I had a custom layout like this: ```fun MyLayout(...

# compose

[dimsuz]

I had a custom layout like this:

fun MyLayout(someState: Int) {
  Layout(content) { measureables, constraints -> 
    if(someState) thisLogic() else thatLogic()
  }
}

It updates layout whenever someState changes, but now I want to factor out MeasurePolicy into a separate class (there will be several of them):

fun MyLayout(someState: Int) {
  val policy = remember { MyMeasurePolicy({ someState }) }
  Layout(content, policy)
}
class MyMeasurePolicy(private val someState: () -> Int)) : MeasurePolicy

I have made someState into a "provider lambda" to avoid recreating policy on each someState change (it will change more or less often).
But now my layout is not re-measured when someState changes.
Any hints on how to do this right?

[ste]

Well, you need to pass the `key`s to remember

[ste]

(this is also how LazyList does it)

[Albert Chang]

Parameters are not states, so you must turn it into a state first:

val state = rememberUpdatedState(someState)
val policy = remember(state) { MyMeasurePolicy(state::value) }
Layout(content, policy)

[dimsuz]

but wouldn't this recreate MyMeasurePolicy on each someState сhange?
Ah, or it won't.
So if I turn it into state it willl work why? Because measure is re-invoked whenever State values inside it change? I didn't know this

[dimsuz]

As a sidenote, it's interesting that

Layout(content) { m, c -> if (someState) this() else that() }

works perfectly and it "captures" someState correctly, and the lambda itself is the same instance across someState changes, but as soon as I try to turn it into a class (which in fact lambda is under the hood IIUC), it's suddenly not as straightforward to achieve without having a lot of allocations of said class.

[Albert Chang]

In your first snippet, the lambda (MeasurePolicy) is recreated every time someState changes. If you do not remember the MyMeasurePolicy in the second snippet it will also work

[dimsuz]

no, if I do this

Layout(content) { m, c -> 
  Timber.d("$this, someState=$someState")
  if (someState) this() else that() 
}

it prints the same this but different `someState`'s, e.g.

androidx.compose.ui.node.LayoutNode$measureScope$1@98c1123, someState=1
androidx.compose.ui.node.LayoutNode$measureScope$1@98c1123, someState=2

pointer stays the same.

Could it be because I'm running this on API 30 and it's not being compiled to an anonymous class?

[shikasd]

In your example, this seems to be the measure scope, not lambda instance
And to answer your previous question, if state is read in measure or draw, it invalidates related layouts on write

[dimsuz]

Oh, right. I thought about it being the MeasureScope, but seeing $1 made me think its a lambda's this. But it should be the measure scope, indeed, that's the receiver.

Good to know about measure-invalidation thing, thanks! Makes sense now why it works, I didn't know this bit.