Hey, if anyone has a moment to take a look at this...

# compose

[Ben Abramovitch]

Hey, if anyone has a moment to take a look at this issue I’m having that would be wonderful.

https://stackoverflow.com/questions/68187240/custom-layout-draws-position-0-off-screen-when-content-is-larger-than-screen

Essentially when I try to place a placeable off screen, instead of position 0 drawing at position 0, it’s drawing as if the total content was centered, so the further i am off screen, the further x=0 gets drawn, such as -300

I feel like there is maybe a way to handle this with AlignmentLines but i can’t figure it out if so.

Also, dividing the difference by two and tacking that on seemed to work for width, but it’s not working for height even if i try to account for the statusbar/toolbars etc, and it. seems pretty hacky anyway

[Tash]

isn’t that just the coordinate system, i.e. “offscreen to the left” for X has to be a negative value?

[Ben Abramovitch]

If i go

a b c d e f

and e/f are off screen it draws

b c d e

[Ben Abramovitch]

i can use the drag gesture to go left/right and see a and f, but it isn’t drawing it uncentered

[Tash]

i see, so you want a to be at 0,0

[Ben Abramovitch]

correct

[Tash]

i see, so you’re saying when you do width = totalWidth in your layout, (0,0) is actually offscreen, and what you need is almost a translation of the “canvas” to bring 0,0 to the top-left of the visible window

[Tash]

you said you’d tried AlignmentLine, how’d that go?

[Ben Abramovitch]

Well, if totalWidth = Screenwidth then 0 is 0

its when totalWidth > ScreenWidth it starts inching off screen

I don’t understand what it wants me to do with the alignment lines.

I’ve been doing translations with width / 2 as the offset but it doesn’t work accurately on the height

[Ben Abramovitch]

and i feel like having manually translate it is the wrong way

[Ben Abramovitch]

Where i set the layout height and width

It can take in this map of alignment lines, but I can’t find a good explanation of what I’m supposed to do with them….

package androidx.compose.ui.layout

 fun layout(
        width: Int,
        height: Int,
        alignmentLines: Map<AlignmentLine, Int> = emptyMap(),
        placementBlock: Placeable.PlacementScope.() -> Unit
    ) = object : MeasureResult {
        override val width = width
        override val height = height
        override val alignmentLines = alignmentLines
        override fun placeChildren() {
            Placeable.PlacementScope.executeWithRtlMirroringValues(
                width,
                layoutDirection,
                placementBlock
            )
        }
    }
}

[Ben Abramovitch]

also i’m not sure if thats even what can solve it… but setting normal alignment on on the columns didn’t help

[Adam Powell]

if a custom layout composable or layout modifier attempts to declare a size outside of the bounds of the constraints it was given, it will center its desired sized content in a space that satisfies the constraints

[Ben Abramovitch]

I declared that the height and width when creating the layout, how else would i do it otherwise?

[Adam Powell]

this allows the parent of the misbehaving element to work with a placeable that meets the constraints it was measured with so that it doesn't have to defensively account for misbehaving child measurables

[Adam Powell]

looks like you're ignoring constraints.maxWidth in that code

[Ben Abramovitch]

@Adam Powell Where?

The stack example has

constraints.copy(
                    minWidth = headerWidth,
                    maxWidth = headerWidth,
                    minHeight = height,
                    maxHeight = height,
                )

[Ben Abramovitch]

Adding a fillMaxWidth modifier here doesn’t help either

Layout(
    modifier = Modifier.fillMaxWidth(),
    content = {
        columnList.forEach { header ->
            Box {
                headerContent(header)
            }
        }
    },
)

[Ben Abramovitch]

or here on the text box

headerContent: @Composable (title: String) -> Unit = { title ->
    Text(title, modifier = Modifier.fillMaxWidth())
},

[Adam Powell]

the code in the SO question has:

layout(width = totalWidth, height = height) {

and those are calculated as

val height = 45.dp.roundToPx()
val totalWidth = headerWidth * numberOfHeaders

both of which completely ignore the incoming constraints

[Adam Powell]

the contract of layout is that the width/height passed to layout must be in range of the constraints, if you violate that, your measurable gets clamped to the constraints and your placement is centered in the clamped space where your parent placed you

[Ben Abramovitch]

I’m sorry but I don’t understand. Which incoming constraints?

[Adam Powell]

this: `

) { measureables, constraints ->

[Ben Abramovitch]

Those are vals and I can’t edit them?

[Adam Powell]

that is correct

[Ben Abramovitch]

And then i set up each placeable with the widths i want?

[Adam Powell]

you can do what you wish in your own space, yes, but your layout size must satisfy those constraints

[Ben Abramovitch]

So i have to set the width/height on the layout ?

Layout(
    modifier = Modifier.fillMaxWidth(headerWidth * numberOfHeaders.toFloat()).fillMaxHeight(100f),
    content = {
        columnList.forEach { header ->
            Box {
                headerContent(header)
            }
        }
    }
)

This doesn’t work either (i changed height to also be 100 further below)

[Ben Abramovitch]

Doesn’t work when i try width/height either

[Ben Abramovitch]

Same problem with this

@Composable
fun HeaderTest(
    columnList: List<String>,
    headerWidth: Dp,
    headerContent: @Composable (title: String) -> Unit = { title ->
        Text(title, modifier = Modifier.fillMaxWidth())
    },
) {
    val numberOfHeaders = columnList.count()

    Layout(
        modifier = Modifier
            .width(headerWidth * numberOfHeaders)
            .height(100.dp)
            .fillMaxHeight()
            .fillMaxWidth(),
        content = {
            columnList.forEach { header ->
                Box {
                    headerContent(header)
                }
            }
        }
    ) { measureables, constraints ->

        val height = 100.dp
        val totalWidth = headerWidth * numberOfHeaders

        val placeWithHeader = measureables.map { measurable ->
            val constraint = constraints.copy(
                minWidth = headerWidth.roundToPx(),
                maxWidth = headerWidth.roundToPx(),
                minHeight = height.roundToPx(),
                maxHeight = height.roundToPx()
            )

            measurable.measure(constraint)
        }

        layout(width = totalWidth.roundToPx(), height = height.roundToPx()) {
            placeWithHeader.forEachIndexed { index, placeable ->
                val offset = index * headerWidth.roundToPx()
                placeable.place(offset, 0)
            }
        }
    }
}

[Adam Powell]

you are still ignoring the constraints

[Adam Powell]

something that would satisfy the constraints could be:

val height = 100.dp.roundToPx().coerceIn(constraints.minHeight, constraints.maxHeight)
val totalWidth = (headerWidth * numberOfHeaders).coerceIn(constraints.minWidth, constraints.maxWidth)

[Ben Abramovitch]

Okay that does work on that, but I’m still not really sure I understand what was going…. It seemed like I was doing the right thing since I was constraining the placable and I was within the bounds I was setting.

I’ll have to do more reading, but I haven’t really seen that in custom layout examples.

E.g this one says don’t constrain them

Layout(
        modifier = modifier,
        content = content
    ) { measurables, constraints ->
        // Don't constrain child views further, measure them with given constraints
        // List of measured children
        val placeables = measurables.map { measurable ->
            // Measure each children
            measurable.measure(constraints)
        }

But it also does the layout different….

// Set the size of the layout as big as it can
        layout(constraints.maxWidth, constraints.maxHeight) {

Lots more reading to do, but this is why i’m starting sooner!

[Ben Abramovitch]

maybe if i’d done one of the code labs….

[Ben Abramovitch]

Oh and thank you! I spent a whole day trying to narrow down what was going on and then trying things out =(

[Adam Powell]

no problem 🙂

[Adam Powell]

what you're seeing here is the bridge between composition and layout. The modifiers you pass to the Layout composable influence but don't completely control the constraints your layout receives

[Adam Powell]

when you call measurable.measure(constraints), it's calling the same kind of measuring code you're writing right now, but on the child element

[Adam Powell]

when you call the layout(width, height) { function in that block, you're telling it the desired size of the Placeable that the measure call returns

[Adam Powell]

and then you can place your own children however you like

[Adam Powell]

things like the horizontal and vertical scroll modifiers measure their child with infinite max constraints, and if the child measures larger than the max the scroll modifier was given, the scroll modifier occupies that max space and the difference becomes the amount you can scroll by

[Adam Powell]

we don't have as much documentation around this as I'd like 🙂

[Ben Abramovitch]

Oh that makes more sense with the layout now. I thought we were coercing it for the childen, but the same values go into the layout

[Ben Abramovitch]

Ya, well this is pretty new, not even stable yet (soon) AND i’m trying to do something custom hehe

[Adam Powell]

eh, this part has been stable for a while 🙂 after years of writing Views and ViewGroups with custom measure logic we wanted to make sure doing it was a lot more straightforward here

[Ben Abramovitch]

I meant compose in general

[Ben Abramovitch]

it’s still beta (although ya, components can be more stable. than others)

[Ben Abramovitch]

Also as in, not enough time to fully document everything as you want because things are still being worked on elsewhere