Re: Foreach and add_watch not working as I would expect

["Mike Martin" <redtux1 googlemail com>]

On 5/13/06, muppet <scott asofyet org> wrote:
> On May 12, 2006, at 10:14 AM, Mike Martin wrote:
>
> > I am having problems with using IO::Watch Gtk2::Helper etc under a
> > foreach loop.
> >
> > For some reason it will only run the last element of the array. Here
> > is example code.
> [...]
> > All that seems to happen is that if there are two elements of the
> > array then two identical scrooll-windows open running the last $var.
>
> - In watch2(), you're using IN as the file handle.  Remember that
> file handles like this are global in perl.  Although you're grabbing
> the fileno(IN) when installing the watch, you're still reading from
> IN; if watch2() is run more than once, IN will always have the value
> of the last file opened.  So, your watch handler will fire for the
> old fileno, but you'll read from the new file handle.  That's why you
> always want to use lexical file handles, e.g.
>
>      sub watch2 {
>          open (my $in, ...) or die ...;
>          Glib::IO->add_watch (fileno ($in), ['in', 'hup'], sub {
>              # Since $in is a lexical in the enclosing block, this
>              # closure will trap the correct $in.
>              sysread $in, ...;
>          });
>      }


thanks that worked

> If using the closure to trap $in makes you uncomfortable, you can
> pass $in as user data to the watch handler, like this:
>
>          Glib::IO->add_watch (fileno($in), ['in', 'hup'],
> \&watch_handler, $in);
>      ...
>      sub watch_handler {
>          my ($fileno, $condition, $in) = @_;
>          # $in is the user data passed to add_watch(), which in this
> case is our file handle.
>      }
>
>
> - The first thing your watch handler does is check for $condition eq
> 'hup'.  However, $condition is a bitfield, which may contain more
> than just 'hup'.  A better way to do this is
>
>      sub watch_handler {
>          my ($fileno, $condition) = @_;
>
>          if ($condition >= 'in') {
>              # handle input data...
>          }
>          # note: *not* mutually exclusive!
>          if ($condition >= 'hup') {
>              # got a hang up -- clean up and uninstall.
>              return FALSE;
>          }
>          return TRUE;
>      }
>
>
>
> - In your watch handler, you're connecting to "insert-text".  That
> means, that you'll connect a new signal handler to the buffer every
> time a new chunk of data arrives on IN.  If it takes 50 reads to get
> all the data from IN, you'll wind up with 50 signal handlers piled up
> on $buffer.  This is not what you want.  Three alternatives:
>
>     1.  Install the insert-text handler outside of the watch handler,
> e.g., after creating the buffer.  This will leave the handler
> installed even after you're finished with the read loop, which is
> probably not what you want.
>     2.  Scroll to mark directly after inserting.  This is the
> simplest.  However, if the view actually updates in an idle, then the
> scrolling will not quite work right.
>     3.  Immediately after reading some data in the watch handler,
> install a single-shot idle handler to scroll to the mark.  This is
> like #2, but gives the view a chance to recalculate itself first.
>
> - The same thing applies to the mark; since you create the mark in
> the watch handler, you're piling up marks every time there's a chunk
> of data.  With this one, the best solution is just to create the mark
> immediately after creating the buffer.
>
>
> --
> Package contains eight 13-inch aliens in assorted colors.
>   -- Catalog copy for space invaders wall decals.