RelocationRequesterModifier.kt
/*
* 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.ui.layout
import androidx.compose.runtime.DisposableEffect
import androidx.compose.runtime.remember
import androidx.compose.ui.ExperimentalComposeUiApi
import androidx.compose.ui.Modifier
import androidx.compose.ui.composed
import androidx.compose.ui.geometry.Offset
import androidx.compose.ui.geometry.Offset.Companion.Zero
import androidx.compose.ui.input.nestedscroll.NestedScrollDelegatingWrapper
import androidx.compose.ui.input.nestedscroll.NestedScrollSource.Companion.Relocate
import androidx.compose.ui.node.LayoutNodeWrapper
import androidx.compose.ui.platform.debugInspectorInfo
import kotlin.math.abs
// This modifier keeps track of the coordinates when the item is positioned, and when
// bringIntoParentBounds() is called, it asks parents to scroll to bring this item into view.
internal class RelocationRequesterModifier : OnGloballyPositionedModifier {
lateinit var coordinates: LayoutCoordinates
fun bringIntoView() {
val layoutNodeWrapper = coordinates
check(layoutNodeWrapper is LayoutNodeWrapper)
// Recursively scroll parents so that the item is visible.
layoutNodeWrapper.findPreviousNestedScrollWrapper()?.bringIntoView(coordinates)
// Ask the owner to send a request to its parents to make sure this item is visible.
layoutNodeWrapper.layoutNode.owner?.requestRectangleOnScreen(coordinates.boundsInRoot())
}
override fun onGloballyPositioned(coordinates: LayoutCoordinates) {
this.coordinates = coordinates
}
}
/**
* This is a modifier that can be used to send relocation requests.
*
* Here is an example where the a [relocationRequester] can be used to bring an item into parent
* bounds. It demonstrates how a composable can ask its parents to scroll so that the component
* using this modifier is brought into the bounds of all its parents.
* @sample androidx.compose.ui.samples.BringIntoViewSample
*
* @param relocationRequester an instance of [RelocationRequester]. This hoisted object can be
* used to send relocation requests to parents of the current composable.
*/
@ExperimentalComposeUiApi
fun Modifier.relocationRequester(relocationRequester: RelocationRequester): Modifier = composed(
inspectorInfo = debugInspectorInfo {
name = "relocationRequester"
properties["relocationRequester"] = relocationRequester
}
) {
val modifier = remember { RelocationRequesterModifier() }
DisposableEffect(relocationRequester) {
relocationRequester.modifiers += modifier
onDispose { relocationRequester.modifiers -= modifier }
}
modifier
}
// Scroll this nested scroll parent to bring the child into view. Then find the nested scroll parent
// of this nested scroll parent and ask them to do the same. This results in scrolls propagating
// up to all the nested scroll parents to bring the specified child into view.
private fun NestedScrollDelegatingWrapper.bringIntoView(child: LayoutCoordinates) {
val childBounds = localBoundingBoxOf(child, false)
val offset = Offset(
calculateOffset(childBounds.left, childBounds.right, size.width.toFloat()),
calculateOffset(childBounds.top, childBounds.bottom, size.height.toFloat())
)
// TODO(b/187432148): We ideally shouldn't be using internal connection functions. We need to
// build a better system with a more granular API that allows us to send scroll requests to
// specific parents.
@Suppress("DEPRECATION")
modifier.connection.onPostScroll(Zero, offset, @OptIn(ExperimentalComposeUiApi::class) Relocate)
wrappedBy?.findPreviousNestedScrollWrapper()?.bringIntoView(child)
}
// Calculate the offset needed to bring one of the edges into view. The leadingEdge is the side
// closest to the origin (For the x-axis this is 'left', for the y-axis this is 'top').
// The trailing edge is the other side (For the x-axis this is 'right', for the y-axis this is
// 'bottom').
private fun calculateOffset(leadingEdge: Float, trailingEdge: Float, parentSize: Float) = when {
// If the item is already visible, no need to scroll.
leadingEdge >= 0 && trailingEdge <= parentSize -> 0f
// If the item is visible but larger than the parent, we don't scroll.
leadingEdge < 0 && trailingEdge > parentSize -> 0f
// Find the minimum scroll needed to make one of the edges coincide with the parent's edge.
abs(leadingEdge) < abs(trailingEdge - parentSize) -> -leadingEdge
else -> parentSize - trailingEdge
}