Simplify calculating showed month, add animations when changing month (…

…#86)

library/src/main/java/io/github/boguszpawlowski/composecalendar/Calendar.kt

@@ -178,7 +178,7 @@ public fun <T : SelectionState> Calendar(
     Box { content(PaddingValues()) }
   },
 ) {
-
+  val initialMonth = remember { calendarState.monthState.currentMonth }
   val daysOfWeek = remember(firstDayOfWeek) {
     DayOfWeek.values().rotateRight(DaysOfWeek - firstDayOfWeek.ordinal)
   }
@@ -189,6 +189,7 @@ public fun <T : SelectionState> Calendar(
     monthHeader(calendarState.monthState)
     if (horizontalSwipeEnabled) {
       MonthPager(
+        initialMonth = initialMonth,
         showAdjacentMonths = showAdjacentMonths,
         monthState = calendarState.monthState,
         selectionState = calendarState.selectionState,

library/src/main/java/io/github/boguszpawlowski/composecalendar/month/MonthContent.kt

@@ -22,8 +22,6 @@ import dev.chrisbanes.snapper.SnapperLayoutInfo
 import dev.chrisbanes.snapper.rememberSnapperFlingBehavior
 import io.github.boguszpawlowski.composecalendar.day.DayState
 import io.github.boguszpawlowski.composecalendar.header.MonthState
-import io.github.boguszpawlowski.composecalendar.pager.PagerItemCount
-import io.github.boguszpawlowski.composecalendar.pager.toIndex
 import io.github.boguszpawlowski.composecalendar.selection.SelectionState
 import io.github.boguszpawlowski.composecalendar.week.WeekContent
 import io.github.boguszpawlowski.composecalendar.week.getWeeks
@@ -35,8 +33,9 @@ internal const val DaysOfWeek = 7
 
 @OptIn(ExperimentalSnapperApi::class)
 @Composable
-@Suppress("LongMethod")
+@Suppress("LongMethod", "LongParameterList")
 internal fun <T : SelectionState> MonthPager(
+  initialMonth: YearMonth,
   showAdjacentMonths: Boolean,
   selectionState: T,
   monthState: MonthState,
@@ -47,11 +46,10 @@ internal fun <T : SelectionState> MonthPager(
   weekHeader: @Composable BoxScope.(List<DayOfWeek>) -> Unit,
   monthContainer: @Composable (content: @Composable (PaddingValues) -> Unit) -> Unit,
 ) {
-  val startIndex = PagerItemCount / 2
   val coroutineScope = rememberCoroutineScope()
 
   val listState = rememberLazyListState(
-    initialFirstVisibleItemIndex = startIndex,
+    initialFirstVisibleItemIndex = StartIndex,
   )
   val flingBehavior = rememberSnapperFlingBehavior(
     lazyListState = listState,
@@ -64,6 +62,7 @@ internal fun <T : SelectionState> MonthPager(
   val monthListState = remember {
     MonthListState(
       coroutineScope = coroutineScope,
+      initialMonth = initialMonth,
       monthState = monthState,
       listState = listState,
     )
@@ -80,7 +79,7 @@ internal fun <T : SelectionState> MonthPager(
         modifier = Modifier.fillParentMaxWidth(),
         showAdjacentMonths = showAdjacentMonths,
         selectionState = selectionState,
-        currentMonth = monthListState.getMonthForPage(index.toIndex()),
+        currentMonth = monthListState.getMonthForPage(index),
         today = today,
         daysOfWeek = daysOfWeek,
         dayContent = dayContent,

library/src/main/java/io/github/boguszpawlowski/composecalendar/month/MonthListState.kt

@@ -1,68 +1,49 @@
 package io.github.boguszpawlowski.composecalendar.month
 
-import android.annotation.SuppressLint
-import androidx.annotation.IntRange
 import androidx.compose.foundation.lazy.LazyListState
 import androidx.compose.runtime.Stable
+import androidx.compose.runtime.derivedStateOf
 import androidx.compose.runtime.getValue
-import androidx.compose.runtime.mutableStateMapOf
-import androidx.compose.runtime.mutableStateOf
-import androidx.compose.runtime.setValue
 import androidx.compose.runtime.snapshotFlow
 import io.github.boguszpawlowski.composecalendar.header.MonthState
-import io.github.boguszpawlowski.composecalendar.pager.toIndex
-import io.github.boguszpawlowski.composecalendar.util.dec
-import io.github.boguszpawlowski.composecalendar.util.inc
 import kotlinx.coroutines.CoroutineScope
-import kotlinx.coroutines.flow.distinctUntilChanged
 import kotlinx.coroutines.flow.launchIn
 import kotlinx.coroutines.flow.onEach
-import kotlinx.coroutines.flow.runningFold
+import kotlinx.coroutines.launch
 import java.time.YearMonth
-
-internal const val PageCount = 3
+import java.time.temporal.ChronoUnit
 
 @Stable
 internal class MonthListState(
-  coroutineScope: CoroutineScope,
+  private val coroutineScope: CoroutineScope,
+  private val initialMonth: YearMonth,
   private val monthState: MonthState,
   private val listState: LazyListState,
 ) {
 
-  private var monthProvider by mutableStateOf(
-    MonthProvider(
-      initialMonth = monthState.currentMonth,
-      currentIndex = listState.currentIndex,
-    )
-  )
+  private val currentlyVisibleMonth by derivedStateOf {
+    getMonthForPage(listState.firstVisibleItemIndex)
+  }
 
   init {
     snapshotFlow { monthState.currentMonth }.onEach { month ->
       moveToMonth(month)
     }.launchIn(coroutineScope)
 
-    snapshotFlow { listState.currentIndex }.runningFold(1 to 1) { oldIndices, newIndex ->
-      oldIndices.second to newIndex
-    }.distinctUntilChanged().onEach { (oldIndex, newIndex) ->
-      onScrolled(oldIndex, newIndex)
-      monthState.currentMonth = getMonthForPage(newIndex)
+    snapshotFlow { currentlyVisibleMonth }.onEach { newMonth ->
+      monthState.currentMonth = newMonth
     }.launchIn(coroutineScope)
   }
 
-  fun getMonthForPage(@IntRange(from = 0, to = 2) index: Int) = monthProvider.cache[index]!!
+  fun getMonthForPage(index: Int): YearMonth =
+    initialMonth.plusMonths((index - StartIndex).toLong())
 
-  @SuppressLint("Range")
   private fun moveToMonth(month: YearMonth) {
-    if (month - getMonthForPage(listState.currentIndex) == 0) return
-    monthProvider = MonthProvider(monthState.currentMonth, listState.currentIndex)
-  }
-
-  private fun onScrolled(oldIndex: Int, newIndex: Int) {
-    when (oldIndex - newIndex) {
-      -1, PageCount - 1 -> monthProvider.cache[(newIndex + 1) % PageCount] =
-        monthProvider.cache[newIndex]!!.inc()
-      else -> monthProvider.cache[(newIndex - 1 + PageCount) % PageCount] =
-        monthProvider.cache[newIndex]!!.dec()
+    if (month == currentlyVisibleMonth) return
+    initialMonth.minus(month).let { offset ->
+      coroutineScope.launch {
+        listState.animateScrollToItem((StartIndex - offset).toInt())
+      }
     }
   }
 
@@ -74,46 +55,19 @@ internal class MonthListState(
 
     if (monthState != other.monthState) return false
     if (listState != other.listState) return false
-    if (monthProvider != other.monthProvider) return false
 
     return true
   }
 
   override fun hashCode(): Int {
     var result = monthState.hashCode()
     result = 31 * result + listState.hashCode()
-    result = 31 * result + monthProvider.hashCode()
     return result
   }
 }
 
-@Stable
-internal class MonthProvider(initialMonth: YearMonth, currentIndex: Int) {
-  val cache = mutableStateMapOf<Int, YearMonth>()
-
-  init {
-    cache[(currentIndex - 1 + PageCount) % PageCount] = initialMonth.dec()
-    cache[currentIndex] = initialMonth
-    cache[(currentIndex + 1) % PageCount] = initialMonth.inc()
-  }
-
-  override fun equals(other: Any?): Boolean {
-    if (this === other) return true
-    if (javaClass != other?.javaClass) return false
-
-    other as MonthProvider
-
-    if (cache != other.cache) return false
-
-    return true
-  }
-
-  override fun hashCode(): Int {
-    return cache.hashCode()
-  }
-}
-
 private operator fun YearMonth.minus(other: YearMonth) =
-  year - other.year + month.value - other.month.value
+  ChronoUnit.MONTHS.between(other, this)
 
-private val LazyListState.currentIndex get() = firstVisibleItemIndex.toIndex()
+internal const val PagerItemCount = 20_000
+internal const val StartIndex = PagerItemCount / 2