Hi. What you'd use to get the position of a composable on screen relative to the parent? as in: <vie...

# compose

[Jorge Castillo]

Hi. What you'd use to get the position of a composable on screen relative to the parent? as in: view.top / view.bottom etc. Layout?

[Adam Powell]

As this is a hot topic for further API iteration right now, what's the use case?

[Jorge Castillo]

Drawing on Canvas, want to clip the canvas only up to a specific height, and clipRect would require absolute position in px if I'm not mistaken?

[Jorge Castillo]

Let me paste code so it's easier.

fun DrawScope.plainClip(currentFillPhase: Float, block: DrawScope.() -> Unit) {
  val halfWidth = (size.width / 2f)
  val halfHeight = (size.height / 2f)

  val left = center.dx - halfWidth
  val top = center.dy - halfHeight
  val right = center.dx + halfWidth
  val bottom = center.dy + halfHeight

  this.clipRect(
    left = left,
    top = (bottom - top) * (1f - currentFillPhase),
    right = right,
    bottom = bottom,
    clipOp = ClipOp.intersect,
    block = block
  )
}

I'd like to clip up to a given height percent on the canvas, but not 100% sure that'd need to require global y for the clipping rect or should it be relative like this.

[Jorge Castillo]

I feel like I should be using relative measures here, so might have something wrong regarding some scaling and translation of the canvas I'm doing.

[Adam Powell]

clipping like that is going to be relative to your current coordinate space

[Adam Powell]

so I don't think you should need to know additional info about the parent positioning? maybe I misunderstood something or need another ☕ 😅

[Jorge Castillo]

right, that's the answer I needed to hear tbh

[Jorge Castillo]

if you can confirm clipping needs to use relative coordinates to the canvas (as in 0,0 would be top left position on the canvas regardless of where it's positioned) then I suspect I've got some other issues caused by the scaling / translation I'm doing. Will keep experimenting a bit

[Adam Powell]

please file bugs if this doesn't work in expected ways. I can't think of a way to make any of this sort of thing compose without working from relative frames of reference.

[Adam Powell]

otherwise, as you suggest, the alternative would be trying to determine what your surroundings are doing to undo them along the way; let's skip that 🙂

[Jorge Castillo]

yep I'll remove those transformations and work over the canvas as is first to double check it does what I'd expect

[Jorge Castillo]

Worked like a charm. Was able to detect where the problem came from. My assumptions weren't correct and I didn't really need to clip a height % of the canvas but a height % of what I'm drawing on it, which is smaller. That was causing a lot of confusion. Thanks for the suggestion 👍

[Jorge Castillo]

Results so far @Adam Powell

device-2020-07-24-174444.mp4