How to support signals in dl.template

[arvind]

Right now, the function produced by dl.template.source take a single argument for the data object to parse the handlebars against. In Vega, signals are not namespaced, and I'm wondering how best we should support them with template. Some ideas:

• Use dl.extend to pass dl.template an object containing both item.datum properties and signals.
• Switch the dl.template function to take vargs, and iterate through them until the property is found.

I prefer the second option, as running dl.extend on every tuple every time an encoder is overly expensive.

In either case, I also need to know which signals are being used within a template expression, in order to compile the signal values and send them into the encoder. i.e., I need from dl.template a list of the handlebar strings used.

[jheer]

Collecting the properties accessed is straightforward, so that's not problem. The API I am thinking of is to pass a hash as an additional optional argument to source. If provided, the hash will be populated with all the accessed properties.

I agree that running dl.extend would be overkill. However, I'm reticent to add the varargs functionality as it will complicate the internal logic in ways I'd prefer to avoid.

An alternative might be to "wrap" the signals and datum in a shared object that you can reuse (no new object allocations): var ctx = {}; ctx.signals = signals; ctx.data = item.datum; template_fn(ctx).... Of course, you would only create ctx once. The complication is that the template variables will need to be updated with the correct access paths due to the name spacing.

In any case, how were you planning to differentiate data variable names from signal names? We must use some form of name spacing otherwise collisions might occur.

[arvind]

So, following up our discussion yesterday of handlebar syntax to support datum, group, parent composition and de-referencing, here are some ideas. Whatever syntax we settle on, the thing I need support for in datalib is finding/parsing properties within [ ].

dl.template.source("{{datum[signalA.foo]}}", "hello", props={})
props = { 'datum[signalA.foo]': 1 }; // should be {'datum': 1, 'signalA': 1}

Alright, so CSS doesn't have parent selectors but proposals have coalesced around two options: parent < child and :parent. I personally prefer the latter because it is difficult for me to parse the order/result of <. So, for dl.template support in Vega, I propose this:

{{signalA}}
{{datum.price}}
{{datum[:parent.A]}}
{{:group.width}}

And of course, they can be chained like so: :parent:parent.a or :group:group.width. Thoughts?