Extract calendar utilities and use derivedStateOf for reactive state

Move shared constants and helper functions into CalendarUtils.kt,
replace manual state synchronization with derivedStateOf in
CalendarViewModel and UI composables.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

shared/src/commonMain/kotlin/plus/rua/project/CalendarViewModel.kt

@@ -16,6 +16,7 @@ import kotlinx.datetime.number
 import kotlinx.datetime.plus
 import kotlinx.datetime.todayIn
 import kotlin.time.Clock
+import plus.rua.project.ui.COLLAPSE_THRESHOLD
 
 data class CalendarDay(
     val date: LocalDate,
@@ -56,11 +57,11 @@ class CalendarViewModel(private val coroutineScope: CoroutineScope) {
         }
     }
 
-    // 拖拽超过 50% 时自动折叠到周视图，否则回弹到月视图
+    // 拖拽超过阈值时自动折叠到周视图，否则回弹到月视图
     fun onDragEnd() {
         coroutineScope.launch {
             val current = _collapseAnimatable.value
-            if (current > 0.5f) {
+            if (current > COLLAPSE_THRESHOLD) {
                 _collapseAnimatable.animateTo(
                     targetValue = 1f,
                     animationSpec = spring(dampingRatio = 0.8f, stiffness = 400f)
@@ -83,11 +84,11 @@ class CalendarViewModel(private val coroutineScope: CoroutineScope) {
         }
     }
 
-    // 下拉超过 50% 时自动展开到月视图，否则回弹到周视图
+    // 下拉超过阈值时自动展开到月视图，否则回弹到周视图
     fun onExpandDragEnd() {
         coroutineScope.launch {
             val current = _collapseAnimatable.value
-            if (current < 0.5f) {
+            if (current < COLLAPSE_THRESHOLD) {
                 _collapseAnimatable.animateTo(
                     targetValue = 0f,
                     animationSpec = spring(dampingRatio = 0.8f, stiffness = 400f)

shared/src/commonMain/kotlin/plus/rua/project/ui/BottomCard.kt

@@ -32,7 +32,7 @@ fun BottomCard(
     modifier: Modifier = Modifier
 ) {
     val density = LocalDensity.current
-    val dragRange = with(density) { 200.dp.toPx() }
+    val dragRange = with(density) { DRAG_RANGE_DP.dp.toPx() }
 
     Surface(
         modifier = modifier

shared/src/commonMain/kotlin/plus/rua/project/ui/CalendarMonthPage.kt

@@ -111,7 +111,7 @@ fun CalendarMonthPage(
                             else Modifier
                         )
                         .offset(y = yOffsetDp)
-                        .padding(vertical = 4.dp)
+                        .padding(vertical = ROW_PADDING_DP.dp)
                         .then(
                             if (weekIndex == 0 && rowHeightPx == 0) {
                                 Modifier.onSizeChanged { size ->

shared/src/commonMain/kotlin/plus/rua/project/ui/CalendarMonthView.kt

@@ -31,10 +31,6 @@ import kotlin.math.abs
 import kotlin.time.Clock
 import plus.rua.project.CalendarViewModel
 
-private const val START_PAGE = Int.MAX_VALUE / 2
-private const val ROW_PADDING_DP = 4
-private const val HORIZONTAL_PADDING_DP = 16
-
 /**
  * 日历主界面，包含月/周视图切换和折叠动画。
  *
@@ -50,8 +46,8 @@ fun CalendarMonthView(
     val coroutineScope = rememberCoroutineScope()
     val viewModel = remember { CalendarViewModel(coroutineScope) }
     val today = remember { Clock.System.todayIn(TimeZone.currentSystemDefault()) }
-    var currentYear by remember { mutableIntStateOf(viewModel.currentYear) }
+    val currentYear by remember { derivedStateOf { viewModel.selectedDate.year } }
-    var currentMonth by remember { mutableIntStateOf(viewModel.currentMonth) }
+    val currentMonth by remember { derivedStateOf { viewModel.selectedDate.month.number } }
     val density = LocalDensity.current
 
     var monthHeaderHeightPx by remember { mutableIntStateOf(0) }
@@ -66,24 +62,19 @@ fun CalendarMonthView(
     val headerHeightPx = monthHeaderHeightPx + weekdayHeaderHeightPx
     val rowPaddingPx = with(density) { ROW_PADDING_DP.dp.toPx() }.toInt()
 
-    // 滑动偏移插值行数
+    val interpolatedWeeks by remember {
-    // 以 currentPage 为基准页，offsetFraction 表示基准页与可视区域左边缘的偏移：
+        derivedStateOf {
-    //   offsetFraction > 0：基准页偏右，可视区域露出下一页（page+1）
+            val fraction = pagerState.currentPageOffsetFraction
-    //   offsetFraction < 0：基准页偏左，可视区域露出上一页（page-1）
+            if (abs(fraction) > OFFSET_FRACTION_THRESHOLD) {
-    // 过渡进度 = abs(offsetFraction)，目标页 = page ± 1。
+                val cp = pagerState.currentPage
-    // 当 currentPage 跳变（如从 Jul 跳到 Aug），基准页行数也随之跳变，
+                val baseWeeks = calculateWeeksCountForPage(cp, today)
-    // 但 abs(offsetFraction) 同时从 ~0.5 降到 ~0.5（连续），所以插值结果连续：
+                val targetPage = cp + if (fraction > 0) 1 else -1
-    //   跳变前: cp=Jul(5行), off=+0.49 → base=5, target=Aug(6), lerp(5,6,0.49)=5.49
+                val targetWeeks = calculateWeeksCountForPage(targetPage, today)
-    //   跳变后: cp=Aug(6行), off=-0.47 → base=6, target=Jul(5), lerp(6,5,0.47)=5.47 ← 连续！
+                lerp(baseWeeks.toFloat(), targetWeeks.toFloat(), abs(fraction))
-    val offsetFraction by remember { derivedStateOf { pagerState.currentPageOffsetFraction } }
+            } else {
-    val interpolatedWeeks = if (abs(offsetFraction) > 0.01f) {
+                calculateWeeksCountForPage(pagerState.currentPage, today).toFloat()
-        val cp = pagerState.currentPage
+            }
-        val baseWeeks = calculateWeeksCountForPage(cp, today)
+        }
-        val targetPage = cp + if (offsetFraction > 0) 1 else -1
-        val targetWeeks = calculateWeeksCountForPage(targetPage, today)
-        lerp(baseWeeks.toFloat(), targetWeeks.toFloat(), abs(offsetFraction))
-    } else {
-        calculateWeeksCountForPage(pagerState.currentPage, today).toFloat()
     }
 
     // 预估行高：DayCell aspectRatio=1，宽度 = (screenWidth - horizontalPadding) / 7
@@ -100,14 +91,18 @@ fun CalendarMonthView(
     // gridH = rowH × (1 + (weeks-1) × (1-p))
     val effectiveWeeks = interpolatedWeeks
 
-    val gridHeightPx = if (effectiveRowHeightPx > 0) {
+    val gridHeightPx by remember {
-        val rowH = effectiveRowHeightPx.toFloat()
+        derivedStateOf {
-        if (p > 0.01f) {
+            if (effectiveRowHeightPx > 0) {
-            (rowH * (1 + (effectiveWeeks - 1) * (1f - p))).toInt()
+                val rowH = effectiveRowHeightPx.toFloat()
-        } else {
+                if (p > OFFSET_FRACTION_THRESHOLD) {
-            (rowH * effectiveWeeks).toInt()
+                    (rowH * (1 + (effectiveWeeks - 1) * (1f - p))).toInt()
+                } else {
+                    (rowH * effectiveWeeks).toInt()
+                }
+            } else 0
         }
-    } else 0
+    }
 
     val calendarAreaHeightPx = headerHeightPx + gridHeightPx + rowPaddingPx
     val cardHeightPx = if (screenHeightPx > 0 && calendarAreaHeightPx > 0) screenHeightPx - calendarAreaHeightPx else 0
@@ -154,8 +149,6 @@ fun CalendarMonthView(
                         val weekSunday = weekMonday.plus(DatePeriod(days = 6))
                         val date = if (today in weekMonday..weekSunday) today else weekMonday
                         viewModel.selectDate(date)
-                        currentYear = date.year
-                        currentMonth = date.month.number
                     }
                 )
             } else {
@@ -167,14 +160,12 @@ fun CalendarMonthView(
                         val date = if (year == today.year && today.month.number == month) today
                                    else LocalDate(year, month, 1)
                         viewModel.selectDate(date)
-                        currentYear = year
-                        currentMonth = month
                     },
                     collapseProgress = viewModel.collapseProgress,
                     rowHeightPx = rowHeightPx,
                     effectiveWeeks = effectiveWeeks,
                     onRowHeightMeasured = { h ->
-                        if (h > 0 && rowHeightPx == 0) rowHeightPx = h
+                        if (h > 0) rowHeightPx = h
                     },
                     pagerState = pagerState,
                     modifier = pagerModifier
@@ -193,15 +184,3 @@ fun CalendarMonthView(
         }
     }
 }
-
-private fun lerp(start: Float, end: Float, fraction: Float): Float = start + (end - start) * fraction
-
-private fun calculateWeeksCountForPage(page: Int, today: LocalDate): Int {
-    val initialYear = today.year
-    val initialMonth = today.month.number
-    val offset = page - START_PAGE
-    val totalMonths = initialYear * 12 + (initialMonth - 1) + offset
-    val year = totalMonths / 12
-    val month = totalMonths % 12 + 1
-    return calculateWeeksCount(year, month)
-}

shared/src/commonMain/kotlin/plus/rua/project/ui/CalendarPager.kt

@@ -11,14 +11,9 @@ import androidx.compose.runtime.snapshotFlow
 import androidx.compose.ui.Modifier
 import kotlinx.coroutines.flow.drop
 import kotlinx.coroutines.launch
-import kotlinx.datetime.DatePeriod
 import kotlinx.datetime.LocalDate
-import kotlinx.datetime.minus
 import kotlinx.datetime.number
 
-/** 无限分页中心页，用于 HorizontalPager 的起始位置 */
-private const val START_PAGE = Int.MAX_VALUE / 2
-
 /**
  * 月度日历分页器，HorizontalPager 实现无限左右滑动切换月份。
  *
@@ -43,13 +38,14 @@ fun CalendarPager(
     pagerState: PagerState,
     modifier: Modifier = Modifier
 ) {
-    val initialYearMonth = remember { today.toYearMonth() }
+    val initialYear = remember { today.year }
+    val initialMonth = remember { today.month.number }
     val coroutineScope = rememberCoroutineScope()
 
     // Sync settled page to onMonthChanged (skip initial emission to preserve "today" selection)
     LaunchedEffect(pagerState) {
         snapshotFlow { pagerState.settledPage }.drop(1).collect { page ->
-            val yearMonth = pageToYearMonth(page, initialYearMonth)
+            val yearMonth = pageToYearMonth(page, initialYear, initialMonth)
             onMonthChanged(yearMonth.first, yearMonth.second)
         }
     }
@@ -60,7 +56,7 @@ fun CalendarPager(
         flingBehavior = PagerDefaults.flingBehavior(state = pagerState),
         modifier = modifier
     ) { page ->
-        val (year, month) = pageToYearMonth(page, initialYearMonth)
+        val (year, month) = pageToYearMonth(page, initialYear, initialMonth)
         CalendarMonthPage(
             year = year,
             month = month,
@@ -69,9 +65,10 @@ fun CalendarPager(
             onDateClick = { date ->
                 onDateClick(date)
                 // If clicking a date in a different month, scroll to that page
-                val clickedYearMonth = date.toYearMonth()
+                val clickedYear = date.year
-                if (clickedYearMonth != pageToYearMonth(page, initialYearMonth)) {
+                val clickedMonth = date.month.number
-                    val targetPage = yearMonthToPage(clickedYearMonth, initialYearMonth)
+                if (clickedYear != year || clickedMonth != month) {
+                    val targetPage = yearMonthToPage(clickedYear, clickedMonth, initialYear, initialMonth)
                     if (targetPage != pagerState.currentPage) {
                         coroutineScope.launch {
                             pagerState.animateScrollToPage(targetPage)
@@ -86,28 +83,3 @@ fun CalendarPager(
         )
     }
 }
-
-private fun LocalDate.toYearMonth(): Pair<Int, Int> = Pair(year, month.number)
-
-// 页码→年月：偏移量 + 初始月份的绝对月数，再拆分回年月
-private fun pageToYearMonth(page: Int, initial: Pair<Int, Int>): Pair<Int, Int> {
-    val offset = page - START_PAGE
-    val totalMonths = initial.first * 12 + (initial.second - 1) + offset
-    return Pair(totalMonths / 12, totalMonths % 12 + 1)
-}
-
-// 年月→页码：目标与初始的绝对月数差 + 起始页
-private fun yearMonthToPage(yearMonth: Pair<Int, Int>, initial: Pair<Int, Int>): Int {
-    val targetTotal = yearMonth.first * 12 + (yearMonth.second - 1)
-    val initialTotal = initial.first * 12 + (initial.second - 1)
-    return START_PAGE + (targetTotal - initialTotal)
-}
-
-// 计算月份在日历网格中需要的行数（4/5/6）
-internal fun calculateWeeksCount(year: Int, month: Int): Int {
-    val firstOfMonth = LocalDate(year, month, 1)
-    val offset = firstOfMonth.dayOfWeek.ordinal
-    val nextMonth = if (month == 12) LocalDate(year + 1, 1, 1) else LocalDate(year, month + 1, 1)
-    val daysInMonth = nextMonth.minus(DatePeriod(days = 1)).day
-    return ((offset + daysInMonth - 1) / 7) + 1
-}

shared/src/commonMain/kotlin/plus/rua/project/ui/CalendarUtils.kt

@@ -0,0 +1,86 @@
+package plus.rua.project.ui
+
+import kotlinx.datetime.DatePeriod
+import kotlinx.datetime.LocalDate
+import kotlinx.datetime.minus
+import kotlinx.datetime.number
+import kotlinx.datetime.plus
+
+/** 无限分页中心页，用于 HorizontalPager 的起始位置 */
+const val START_PAGE = Int.MAX_VALUE / 2
+
+/** 折叠判定阈值：progress > 此值时折叠，< 此值时展开 */
+const val COLLAPSE_THRESHOLD = 0.5f
+
+/** 滑动偏移插值阈值：abs(offsetFraction) > 此值时启用插值 */
+const val OFFSET_FRACTION_THRESHOLD = 0.01f
+
+/** 行内 vertical padding (dp) */
+const val ROW_PADDING_DP = 4
+
+/** 日历网格水平 padding (dp) */
+const val HORIZONTAL_PADDING_DP = 16
+
+/** BottomCard 拖拽手势范围 (dp) */
+const val DRAG_RANGE_DP = 200
+
+/** 线性插值 */
+fun lerp(start: Float, end: Float, fraction: Float): Float = start + (end - start) * fraction
+
+/**
+ * 计算月份在日历网格中需要的行数（4/5/6）。
+ */
+fun calculateWeeksCount(year: Int, month: Int): Int {
+    val firstOfMonth = LocalDate(year, month, 1)
+    val offset = firstOfMonth.dayOfWeek.ordinal
+    val nextMonth = if (month == 12) LocalDate(year + 1, 1, 1) else LocalDate(year, month + 1, 1)
+    val daysInMonth = nextMonth.minus(DatePeriod(days = 1)).day
+    return ((offset + daysInMonth - 1) / 7) + 1
+}
+
+/**
+ * 根据 pager 页码计算该页月份的行数。
+ */
+fun calculateWeeksCountForPage(page: Int, today: LocalDate): Int {
+    val initialYear = today.year
+    val initialMonth = today.month.number
+    val offset = page - START_PAGE
+    val totalMonths = initialYear * 12 + (initialMonth - 1) + offset
+    val year = totalMonths / 12
+    val month = totalMonths % 12 + 1
+    return calculateWeeksCount(year, month)
+}
+
+/**
+ * 页码转年月。
+ */
+fun pageToYearMonth(page: Int, initialYear: Int, initialMonth: Int): Pair<Int, Int> {
+    val offset = page - START_PAGE
+    val totalMonths = initialYear * 12 + (initialMonth - 1) + offset
+    return Pair(totalMonths / 12, totalMonths % 12 + 1)
+}
+
+/**
+ * 年月转页码。
+ */
+fun yearMonthToPage(year: Int, month: Int, initialYear: Int, initialMonth: Int): Int {
+    val targetTotal = year * 12 + (month - 1)
+    val initialTotal = initialYear * 12 + (initialMonth - 1)
+    return START_PAGE + (targetTotal - initialTotal)
+}
+
+/**
+ * 获取日期所在周的周一。
+ */
+fun LocalDate.toWeekMonday(): LocalDate {
+    val dayOfWeekOrdinal = dayOfWeek.ordinal
+    return minus(DatePeriod(days = dayOfWeekOrdinal))
+}
+
+/**
+ * 根据 pager 页码计算该页对应的周周一日期。
+ */
+fun pageToWeekMonday(page: Int, initial: LocalDate): LocalDate {
+    val offset = page - START_PAGE
+    return initial.plus(DatePeriod(days = offset * 7))
+}

shared/src/commonMain/kotlin/plus/rua/project/ui/WeekPager.kt

@@ -9,20 +9,14 @@ import androidx.compose.foundation.pager.rememberPagerState
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.remember
-import androidx.compose.runtime.rememberCoroutineScope
 import androidx.compose.runtime.snapshotFlow
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.unit.dp
 import kotlinx.coroutines.flow.drop
-import kotlinx.coroutines.launch
 import kotlinx.datetime.DatePeriod
 import kotlinx.datetime.LocalDate
-import kotlinx.datetime.minus
 import kotlinx.datetime.plus
 
-/** 无限分页中心页，用于 HorizontalPager 的起始位置 */
-private const val START_PAGE = Int.MAX_VALUE / 2
-
 /**
  * 周视图分页器，折叠状态下显示选中日期所在周，支持左右滑动切换周。
  *
@@ -79,13 +73,3 @@ fun WeekPager(
         }
     }
 }
-
-private fun LocalDate.toWeekMonday(): LocalDate {
-    val dayOfWeekOrdinal = dayOfWeek.ordinal // Monday=0 ... Sunday=6
-    return minus(DatePeriod(days = dayOfWeekOrdinal))
-}
-
-private fun pageToWeekMonday(page: Int, initial: LocalDate): LocalDate {
-    val offset = page - START_PAGE
-    return initial.plus(DatePeriod(days = offset * 7))
-}