Remove the need for fixed height in BottomSheet with scrollable content

[ProgrammerDMD]

Expose the sheet’s current visible height (in Dp), or a way that allows child composables to size themselves according to the sheet’s dynamically changing height.

Discussed in #122

[firekesti]

+1, I'm currently attempting to get a fixed "footer" working within the sheet that sticks to the bottom of the screen regardless of current detent. I've got a partial solution (it flickers quite a bit) by using an offset modifier that essentially moves the footer up by screenHeight - modalBottomSheetState.offset so that it's visible when the sheet is not fully expanded, since the entire sheet is laid out below the bottom of the screen in that case.

[alexstyl]

@firekesti if this is for a modal sheet, you can place your footer outside the Sheet so it is always fixed at the bottom

[firekesti]

@alexstyl yes, it is for a modal sheet – doing e.g.

ModalBottomSheet {
    Scrim()
    Box {
        Sheet { }
        Footer { } 
    }
}

does work well for any non-hidden detents, but when collapsing the sheet to hidden, the footer should also "collapse" (animate off-screen) along with the sheet. I can try to animate it with the sheet based on its offset possibly...ideally checking if the target detent is Hidden would work, but it doesn't quite work as I'd hope. I will keep poking at a solution keeping the footer outside the sheet and let you know if I get something working.

--

Edit: I got something working! Here's some sample code for anyone else who runs into this issue:

ModalBottomSheet {
    Scrim()
    Box {
        Sheet { }
        val footerHeight = /* your logic here to get the height of the footer */
        Box(
            modifier = Modifier
                // We only want the footer to slide down if the sheet is below half height (since that means it's going off screen)
                .conditional(modalBottomSheetState.progress < 0.5f) {
                    // This isn't *exact* but it's convincingly close – under half height, slide the footer down at roughly the same speed as the sheet
                    offset(y = ((0.5f - modalBottomSheetState.progress) * 2f * footerHeight).dp)
                }
                // any other footer container modifiers you have, background etc
                .align(Alignment.BottomCenter),
        ) {
            FooterContent()
        }
    }
}

Edit 2: Note that you'll also have to keep that footer height handy earlier in the tree so you can pad the bottom of the sheet contents as well so the footer doesn't cover them!