In the context of a recent technical presentation, I was experimenting different layouts in Jetpack Compose, from which SubComposeLayout was one of the most challenging.
Theory
SubComposeLayout is a low level API for building layout in Jetpack Compose, by overriding the standard steps of composition-layout-drawing with a subcomposition step for child nodes of a layout (that is obviously a SubComposeLayout).
This way, we engineers, can decide at runtime whether we want to display (place) a subsequent child node on the UI based on the measurement (size and position) of a preceeding child node.
More about the theory can be found in Advanced layout concepts — MAD Skills and SubcomposeLayout — breaking the Compose phases rule.
Use case
Some of the good uses cases are LazyLayout together with it’s derivatives and BoxWithConstraints, however in the Compose Layouts and Modifiers: Live Q&A — MAD Skills discussion the experts say that there are not that many cases for the custom SubComposeLayout, however I wanted to enforce one for my presentation so, here MultiItemPager comes 🙂
(do not miss the nice and actress-like look and behaviour of Simona from 41:25 in the video 🙂
Back to the use case, let’s consider that the business says the following:
– display a list of items on the UI that have different sizes
– place as many item on the UI as many can be fit vertically
– moving to the next page of items should be done by clicking on a button
– neither vertical nor horizontal scrolling cannot be used
After implementing, it looks this way:
Show me the code
The main point in the code is that it is only placing subsequent child items of SubComposeLayout, until there is space left by preceeding children (line 124).
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| package david.composesimulation.ui.subcompose | |
| import androidx.compose.foundation.background | |
| import androidx.compose.foundation.layout.Box | |
| import androidx.compose.foundation.layout.Column | |
| import androidx.compose.foundation.layout.fillMaxWidth | |
| import androidx.compose.foundation.layout.padding | |
| import androidx.compose.foundation.layout.size | |
| import androidx.compose.material3.Button | |
| import androidx.compose.material3.Text | |
| import androidx.compose.runtime.Composable | |
| import androidx.compose.runtime.getValue | |
| import androidx.compose.runtime.mutableIntStateOf | |
| import androidx.compose.runtime.mutableStateOf | |
| import androidx.compose.runtime.saveable.rememberSaveable | |
| import androidx.compose.runtime.setValue | |
| import androidx.compose.ui.Alignment | |
| import androidx.compose.ui.Modifier | |
| import androidx.compose.ui.graphics.Color | |
| import androidx.compose.ui.layout.Placeable | |
| import androidx.compose.ui.layout.SubcomposeLayout | |
| import androidx.compose.ui.platform.LocalDensity | |
| import androidx.compose.ui.tooling.preview.Preview | |
| import androidx.compose.ui.unit.dp | |
| import androidx.compose.ui.unit.sp | |
| private data class Element( | |
| val text: String, | |
| val widthInPixel: Int, | |
| val heightInPixel: Int, | |
| val color: Color, | |
| ) | |
| private val elements = | |
| listOf( | |
| Element(text = "1", widthInPixel = 800, heightInPixel = 1200, color = Color.Green), | |
| Element(text = "2", widthInPixel = 1000, heightInPixel = 800, color = Color.Red), | |
| Element(text = "3", widthInPixel = 900, heightInPixel = 1200, color = Color.Gray), | |
| Element(text = "4", widthInPixel = 800, heightInPixel = 1300, color = Color.Blue), | |
| Element(text = "5", widthInPixel = 1100, heightInPixel = 900, color = Color.Cyan), | |
| Element(text = "6", widthInPixel = 1100, heightInPixel = 1200, color = Color.Magenta), | |
| Element(text = "7", widthInPixel = 300, heightInPixel = 300, color = Color.Yellow), | |
| Element(text = "8", widthInPixel = 6000, heightInPixel = 200, color = Color.DarkGray), | |
| Element(text = "9", widthInPixel = 500, heightInPixel = 300, color = Color.LightGray), | |
| ) | |
| @Composable | |
| fun MultiItemPagerUseCase() { | |
| Column { | |
| var lastDisplayedElementIndex by rememberSaveable { mutableStateOf<Int?>(null) } | |
| var elementIndex by rememberSaveable { mutableIntStateOf(0) } | |
| Button( | |
| enabled = (lastDisplayedElementIndex ?: 0) < elements.lastIndex, | |
| onClick = { | |
| lastDisplayedElementIndex?.let { | |
| elementIndex = it + 1 | |
| } | |
| }, | |
| modifier = Modifier | |
| .padding(all = 16.dp) | |
| .fillMaxWidth() | |
| ) { | |
| Text(text = "Next page") | |
| } | |
| MultiItemPager( | |
| firsElementIndexToDisplay = elementIndex, | |
| elements = elements, | |
| elementContent = { element -> | |
| val density = LocalDensity.current | |
| Box( | |
| contentAlignment = Alignment.Center, | |
| modifier = Modifier | |
| .padding(top = 16.dp) | |
| .padding(horizontal = 16.dp) | |
| .size( | |
| width = density.run { element.widthInPixel.toDp() }, | |
| height = density.run { element.heightInPixel.toDp() }, | |
| ) | |
| .background(color = element.color) | |
| ) { | |
| Text( | |
| text = element.text, | |
| fontSize = 29.sp, | |
| ) | |
| } | |
| }, | |
| onPageIsFull = { page -> | |
| lastDisplayedElementIndex = page | |
| } | |
| ) | |
| } | |
| } | |
| @Composable | |
| private fun MultiItemPager( | |
| firsElementIndexToDisplay: Int, | |
| elements: List<Element>, | |
| elementContent: @Composable (element: Element) -> Unit, | |
| onPageIsFull: (lastDisplayedElementIndex: Int) -> Unit, | |
| modifier: Modifier = Modifier, | |
| ) { | |
| SubcomposeLayout( | |
| modifier = modifier | |
| ) { constraints -> | |
| val maxHeight = constraints.maxHeight | |
| val placeables = mutableListOf<Placeable>() | |
| var isAvailableSpaceLeft = true | |
| var elementIndex = firsElementIndexToDisplay | |
| println("SegmentedLayout maxHeight $maxHeight elementIndex $elementIndex") | |
| while (isAvailableSpaceLeft) { | |
| val measurables = subcompose(slotId = elementIndex, content = { elementContent(elements[elementIndex]) }) | |
| val currentPlaceables = measurables.map { measurable -> measurable.measure(constraints) } | |
| val currentPlaceablesAggregatedHeight = | |
| currentPlaceables.fold(0) { accumulator, element -> accumulator + element.height } | |
| val previousPlaceablesAggregatedHeight = | |
| placeables.fold(0) { accumulator, element -> accumulator + element.height } | |
| if (maxHeight - previousPlaceablesAggregatedHeight < currentPlaceablesAggregatedHeight) { | |
| isAvailableSpaceLeft = false | |
| onPageIsFull(elementIndex - 1) | |
| } else { | |
| placeables.addAll(currentPlaceables) | |
| elementIndex++ | |
| if (elementIndex == elements.size) { | |
| isAvailableSpaceLeft = false | |
| onPageIsFull(elementIndex - 1) | |
| } | |
| } | |
| } | |
| val layoutWidth = constraints.maxWidth | |
| val layoutHeight = placeables.sumOf { placeable -> placeable.height } | |
| layout( | |
| width = layoutWidth, | |
| height = layoutHeight, | |
| ) { | |
| var y = 0 | |
| placeables.forEach { placeable -> | |
| val horizontalSpace = (layoutWidth - placeable.width) / 2 | |
| placeable.placeRelative(x = horizontalSpace, y = y) | |
| y += placeable.height | |
| } | |
| } | |
| } | |
| } | |
| @Preview | |
| @Composable | |
| private fun MultiItemPagerUseCasePreview() { | |
| MultiItemPagerUseCase() | |
| } |
The code is part of the ComposeSimulation draft project.
This article is previously published on proandroiddev.com
