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/*
* Copyright 2021 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package androidx.compose.material3
import androidx.compose.foundation.layout.PaddingValues
import androidx.compose.runtime.Composable
import androidx.compose.runtime.CompositionLocalProvider
import androidx.compose.runtime.Immutable
import androidx.compose.runtime.staticCompositionLocalOf
import androidx.compose.ui.Modifier
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.layout.SubcomposeLayout
import androidx.compose.ui.unit.LayoutDirection
import androidx.compose.ui.unit.dp
/**
* Material Design layout.
*
* Scaffold implements the basic material design visual layout structure.
*
* This component provides API to put together several material components to construct your
* screen, by ensuring proper layout strategy for them and collecting necessary data so these
* components will work together correctly.
*
* Simple example of a Scaffold with [SmallTopAppBar], [FloatingActionButton]:
*
* @sample androidx.compose.material3.samples.SimpleScaffoldWithTopBar
*
* To show a [Snackbar], use [SnackbarHostState.showSnackbar].
*
* @sample androidx.compose.material3.samples.ScaffoldWithSimpleSnackbar
*
* @param modifier the [Modifier] to be applied to this scaffold
* @param topBar top app bar of the screen, typically a [SmallTopAppBar]
* @param bottomBar bottom bar of the screen, typically a [NavigationBar]
* @param snackbarHost component to host [Snackbar]s that are pushed to be shown via
* [SnackbarHostState.showSnackbar], typically a [SnackbarHost]
* @param floatingActionButton Main action button of the screen, typically a [FloatingActionButton]
* @param floatingActionButtonPosition position of the FAB on the screen. See [FabPosition].
* @param containerColor the color used for the background of this scaffold. Use [Color.Transparent]
* to have no color.
* @param contentColor the preferred color for content inside this scaffold. Defaults to either the
* matching content color for [containerColor], or to the current [LocalContentColor] if
* [containerColor] is not a color from the theme.
* @param content content of the screen. The lambda receives a [PaddingValues] that should be
* applied to the content root via [Modifier.padding] to properly offset top and bottom bars. If
* using [Modifier.verticalScroll], apply this modifier to the child of the scroll, and not on
* the scroll itself.
*/
@ExperimentalMaterial3Api
@Composable
fun Scaffold(
modifier: Modifier = Modifier,
topBar: @Composable () -> Unit = {},
bottomBar: @Composable () -> Unit = {},
snackbarHost: @Composable () -> Unit = {},
floatingActionButton: @Composable () -> Unit = {},
floatingActionButtonPosition: FabPosition = FabPosition.End,
containerColor: Color = MaterialTheme.colorScheme.background,
contentColor: Color = contentColorFor(containerColor),
content: @Composable (PaddingValues) -> Unit
) {
Surface(modifier = modifier, color = containerColor, contentColor = contentColor) {
ScaffoldLayout(
fabPosition = floatingActionButtonPosition,
topBar = topBar,
bottomBar = bottomBar,
content = content,
snackbar = snackbarHost,
fab = floatingActionButton
)
}
}
/**
* Layout for a [Scaffold]'s content.
*
* @param fabPosition [FabPosition] for the FAB (if present)
* @param topBar the content to place at the top of the [Scaffold], typically a [SmallTopAppBar]
* @param content the main 'body' of the [Scaffold]
* @param snackbar the [Snackbar] displayed on top of the [content]
* @param fab the [FloatingActionButton] displayed on top of the [content], below the [snackbar]
* and above the [bottomBar]
* @param bottomBar the content to place at the bottom of the [Scaffold], on top of the
* [content], typically a [NavigationBar].
*/
@OptIn(ExperimentalMaterial3Api::class)
@Composable
private fun ScaffoldLayout(
fabPosition: FabPosition,
topBar: @Composable () -> Unit,
content: @Composable (PaddingValues) -> Unit,
snackbar: @Composable () -> Unit,
fab: @Composable () -> Unit,
bottomBar: @Composable () -> Unit
) {
SubcomposeLayout { constraints ->
val layoutWidth = constraints.maxWidth
val layoutHeight = constraints.maxHeight
val looseConstraints = constraints.copy(minWidth = 0, minHeight = 0)
layout(layoutWidth, layoutHeight) {
val topBarPlaceables = subcompose(ScaffoldLayoutContent.TopBar, topBar).map {
it.measure(looseConstraints)
}
val topBarHeight = topBarPlaceables.maxByOrNull { it.height }?.height ?: 0
val snackbarPlaceables = subcompose(ScaffoldLayoutContent.Snackbar, snackbar).map {
it.measure(looseConstraints)
}
val snackbarHeight = snackbarPlaceables.maxByOrNull { it.height }?.height ?: 0
val snackbarWidth = snackbarPlaceables.maxByOrNull { it.width }?.width ?: 0
val fabPlaceables =
subcompose(ScaffoldLayoutContent.Fab, fab).mapNotNull { measurable ->
measurable.measure(looseConstraints).takeIf { it.height != 0 && it.width != 0 }
}
val fabPlacement = if (fabPlaceables.isNotEmpty()) {
val fabWidth = fabPlaceables.maxByOrNull { it.width }!!.width
val fabHeight = fabPlaceables.maxByOrNull { it.height }!!.height
// FAB distance from the left of the layout, taking into account LTR / RTL
val fabLeftOffset = if (fabPosition == FabPosition.End) {
if (layoutDirection == LayoutDirection.Ltr) {
layoutWidth - FabSpacing.roundToPx() - fabWidth
} else {
FabSpacing.roundToPx()
}
} else {
(layoutWidth - fabWidth) / 2
}
FabPlacement(
left = fabLeftOffset,
width = fabWidth,
height = fabHeight
)
} else {
null
}
val bottomBarPlaceables = subcompose(ScaffoldLayoutContent.BottomBar) {
CompositionLocalProvider(
LocalFabPlacement provides fabPlacement,
content = bottomBar
)
}.map { it.measure(looseConstraints) }
val bottomBarHeight = bottomBarPlaceables.maxByOrNull { it.height }?.height ?: 0
val fabOffsetFromBottom = fabPlacement?.let {
if (bottomBarHeight == 0) {
it.height + FabSpacing.roundToPx()
} else {
// Total height is the bottom bar height + the FAB height + the padding
// between the FAB and bottom bar
bottomBarHeight + it.height + FabSpacing.roundToPx()
}
}
val snackbarOffsetFromBottom = if (snackbarHeight != 0) {
snackbarHeight + (fabOffsetFromBottom ?: bottomBarHeight)
} else {
0
}
val bodyContentPlaceables = subcompose(ScaffoldLayoutContent.MainContent) {
val innerPadding = PaddingValues(
top = topBarHeight.toDp(),
bottom = bottomBarHeight.toDp()
)
content(innerPadding)
}.map { it.measure(looseConstraints) }
// Placing to control drawing order to match default elevation of each placeable
bodyContentPlaceables.forEach {
it.place(0, 0)
}
topBarPlaceables.forEach {
it.place(0, 0)
}
snackbarPlaceables.forEach {
it.place(
(layoutWidth - snackbarWidth) / 2,
layoutHeight - snackbarOffsetFromBottom
)
}
// The bottom bar is always at the bottom of the layout
bottomBarPlaceables.forEach {
it.place(0, layoutHeight - bottomBarHeight)
}
// Explicitly not using placeRelative here as `leftOffset` already accounts for RTL
fabPlacement?.let { placement ->
fabPlaceables.forEach {
it.place(placement.left, layoutHeight - fabOffsetFromBottom!!)
}
}
}
}
}
/**
* The possible positions for a [FloatingActionButton] attached to a [Scaffold].
*/
// TODO(b/200553810): Mark as experimental
@ExperimentalMaterial3Api
@kotlin.jvm.JvmInline
value class FabPosition internal constructor(@Suppress("unused") private val value: Int) {
companion object {
/**
* Position FAB at the bottom of the screen in the center, above the [NavigationBar] (if it
* exists)
*/
val Center = FabPosition(0)
/**
* Position FAB at the bottom of the screen at the end, above the [NavigationBar] (if it
* exists)
*/
val End = FabPosition(1)
}
override fun toString(): String {
return when (this) {
Center -> "FabPosition.Center"
else -> "FabPosition.End"
}
}
}
/**
* Placement information for a [FloatingActionButton] inside a [Scaffold].
*
* @property left the FAB's offset from the left edge of the bottom bar, already adjusted for RTL
* support
* @property width the width of the FAB
* @property height the height of the FAB
*/
@Immutable
internal class FabPlacement(
val left: Int,
val width: Int,
val height: Int
)
/**
* CompositionLocal containing a [FabPlacement] that is used to calculate the FAB bottom offset.
*/
internal val LocalFabPlacement = staticCompositionLocalOf { null }
// FAB spacing above the bottom bar / bottom of the Scaffold
private val FabSpacing = 16.dp
private enum class ScaffoldLayoutContent { TopBar, MainContent, Snackbar, Fab, BottomBar }
