[Rock-dev] improvement TaskContext.get

[Sylvain Joyeux]

On 11/03/2011 03:59 PM, Alexander Duda wrote:
> I agree that this behavior is not useful for starting components on the
> robot but for a user interface it is a little bit different. And by the
> way the method port is playing the same game.
>
> def port(name, verify = true)
It's not meant to be a public fact. The reason lies in the blame:

====
allow to bypass the port existence check in TaskContext#port

This is to be used in disconnection code: someone can do a
output_port.disconnect(input_port) while the input port does not exist 
anymore
to remove the dangling connection
====

>> I also don't like the caching behaviour ... I'm not sure of seeing all
>> the implications of it, and that's a too important method to play that game.
>>
>> Here's what I would propose: implement TaskContext.find(expression)
>> which returns nil if the task cannot be found (but no caching !).

> What's bad with caching?
If not done properly, it can hide that things changed. In the port case, 
you would start having a problem if the task unregisters a port and then 
re-registers a new one with the same name. It is actually not only 
possible, but problematic.

I think you have a point: the #port method should be refactored to (1) 
not cache anymore and (2) still allow the disconnection behavior 
mentioned in the blame.

> task.ping is verifying if the task is still valid. If this kind of
> caching is a problem all scripts will run into this because they all
> cache tasks,ports etc.
Yep, but at least it's not hidden deep down in the framework. As I said: 
I'm being careful here, as I'm usually very wary of caching behaviours.

> by the way the corba name service is heavily caching as well.
??? Where ?

In general, for the UI stuff, you should maybe consider creating a proxy 
class. It could even handle the #connect() method to create 
"name-to-name connections" in vizkit.

class TaskProxy
    def initialize(task_name)
       @__task_name = task_name
    end

    def method_missing(m, *args, &block)
        if @__task && !@__task.ping
           @__task = nil
        end

        @__task =
           begin TaskContext.get(@__task_name)
           rescue Orocos::NotFound
           end

        if !@__task
            return
        end
        @__task.send(m, *args, &block)
    end
end
-- 
Sylvain Joyeux (Dr.Ing.)
Space & Security Robotics

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