So I guess another question is where "handle-command" is being called. What
about providing the component as argument to the multimethod (there are
other approaches too depending on your need). So:
(defmulti handle-command(fn [component command] (first command)))
(defmethod handle-command :add-customer [{:keys [db eventstore} [[_
customer]]]
... )
Isn't something like this enough?
Jeroen
On Wed, Feb 25, 2015 at 1:34 PM, Colin Yates <[email protected]> wrote:
> Hi Jeroen - sure.
>
> I have a gateway which can handle a number of 'commands', let's say:
>
> AddCustomerCommand
> ExportQueryCommand
>
> to handle the AddCustomerCommand I need access to a database and an
> eventstore. To handle ExportQueryCommand I need access to the
> database, a PDF service and so on.
>
> One way of achieving this is (assuming commands are of the form
> [:command-discriminator payload]:
>
> (defmulti handle-command first)
>
> (defn add-customer [db eventstore customer]
> ...)
>
> (defrecord AddCustomerComponent [db eventstore]
> ...
> (start [this] (defmethod handle-command :add-customer [[_ customer]]
> (add-customer db eventstore customer))..)
>
> (defn export-reports [db pdf-generator]
> ...)
>
> (defrecord ExportQueryCommand [db pdf-generator]
> ...
> (start [this] (defmethod handle-command :export-query [[_]]
> (export-query db pdf-generator))..)
>
> and so on.
>
> Fundamentally my question is one of dispatching where the target of
> the dispatch requires collaborators that aren't part of the dispatch
> payload.
>
> Does that clarify?
>
> On 25 February 2015 at 12:11, Jeroen van Dijk
> <[email protected]> wrote:
> > I would try to stay away from any nested def's in components (but also in
> > other functions). I'm using component a lot and I never had the need for
> > this.
> >
> > Maybe you can elaborate on why you think you need a multimethod inside
> the
> > component? Maybe a full example?
> >
> > Cheers,
> > Jeroen
> >
> >
> >
> > On Wed, Feb 25, 2015 at 12:08 PM, Colin Yates <[email protected]>
> wrote:
> >>
> >> Hi,
> >>
> >> I have a number of commands flying around which need to be handled.
> >> defmulti/defmethod seem a nice answer. The problem is that each command
> >> handler will need different collaborators (those with long memories will
> >> realise this isn't new ground here ;)).
> >>
> >> I want to do something like:
> >>
> >> (ns one)
> >> (defmulti handle first)
> >>
> >> (ns two)
> >> (defrecord CommandAHandler [collab-1 collab-2]
> >> component/Lifecycle
> >> (start [this]
> >> (defmethod handle :command-a ....
> >> (collab-1 ...)))
> >>
> >> I realise that the lifecycle of defmethod doesn't match the component
> >> (i.e. stopping can't unregister the defmethod), but my main reservation
> is
> >> that a nested defmethod feels a bit weird. Has anyone done anything like
> >> this before?
> >>
> >> Thanks!
> >>
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