Can I access the laid out width or the constraints from within a modifier?

# compose

[Daniel]

Can I access the laid out width or the constraints from within a modifier?

[Adam Powell]

Yes, but there are inefficient or feedback-loop patterns to avoid depending on what you're doing with that data. What's your use case?

[Se7eN]

Modifier.onSizeChanged

[Adam Powell]

yes, that is one 🙂

[Daniel]

I have a modifier that creates a turned page effect based on a turn percent parameter

[Daniel]

I need to convert the percent to pixels

[Daniel]

In my case I happen to know that the width and height will always be the exact size of the parent (I require this anyway because I'm custom drawing a very specific thing in this component)

[Daniel]

Right now I'm wrapping like so, which I think avoids the inefficiencies?

BoxWithConstraints {
  Box(Modifier
    .myCustomDraw()
    .myCustomTurn(percent, constraints.minWidth))
}

[Adam Powell]

this is why I asked 🙂 all of the custom drawing APIs have the size available in the receiver scope already - you shouldn't need to use any additional modifiers to get it

[Adam Powell]

e.g.

Canvas(...) {
  this.size
}

or

Modifier.drawBehind {
  this.size
}

[Daniel]

I'm using graphicsLayer, and I didn't find anything like size on it when I checked @Adam Powell

[Daniel]

I'm separating the turn animation from drawing the page for performance reasons. I draw the page once, and then use graphicsLayer to translate it efficiently

[Adam Powell]

ah, could be an omission there, can you file an issue for it please? We can evaluate adding it there. In the meantime, yes, as @Se7eN said you can do this:

fun Modifier.myModifier(...) = composed {
  var size by remember { mutableStateOf(IntSize.Zero) }
  // ...
  Modifier.onSizeChanged { size = it }
    .graphicsLayer {
      size...
    }
}

[Daniel]

It feels less hacky to me to just pass it in, so if that has similar characteristics I'll do that. (In my case if the size changes that would necessitate re-laying out the entire section of the book, which would involve throwing out the grandparent of the component with this modifier)

[Daniel]

I'll do that @Adam Powell. graphicsLayer provides very little, I had assumed that was because getting the other information was impossible