[Lazarus] Dispatching GUI messages from background threads

[David W Noon]

On Sun, 2009-01-11 at 01:17 +0100, Hans-Peter Diettrich wrote:

> David W Noon schrieb:
> 
> > This looks rather like some sort of deadlock on something like a mutex
> > semaphore -- but that's just a guess. Is there some "knack" to calling
> > Dispatch() that avoids this problem?
> 
> The solution is simple: don't use Dispatch() from a thread, use the 
> thread-safe SendMessage instead.

The only function like that I could find was SimpleSendMessage(), which
produced the same result as Dispatch() and DeliverMessage(), but without
supplying any message data. In which unit is SendMessage() defined?

I finally resolved the issue by changing the pointer to the form into an
object reference to a TObject. After that, Dispatch() worked as
documented (such as it is documented).

Moreover, the action I needed was a "post", not a "send". It is the
"send" style of message transfer that causes deadlocks between the
worker thread and the user interface thread. This is because "send"
waits for a return code from the message handler, whereas "post" just
places the message on the queue and proceeds regardless.

-- 
Regards,

Dave  [RLU #314465]
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david.w.noon at ntlworld.com (David W Noon)
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