Re: no exit!
Re: no exit!
- Subject: Re: no exit!
- From: Christopher Nebel <email@hidden>
- Date: Thu, 5 Jun 2008 12:12:20 -0700
On Jun 3, 2008, at 3:14 PM, Joshua Whalen wrote:
This routine controls a fade-out timer for itunes. It works nicely
most of the time, but on slower machines, accumulated latency will
cause it to run a second or two too long, with catastrophic results:
The new playlist starts, sets the volume to 100, and the fader never
reaches zero, thus causing the playlist to fadeout on launch. Not
good.
To compensate for this, I added a variable and a check for the
variable's status. The playlist sets NewShow to true on launch, and
the fader sets it to false, and then checks on each loop to see if
it's true or false. If it's true, it should quit the loop. There's
just one problem: It doesn't.
I have tried setting it as a property that defaults to false, I have
tried setting it as a property that defaults to true. I have tried
putting the check in different parts of the loop, I have tried
putting multiple checks all through the loop, I have tried all kinds
of variations, it never gets the variable status.
The basic problem is that the second script invocation fires while the
first one is still running, and they start stomping on each other's
changes. The usual solution is to create some sort of "semaphore"
that allows the two invocations to communicate state and keep out of
each other's hair. That much you already figured out.
The trick is, script properties make terrible cross-process
semaphores. You're assuming that the property value is shared between
different simultaneous executions of the script, and that isn't the
case. Properties (and, by what I believe to be an accident, global
variables) are "persistent", that is, their values are written back to
the script file, but not until the script finishes running, and the
whole point here is that the first script *hasn't* finished running yet.
If you're going to use a semaphore, it needs to be in some location
that's accessible to both processes, and, more to the point, if one
process changes it, the other process needs to be able to see the
change. The old-school way to do this is to write a temporary file
somewhere and then delete it when you're done -- the existence of the
file itself can be the semaphore. Another possibility would be to use
a user preference (a la "defaults write" or some such).
Alternatively, it might also be possible to write your script to be
less vulnerable to interference. For instance, your fader could check
if the volume is still the last thing it set it to. If it's not, then
obviously someone else is changing the volume, and it should quit
trying to fade. Or you could write an actual application to do the
scheduling instead of making cron(8) do it, and then you aren't
obliged to run each script as a separate process.
--Chris Nebel
AppleScript Engineering
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| References: | |
| | >no exit! (From: Joshua Whalen <email@hidden>) |