Consider a more general key toggle method

[raquo]

In line with #89...

Currently we can toggle only composite attributes like cls:

cls.toggle("class1 class2")(myBoolean)
cls.toggle("class1 class2") := myBoolean
cls.toggle("class1", "class2") <-- myBooleanStream

Would be nice to be able to use similar pattern with other types of keys, but there's a challenge with styles – they use constants like display.none, the existing cls syntax doesn't have an equivalent in this case.

Maybe we should change it to be a postfix:

cls("class1 class2").when(myBoolean)
cls("class1 class2").when := myBoolean
cls("class1", "class2").when <-- myBooleanStream

colSpan(2).when(myBoolean)
colSpan(2).when <-- myBooleanStream

display.none.when(myBoolean)
display.none.when <-- myBooleanStream

Same question as in #89 regarding using default value vs prior value:

• Should when(false) do nothing, or set an "empty" / "default" value?
• What about if when <-- $bool emits false, what should happen then?
• Seems like it's going to be either weird or not super consistent...

[Lasering]

I have this in my codebase:

val active: Signal[Boolean] = ...
aria.current <-- active.map(if _ then "page" else "")

Using when it would probably become:

aria.current("page").when <-- active

In this scenario I would expect that when active emits false the aria-current should be removed from the element. Setting an empty/default value would also be appropriate.