Re: Async processing issue within an event handler

Hi James,

On 21/09/15 23:43, James Lynes wrote:
> I apologize for the length of this question in advance.
>
> Background: The July 2015 issue of Nuts and Volts magazine had an 
> article on using an inexpensive DVB-T USB dongle along with a 24MHz 
> upconverter as the basis for an HF and higher Software Defined 
> Receiver(SDR). 
Amazing, I didn't know there were such things as SDRs, although I did 
play around with the idea of using a microprocessor to act as a video 
capture card, which I guess is a similar idea.  So you're using a TV 
receiver as a radio receiver? Slightly inverted, but cool.
> The software used in the article was SDR# an MS-Windows application. 
> As I am an Ubuntu user, I looked for a similar Linux application. I 
> found gqrx. While waiting on parts to build the upconverter, I 
> installed gqrx-sdr from the Ubuntu Software Center. It ran and pulled 
> in various signals from 24MHz up, but was unstable. Turns out this was 
> a pretty old version and after uninstalling and then installing sudo 
> add-apt-repository ppa:gqrx/snapshots, a stable version was found.
>
> Well, gqrx has a Remote Control feature that can accept commands over 
> a Telnet connection and more digging turned up a perl 
> script(gqrx-scan) that uses this interface, through Net::Telnet, to 
> implement a scanner feature. gqrx-scan has many bells and whistles 
> that I wouldn't ever use so I wrote a lite/crude version(lite.pl 
> <http://lite.pl>) that just scans from F1 to F2 repeatedly. For more 
> flexibility(and practice) I decided to expand lite.pl <http://lite.pl> 
> into a wxPerl application(wxgqrxLite.pl), mostly a bunch of text 
> controls and sizers.
>
> Problem 1: lite.pl <http://lite.pl> is basically a while(1) loop 
> scanning from F1 to F2, listening for a second when a strong signal is 
> detected.
> Question 1: Where should the non-GUI stuff fit into a wxPerl application?
> Current Solution 1: I created two timer events and put the scan code 
> in one and the listen code in the other. They trigger each other 
> alternately at separate rates(30msec and 2 seconds). Only one of these 
> events/timers are active at a time. This actually works!
This depends on how decoupled you are.  If you are completely decoupled 
and just want an on or off signal (and I have a number of cases where I 
do this) then a parallel process will do it.  For instrance I have a 
process to check network availability.  Sometimes the network 
availability is mixed, like on the edge of a mobile or wifi signal, in 
this case wx::Timer will appear to hang and your process will freeze.  
If this is not what you want, use a separate process.  Here is an 
example where I use a temporary file with 1 byte in it.

    `echo 0 > /home/image/Documents/Endoscopia/DB/nw.txt`;     # Create 
base file with '0' (network not available)
                                         # if in case previous instance 
crashed with nw available
                                         # and it now isn't.

    $gl_nw_process_id_int=Wx::ExecuteCommand("perl iM_nw.pl", 
wxEXEC_ASYNC);    # Maintain network status file asynchronously.
.
.
.
call other stuff.
.
.
.
     # on exit or crash
    # Switch off nw monitor.
    Wx::Process::Kill( $gl_nw_process_id_int, Wx::wxSIGKILL() ) if 
$gl_nw_process_id_int;

I then use wx::Timer to reread the file and there is no pause.  It 
always returns 1 or 0.
>
> Problem 2: You can't start scanning unless the Telnet connection is 
> open, otherwise it throws a connection refused error.
> Solution 2: Add code to track scanning and connection status to 
> prevent the user from doing illegal operations or multiple consecutive 
> hits of the same button, and popup message boxes to flag these errors.
You can use a similar solution to the above.
>
> Problem 3: The error checks, interlocks and message boxes between the 
> {startbutton} and {stopbutton} events seems to be ok. The same cannot 
> be said for the {connectbutton} and {disconnectbutton} events. There 
> seems to be a race/async processing problem between establishing the 
> Telnet connection(which errors and should set the {tnerror} flag) and 
> the mainline event code. The three individual print Dumper 
> $self->{tnerror} statements all show {tnerror} = 0 (wrong!) while the 
> print Dumper $self statement below shows {tnerror} = 1 (right!). 
> There's a delay in this flag being set in the Net::Telnet call which 
> causes the rest of the {connectbutton} event routine to process a 
> wrong flag value.
> Question 3: HELP ????? I'm out of ideas...
Maybe a similar one a again?
>
> Question 4: Does event->Skip() terminate the current event handler 
> like last terminates a while loop?
You have Skip and Veto, Skip says continue and imagine I am not here.  
Eg if you pressed the escape key out of a dialog box,  if you have code 
there, the dialog box will not exit.  If you use ->Skip, it will, as it 
would as if there was no code there.

Veto says, undo what you just did and is not always applicable.

Neither of these says this routine over-rides and replaces the default 
behaviour.

(People, do you all agree?).
>
> Code attached. It will run without the dongle or gqrx installed. It 
> just fails on the Telnet connection.
>
> Thanks for your input.
Broadly speaking your choices are Wx::Timer, Threads and 
Wx::ExecuteCommand/Wx::Process

Good luck,

Steve
>
> James
> Ubuntu 14.04.2LTS/perl 5.18.2/wxPerl 3.0.1/HP 15 Quadcore Laptop
>