`beforeDraw` hook

[samselikoff]

It'd be nice to have a beforeDraw hook, a place to do chart-wide work not related to any particular layer (e.g. updating a scale's domain), but completely removed from the transform cascade.

Thoughts?

(I took a look at the library myself and was playing around with implementing, but figured I'd message here first.)

[jugglinmike]

Tell me if I'm interpreting your request correctly:

	transform	beforeDraw
argument(s)	chart data	none
return value	passed along to draw method of layers and attachments	ignored
invocation	all implementations on the prototype chain ("cascading")	the first implementation on the prototype chain (typical JavaScript)

Is that right?

[samselikoff]

Almost - I think beforeDraw should have the chart data as its argument, since it's related to drawing (and you draw with data, typically).

This way you could use the data to, for example, update your scale's domains.

[iros]

Sam, that is the intent of the "transform" method. It takes in the data as
the only argument and is called before any of the layers'/attached charts'
draw methods. It must return the data (possibly transformed) because that
is what will be passed to the layers/attached charts, but otherwise you can
do with it as you wish before it exists. The transform method is only
called once.

I use it traditionally to update scale domains, create layouts etc.
Perhaps it's the method name that isn't appropriate for what it actually
does?

-- Irene

On Mon, Apr 28, 2014 at 11:02 AM, Sam Selikoff notifications@github.comwrote:

Almost - I think beforeDraw should have the chart data as its argument,
since it's related to drawing (and you draw with data, typically).

This way you could use the data to, for example, update your scale's
domains.

—
Reply to this email directly or view it on GitHubhttps://github.com//issues/97#issuecomment-41568925
.

[samselikoff]

So there was a situation I ran into that prevented me from being able to use transform for this, and I think it was the cascade. Does a layer's or attached chart's transform method get called before or after the parent chart's?

I think I wanted a layer/attached chart's domain to override the parent's. When I have more time I'll go back and try to get what the exact use case was.

[iros]

That would be incredibly helpful Sam! Let us know when there's an example
of that, that we can look at.

-- Irene

On Mon, Apr 28, 2014 at 11:38 AM, Sam Selikoff notifications@github.comwrote:

So there was a situation I ran into that prevented me from being able to
use transform for this, and I think it was the cascade. Does a layer's or
attached chart's transform method get called before or after the parent
chart's?

I think I wanted a layer/attached chart's domain to override the parent's.
When I have more time I'll go back and try to get what the exact use case
was.

—
Reply to this email directly or view it on GitHubhttps://github.com//issues/97#issuecomment-41573767
.

[samselikoff]

@iros you can actually see the convo here: #78 (comment)

The problem was because transform is called upwards, so it doesn't behave as I would normally expect inheritance to (i.e. super is called last, instead of first).

But perhaps I need to rethink the hierarchy a la Mike's suggestion. Still, having a method that is reliably invoked last on a subchart could be useful.

[jugglinmike]

Another solution would be to build support for constructors into d3.chart (see #76). If it were clear how to access any given chart constructor, then we could remove both initCascade and transformCascade and instead defer to the user for prototype traversal. This would facilitate @samselikoff's use case here, since he could call Parent.prototype.transform(data) and then perform some additional work.

Removing the "cascading" behavior would be a pretty significant breaking change, and I don't suggest it lightly. But it may be that the -cascade methods are too clever for their own good, so I want to at least put that on the table.

@samselikoff In the mean time, you should be able to solve your problem with the refactoring I suggested in that issue.