[Ovmsdev] Addition to OVMS Developer Guide

Greg D. gregd2350 at gmail.com
Sun Jan 3 11:15:43 HKT 2021

Steve, Michael, et al,

So I see the mention of obdii and it got me to pay attention...  :) 

Regarding obdii "complexity"...  I've always wondered why the obdii
command only has 'ecu' as a second and only parameter.  If obdii is
essentially the command front-end to the obd2ecu task, that split is
odd.  What else was considered for the obdii framework?

A few months ago there was a discussion on adding a bus speed parameter
to the obd2ecu task start command.  In general I'm not a fan of
positional parameters, as they're not self-documenting, and parameter
skipping is always hard to deal with.  I also note that in terms of
strings, there's no real difference between '-s', '--speed', and
'speed'.  They're just strings to be matched.  The value associated with
that parameter, if any, would be included after a bit of white space. 
So, the proposal for adding a speed to the can bus selection would look
like 'obdii ecu start can3 speed 500' (with kbits/sec implied). 
Alternatively, I can't imagine what else we'd add to that command, so
just putting the '500' in there without an option prefix would work
too.  So, the use of option prefixes would be, um, "complexity driven"?

All this said, I'm coming in from not really following where the command
line parser discussion started. 


Stephen Casner wrote:
> Michael,
> On Fri, 1 Jan 2021, Michael Balzer wrote:
>> Positional optional parameters don't grow well for multiple options, force
>> users to a fixed argument scheme and tend to produce hardly readable commands.
>> I also think command options are for experienced/advanced users, and those are
>> familiar with the concept.
> As I said, I agree that there are some situations like obdii where the
> variability of operations is too complex for positional optional
> parameters.  However, I don't consider a fixed argument scheme to be a
> bad thing for many of our commands.  Do you consider the scheme I've
> set up for "location action" to be hardly readable?
>> Extending the parser to support command argument templates would be an option,
>> but I had the impression that would add a lot of complexity, and would only
>> benefit a small set of (advanced) commands.
> I agree that it makes sense to implement the flag options directly for
> those commands where it is appropriate.
>> Agreed for the case of a single option and a command that's not supposed to
>> grow any new features. I did so on some of the CANopen commands. But imagine
>> we need to add another numerical option to "event raise": nice and clean by
>> prefix options, a usability nightmare with positional args.
> Depends whether the options are truly independent.
>> I wasn't aware of the tab completion on "event raise"... I normally use the
>> command to raise custom events only. I'll fix that.
> Are those custom events not registered?  RegisterEvent() adds events
> to m_map which makes them available for tab completion.
>>> With the flexibility to have intermediate parameters now, using the
>>> validate function, it may fit reasonably to have the options handled
>>> by a subtree.
>>>                                                          -- Steve
>> Do you mean a secondary option subtree triggered by the "-" prefix, inherited
>> to the regular command subtree?
>> That would allow simple tab completion on options as well... sounds like a
>> nice idea.
> What I was referring to was that when there are multiple options
> needing their own argument schemes that can be handled by the subtree
> of OvmsCommand nodes that can follow an intermediate parameter, as in
> the "location action" command.
> Tab completion wouldn't provide any benefit for single-letter option
> flags, but if options were words, as is the case for the different
> actions that can happen on a location, then it's useful.
> I'm not sure if you were thinking of detatched command subtrees.  I
> considered that idea when designing the validation scheme to allow
> intermediate parameters, but decided against it because I did not
> figure out a way to implement it cleanly including linking back up to
> the root for dynamic generation of the usage message.
>                                                         -- Steve
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