`tinytheme`: Safer hook handling

[vincentarelbundock]

There is a discussion on the evaluate repo about hook handling in tinytheme(). Moving here to discuss the tinyplot-specific issues, instead of polluting everyone's mentions in the other repo. Tag @grantmcdermott

The tinytheme() design uses a before.plot.new hook to set persistent theme parameters. In principle, hooks can be specified as a list, and many can be specified and applied sequentially before drawing the plot. However,

1. Most of the time tinytheme() wants to reset parameters (i.e., delete existing hooks) to default values before applying its own.
2. AFAICT, there is not a great mechanism in R to "tag" or "name" elements of the hook list, that would enable us to identify the tinyplot-specific hooks.

Because of 2, we cannot filter the hook list, that is, we cannot only remove those that we know for sure we have set. Instead, we call:

setHook("before.new.plot", NULL, "replace")

This removes ALL hooks, including some that might have been set by other packages, such as evaluate.

I don't remember all the details, but I do remember trying many alternatives to this when implementing. All had really fiddly edge cases, with theme elements not being reset and such. Frankly, I think this is above my pay grade, as I'm not that great with R graphics.

[grantmcdermott]

Thanks Vincent. I won't be able to look into this for a few days, but hopefully we can use Hadley's evaluate tip as a starting point.

[vincentarelbundock]

#425