[fpc-pascal] Re: Library callback : how do I do that?

Lukasz Sokol el.es.cr at gmail.com
Thu Aug 8 13:07:09 CEST 2013

On 08/08/13 11:52, Michael Schnell wrote:
> On 08/08/2013 12:34 PM, Lukasz Sokol wrote:
>> Hello,
>> I am making a Pascal wrapper for a library, which states in its API
>> description:
>> " Note that the Callback function will be called from an
>> independent child thread, not from the ApplicationÂ’s Window thread.
>> It should not therefore be used to draw data directly to a Windows
>> screen. The safest use of the callback is to use a threadsafe
>> interface (such as Windows messaging) to pass the message to
>> Windows for handling by the Windows thread. "
> AFAIU: The callback is just a callback. So you can handle it in the
> normal way (either in an application, in a command line tool without
> a main form or in a dll.) You just can't do any GUI stuff in that
> callback.
> I understand that you in fact don't want to do that, anyway.
> -Michael

Oh, I missed the whole point ... sorry.

in fact I DO want to be able to call gui-interfacing functions of the 
form. The library I am loading (dynamically) will be giving me callbacks
when something changes in an external USB device...
When that happens, I want to be able to do e.g.

MyObject.OnStateChange := TForm1.ShowStateChange;

and e.g. safely call ShowMessage or display a custom dialog box,

(as well as I will be calling procedures/functions of another non-vis object(s)
doing something else with the data provided by callback)

So it seems, if I call my objects' InternalCallback function from the
installed callback, then it's my object that has to care for thread safety
to the GUI then ?

(Henry Vermaak mentions QueueAsyncCall, going there...

Kind Regards,

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