Add timer-based signals to Vega

One of the constant asks I hear is how to do animations in Vega. Animations require timer-based event mechanism, so one solution is to add a signal that changes on timer. This issue is to discuss the best interface/approach. It doesn’t have to be implemented in core vega, but would be good to get everyone’s opinions.

Simple - name-based

This, somewhat hacky approach, allow addition of timers with the minimal Vega code changes. A Vega plugin could iterate over all signals, and if any signal names match /^timer_([0-9]+)$/, automatically update signal’s value with the new Date() object every (number) milliseconds. The timer can be paused/resumed by setting its value to truthy/falsy value.

signals: [{
  "name": "timer_1000",  // will be updated once a second
  "value": true
}]

Complex - core support

Extend core event parsing mechanism to allow parameter passing. This approach might be more flexible in some cases, but also makes it harder to suspect/resume timer as needed.

signals: [{
  "name": "myTimerSignal",
  "on": [{"events": "timer(1000)", "update": "..."}]
}]

[domoritz]

I believe we could use this to implement games in Vega 😃

[nyurik]

@domoritz good idea, but i think this shouldn’t be a core feature yet - maybe a plugin? Several reasons:

• WS hasn’t been standardized as a data transmission format - each site implements its own custom protocol
• I wouldn’t want Vega update to all of a sudden support a new protocol by default for security reasons. The less functionality is enabled by default, the more secure the product is. (only enable what you need - smaller attack surface).

As for Hans Rosling viz - agree, I think I misunderstood @g3o2 to mean streaming data support, not (looped) time animation.