM object.
Visual Basic supports all callbacks, so compiling a Visual Basic project into ActiveX component and exposing variables which are populated from the callback functions inside VB code makes it possible to have access to callbacks which are not accessible with the technique described above.

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6 comments:

Anonymous said...

hi andrei, i'm trying to implement callback using vb as my activeX dll but i'm having difficulty implementing it in lotusscript. it my activeX dll works perfectly when called from vb but not in lotussript. i would really appreciate if you could help me. thanks

roel

Andrei Kouvchinnikov said...

Send me your code and I'll take a look at it.
Your ActiveX dll should expose a variable which shows a certain state (1,2,3,4) and which is possible to be queried from LotusScript. There can also be some caching problems so that LotusScript can not get the latest state value.

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