Config mechanism for plug defaults

[andrewkaufman]

It should make sense to use the Metadata to specify the values, since that already implements a per-plug-per-node registry. The NodeMenu would apply them, and there'd be a simple method somewhere to allow the same to be done explicitly within a script.

[andrewkaufman]

I've got a commit with this working (referenced above), but I haven't made a pull request yet, because I don't like that the entire thing is in GafferUI.NodeMenu.

You can register a default like this:
Gaffer.Metadata.registerPlugValue( Gaffer.ExecutableNode.staticTypeId(), "dispatcher.batchSize", "defaultValue", IECore.IntData( 7 ) )

and it will be applied when creating the node via the menu. Scripts could also apply the defaults using GafferUI.NodeMenu.setPlugDefaultValues( node ) but as I said, I don't like it being tied to the GafferUI module. Do you have a better recommendation of where this function should live? On Metadata itself? Is "defaultValue" a reasonable key?

[johnhaddon]

I think "defaultValue" is a potentially confusing name, since we already have real default values, and what we're actually doing here is overriding them. Methods starting with "set" should always have a corresponding "get" method as well, so that name is a little confusing.

How about we call the metadata entry "userDefault"? And how about we make a new NodeAlgo.py file and add a method called applyUserDefaults() in it? We have various *Algo files now, and it seems like a pretty decent way of collecting various small utilities in a sensible place - it's also what the Imath library does, so it feels fairly coherent to me.

I think it's only a matter of time before we want this functionality in C++, but it's a bit of a pain to implement there, so I'm happy to keep it in Python for now.

Now we have user defaults, I think we can reconsider the Dispatcher registry mechanism again. The objection to registering creators rather than instances was that it prevented startup files from collaborating, each changing only some settings. Now, each file can set some user defaults, but not others, and we get the collaboration we want.