/*
* Copyright 2023 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.foundation.pager
import androidx.compose.foundation.ExperimentalFoundationApi
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.setValue
import androidx.compose.runtime.snapshots.Snapshot
/**
* Contains the current scroll position represented by the first visible page and the first
* visible page scroll offset.
*/
@OptIn(ExperimentalFoundationApi::class)
internal class PagerScrollPosition(
initialPage: Int = 0,
initialScrollOffset: Int = 0
) {
var firstVisiblePage by mutableStateOf(initialPage)
var currentPage by mutableStateOf(initialPage)
var scrollOffset by mutableStateOf(initialScrollOffset)
private set
private var hadFirstNotEmptyLayout = false
/** The last know key of the page at [firstVisiblePage] position. */
private var lastKnownFirstPageKey: Any? = null
/**
* Updates the current scroll position based on the results of the last measurement.
*/
fun updateFromMeasureResult(measureResult: PagerMeasureResult) {
lastKnownFirstPageKey = measureResult.firstVisiblePage?.key
// we ignore the index and offset from measureResult until we get at least one
// measurement with real pages. otherwise the initial index and scroll passed to the
// state would be lost and overridden with zeros.
if (hadFirstNotEmptyLayout || measureResult.pagesCount > 0) {
hadFirstNotEmptyLayout = true
val scrollOffset = measureResult.firstVisiblePageOffset
check(scrollOffset >= 0f) { "scrollOffset should be non-negative ($scrollOffset)" }
Snapshot.withoutReadObservation {
update(
measureResult.firstVisiblePage?.index ?: 0,
scrollOffset
)
measureResult.closestPageToSnapPosition?.index?.let {
if (it != this.currentPage) {
this.currentPage = it
}
}
}
}
}
/**
* Updates the scroll position - the passed values will be used as a start position for
* composing the pages during the next measure pass and will be updated by the real
* position calculated during the measurement. This means that there is no guarantee that
* exactly this index and offset will be applied as it is possible that:
* a) there will be no page at this index in reality
* b) page at this index will be smaller than the asked scrollOffset, which means we would
* switch to the next page
* c) there will be not enough pages to fill the viewport after the requested index, so we
* would have to compose few elements before the asked index, changing the first visible page.
*/
fun requestPosition(index: Int, scrollOffset: Int) {
update(index, scrollOffset)
// clear the stored key as we have a direct request to scroll to [index] position and the
// next [checkIfFirstVisibleItemWasMoved] shouldn't override this.
lastKnownFirstPageKey = null
}
private fun update(index: Int, scrollOffset: Int) {
require(index >= 0f) { "Index should be non-negative ($index)" }
if (index != this.firstVisiblePage) {
this.firstVisiblePage = index
}
if (scrollOffset != this.scrollOffset) {
this.scrollOffset = scrollOffset
}
}
}