What’s the recommendation around using `derivedSta...

# compose

[Jonathan]

What’s the recommendation around using derivedStateOf? Is it safe and or efficient to “nest” calls ie call another derivedStateOf property inside the calculation block? Is there a diminishing returns on nesting? Will there be recomposition delays by heavily nesting?

[Jonathan]

Working example:

val opp by derivedStateOf { distance(vertexB, vertexC) }

val adj by derivedStateOf { distance(vertexA, vertexC) }

val hyp by derivedStateOf { distance(vertexA, vertexB) }

val sin by derivedStateOf { opp / hyp }

val cos by derivedStateOf { adj / hyp }

val tan by derivedStateOf { opp / adj }

The properties vertextA, vertextB, vertextC are also derivedStateOf properties… This currently works but I’m concerned that I’m introducing subtle bugs and I don’t realize it.

[romainguy]

If the distance function does what I think it does, none of those derivedStateOf are useful. These computations are trivial enough

[Jonathan]

What do you mean useful? distance is just a call to Math.hypot(...)

[romainguy]

Right so the cost of derivedStateOf is going to be greater than just doing the computation always

[romainguy]

(even more so for the simple divisions you have)

[Jonathan]

I see.. The vertex variables are mutableStateOf because I set them from my Composable when the I detect taps/drags. Wouldn't I need "derived" state to allow calling code to observe changes to trig info?

[romainguy]

Using derivedStateOf would make sense if the rate of change of the derived state is (very) different from the rate of change of the source state

[romainguy]

In your case changing a vertex will always change the distance, so there's no benefit

[romainguy]

The cost/sin/tan angles might change at a different rate but I'd be surprised if it's meaningful anyway

[Jonathan]

Should I just make these simple computed properties?

[romainguy]

Indeed

[Jonathan]

Will this be sufficient for other composable code to observe changes to tan or cos since they are not State ?

[shikasd]

You don't have to call state directly in the Composable code for it to be observable
As long as state.value is called somewhere inside the function, even deep on the call stack (e.g. params to trig functions), it is going to be observed
For example, that's how LazyListState scroll index/offset works