Re-evaluate signal's current value when restarting signal

[raquo]

val state =  Var(0)
val x10signal = state.signal.map(_ * 10)
val x100signal = x10signal.map(_ * 10)

Currently, when you first start x100signal, it initializes its initial value (not a typo – signals' initial values are lazy) by pulling the current value of x10signal, which initializes the initial value of x10signal by pulling state.signal's current value. However, if afterwards you stop x100signal, then after some time start it again, this pulling-current-value chain will not happen, x100signal will simply start listening to new x10signal updates from this point on. This means that if state was updated while x100signal was stopped, x100signal will not receive this update when it's started again.

This is documented behavior – observables are lazy, so you shouldn't stop the ones you care about – but it would be more useful if x100signal could recursively pull its parent's current value not just when initializing the signal's initial value, but every time it's started.

This would have convenient practical effects in Laminar when re-mounting unmounted elements – those would be brought up to date when they're re-mounted.

However, very importantly, this measure can't cancel out laziness per se, and specifically this can not possibly replay the EventStream events that fired while the signal was stopped, so we will not be able to pull an updated current value for signals that depend on event streams, such as stream.foldLeft(...) or stream.startWith(...).

This caveat will make undesired behavior more rare, but more specialized and harder to debug. However, it is probably a good tradeoff, since it should be more obvious that any events you've missed will not be re-emitted, than how your signal won't pull a parent's current value when it's reactivated.

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