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RE: [platform-ui-dev] Difference between Action and Command
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Glad
you asked...
I
always considered an "action" as something attached to the user
interface that responds to user interaction, while a "command" is an object that
encapsulates a discrete change against the model.
Generally, the two work together quite nicely; a single instance of an
action is attached to a component like a menu or assigned to a keypress, and
that action will create instances of commands and run them.
I
haven't created many eclipse "actions", but it feels pretty consistent
with the above to me.
I
just skimmed the "Contexts" prop page, and I'm a bit concerned that it's
merging the two above (and quite distinct) concepts. At its most basic, an
"action" would just be a "command factory", simply creating an instance of a
command. For these types of commands, the "Contexts" prop would work fine, as
it's just eliminating the middle-man action.
[NOTE:
Please let me know if I misinterpreted, as I only skimmed it rather
quickly]
However, consider two things: overhead of command object creation, and
inflexibility.
First,
taking this approach, a new command instance would need to be created for each
user interaction. While in many cases this is what we need, as the object needs
to be put on the undo stack, there are plenty of actions that don't need to be
"undoable", such as "find" (arguable..., but get my drift?)
Second, and more importantly, is inflexibility. While many actions would
simply be "command factories", I don't think that can be said for all
actions. I tend to view command objects as acting against a single model; an
action may need to affect multiple models. While we could create a composite
command that encapsulates a multi-model update (with separate commands for
each), that forces us into a specific strategy for "undo".
Granted, there may not be many apps that need the extra flexibility, but
it should be there IMHO. We can create a simple "command factory" action that
can be used all over the place to create certain instances of commands (and
perhaps the "binding" mentioned in the contexts prop could be implemented using
this), but I think we should still leave it open that an action
can be bound as
well, only creating the single instance of the action object that can create
commands or perform other tasks.
Don't get me wrong -- I really like commands. I just feel
this prop is merging two concepts that should remain independent, but encouraged
to be used together.
-- Scott
In the recently
announced "contexts" proposal:
"As Eclipse moves from an action-centric model to
command-centric model, action sets would disappear entirely"
To me
"action" and "command" are the same thing. What is the difference here
between a command and an action? Where can I read more on the new
"command" stuff?
Won't this be
confusing since the term "command" is widely used to mean a change which can
be undone and redone? There is also a bugzilla open tracking the
proposal to add undo/redo to the workbench.
-Randy
