app/src/main/java/de/jeanlucmakiola/calendula/ui/month/MonthMorph.kt

@@ -0,0 +1,98 @@
+package de.jeanlucmakiola.calendula.ui.month
+
+import androidx.compose.animation.AnimatedVisibilityScope
+import androidx.compose.animation.ExperimentalSharedTransitionApi
+import androidx.compose.animation.SharedTransitionScope
+import androidx.compose.animation.fadeIn
+import androidx.compose.animation.fadeOut
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.compositionLocalOf
+import androidx.compose.ui.Modifier
+import kotlinx.datetime.LocalDate
+
+/**
+ * What lets the split style's compact grid *become* the full month rather than be
+ * swapped for it (#53).
+ *
+ * The two grids are separate composables — the compact one draws dots, the
+ * expanded one draws the paged style's bars and pills — so nothing about them is
+ * shared by construction. Tagging the pieces that mean the same thing on both
+ * sides with the same [MonthMorphKey] hands Compose enough to animate one into
+ * the other: a day's cell grows, its number rides along, and each dot travels out
+ * to the bar it stood for.
+ *
+ * It travels as a composition local rather than as parameters because the pieces
+ * sit five levels below where the scopes exist, and because a null default is
+ * exactly the right meaning for everyone else: the paged, continuous, and dense
+ * styles share the same row and cell composables, provide nothing, and pay
+ * nothing. Reduced motion provides nothing either, which leaves the plain
+ * cross-fade underneath.
+ */
+internal sealed interface MonthMorphKey {
+    /** A day's background pill — the structural anchor the rest rides on. */
+    data class Cell(val date: LocalDate) : MonthMorphKey
+
+    data class DayNumber(val date: LocalDate) : MonthMorphKey
+
+    /**
+     * One event *on one day*. Keyed by date as well as instance because a
+     * multi-day event has a dot on every day it covers but only one bar, drawn
+     * from where it starts: the start day's dot is the one that becomes the bar,
+     * and the rest are left unmatched on purpose. They fade where they stand
+     * while the bar sweeps out over them, which is the honest reading — one of
+     * them could not become the bar without the others teleporting into it.
+     */
+    data class Event(val date: LocalDate, val instanceId: Long) : MonthMorphKey
+
+    /** The grab handle, which slides from the pane's seam to the foot of the grid. */
+    data object Handle : MonthMorphKey
+}
+
+@OptIn(ExperimentalSharedTransitionApi::class)
+internal class MonthMorphScope(
+    val shared: SharedTransitionScope,
+    val visibility: AnimatedVisibilityScope,
+)
+
+internal val LocalMonthMorph = compositionLocalOf<MonthMorphScope?> { null }
+
+/**
+ * Tag content that is *the same thing* on both sides — a background pill, a day
+ * number — so it animates between its two positions and sizes.
+ */
+@OptIn(ExperimentalSharedTransitionApi::class)
+@Composable
+internal fun Modifier.morphElement(key: MonthMorphKey): Modifier {
+    val morph = LocalMonthMorph.current ?: return this
+    return with(morph.shared) {
+        this@morphElement.sharedElement(
+            sharedContentState = rememberSharedContentState(key),
+            animatedVisibilityScope = morph.visibility,
+        )
+    }
+}
+
+/**
+ * Tag content that means the same thing but *is drawn differently* on each side —
+ * a 5dp dot and a titled bar — so only the bounds are shared and the contents
+ * cross-fade inside them.
+ *
+ * [ResizeMode.RemeasureToBounds][SharedTransitionScope.ResizeMode] rather than
+ * scaling: a bar's title laid out at the dot's 5dp and then scaled up would
+ * arrive as a smear. Remeasuring keeps the text at its real size throughout and
+ * simply clips it while there is no room, so what grows is the pill, not the type.
+ */
+@OptIn(ExperimentalSharedTransitionApi::class)
+@Composable
+internal fun Modifier.morphBounds(key: MonthMorphKey): Modifier {
+    val morph = LocalMonthMorph.current ?: return this
+    return with(morph.shared) {
+        this@morphBounds.sharedBounds(
+            sharedContentState = rememberSharedContentState(key),
+            animatedVisibilityScope = morph.visibility,
+            enter = fadeIn(),
+            exit = fadeOut(),
+            resizeMode = SharedTransitionScope.ResizeMode.RemeasureToBounds,
+        )
+    }
+}

app/src/main/java/de/jeanlucmakiola/calendula/ui/month/MonthScreen.kt

@@ -1,6 +1,8 @@
 package de.jeanlucmakiola.calendula.ui.month
 
 import androidx.compose.animation.AnimatedContent
+import androidx.compose.animation.ExperimentalSharedTransitionApi
+import androidx.compose.animation.SharedTransitionLayout
 import androidx.compose.animation.core.RepeatMode
 import androidx.compose.animation.core.animateFloatAsState
 import androidx.compose.animation.core.snap
@@ -891,6 +893,7 @@ private val MONTH_EXPAND_THRESHOLD = 48.dp
  * screen to explain why. [rememberSaveable] carries it across a rotation, which
  * is as long as it should live.
  */
+@OptIn(ExperimentalSharedTransitionApi::class)
 @Composable
 private fun SplitMonthContent(
     state: MonthUiState,
@@ -906,6 +909,7 @@ private fun SplitMonthContent(
     onCreateEvent: (LocalDate) -> Unit,
 ) {
     val fadeSpec = rememberCalendarFadeSpec()
+    val reduceMotion = rememberReduceMotion()
     var expanded by rememberSaveable { mutableStateOf(false) }
     // The swipe wraps the grid rather than living inside it: mid-transition
     // there are two grids, and the gesture belongs to neither. The pane is left
@@ -920,31 +924,76 @@ private fun SplitMonthContent(
     when (state) {
         MonthUiState.Loading -> MonthGridLoading()
         is MonthUiState.Failure -> CalendarFailure(reason = state.reason, onRetry = onRetry)
-        is MonthUiState.Success -> AnimatedContent(
+        is MonthUiState.Success -> SharedTransitionLayout(Modifier.fillMaxSize()) {
+            AnimatedContent(
                 targetState = expanded,
                 modifier = Modifier.fillMaxSize(),
                 // Both branches fill the same box — the grid grows into exactly the
                 // room the pane gives up — so there is no size change to contain.
+                // The travel between them is the shared elements' job, not a slide.
                 transitionSpec = { fadeIn(fadeSpec) togetherWith fadeOut(fadeSpec) },
                 label = "split-expand-transition",
             ) { isExpanded ->
-            if (isExpanded) {
+                CompositionLocalProvider(
+                    // Null under reduced motion: nothing is tagged, nothing
+                    // travels, and the cross-fade above is the whole transition.
+                    LocalMonthMorph provides if (reduceMotion) {
+                        null
+                    } else {
+                        MonthMorphScope(this@SharedTransitionLayout, this@AnimatedContent)
+                    },
+                ) {
+                    SplitMonthBody(
+                        expanded = isExpanded,
+                        state = state,
+                        selected = selected,
+                        slideDir = slideDir,
+                        showWeekNumbers = showWeekNumbers,
+                        swipeModifier = swipeModifier,
+                        onSelectDay = onSelectDay,
+                        onOpenDay = onOpenDay,
+                        onEventClick = onEventClick,
+                        onCreateEvent = onCreateEvent,
+                        onSetExpanded = { expanded = it },
+                    )
+                }
+            }
+        }
+    }
+}
+
+/** The two faces of the split style, sharing one set of morph tags. */
+@Composable
+private fun SplitMonthBody(
+    expanded: Boolean,
+    state: MonthUiState.Success,
+    selected: LocalDate,
+    slideDir: Int,
+    showWeekNumbers: Boolean,
+    swipeModifier: Modifier,
+    onSelectDay: (LocalDate) -> Unit,
+    onOpenDay: (LocalDate) -> Unit,
+    onEventClick: (EventInstance) -> Unit,
+    onCreateEvent: (LocalDate) -> Unit,
+    onSetExpanded: (Boolean) -> Unit,
+) {
+    if (expanded) {
         SplitMonthExpanded(
             state = state,
             selected = selected,
             slideDir = slideDir,
             showWeekNumbers = showWeekNumbers,
             swipeModifier = swipeModifier,
-                    // A tap in the expanded grid picks the day and drops back,
-                    // which gives the expanded month a job — a chooser you dip
-                    // into — rather than a mode you can get stranded in. The
-                    // collapse then runs with the selection already set, so the
-                    // pane arrives showing the day you picked.
+            // A tap in the expanded grid picks the day and drops back, which
+            // gives the expanded month a job — a chooser you dip into — rather
+            // than a mode you can get stranded in. The collapse then runs with
+            // the selection already set, so the pane arrives showing the day
+            // you picked.
             onPickDay = {
                 onSelectDay(it)
-                        expanded = false
+                onSetExpanded(false)
             },
-                    onCollapse = { expanded = false },
+            onCollapse = { onSetExpanded(false) },
         )
     } else {
         SplitMonthCollapsed(
@@ -957,11 +1006,9 @@ private fun SplitMonthContent(
             onOpenDay = onOpenDay,
             onEventClick = onEventClick,
             onCreateEvent = onCreateEvent,
-                    onExpand = { expanded = true },
+            onExpand = { onSetExpanded(true) },
         )
     }
-        }
-    }
 }
 
 /** The split style at rest: compact grid, handle, then the selected day's events. */
@@ -1093,6 +1140,9 @@ private fun SplitExpandHandle(
         Box(
             Modifier
                 .size(width = SPLIT_HANDLE_WIDTH, height = SPLIT_HANDLE_HEIGHT)
+                // Tagged too, so it slides from the pane's seam down to the foot
+                // of the grid rather than blinking out at one and in at the other.
+                .morphElement(MonthMorphKey.Handle)
                 .background(MaterialTheme.colorScheme.outlineVariant, CircleShape),
         )
     }
@@ -1234,19 +1284,28 @@ private fun SplitDayCell(
         animationSpec = if (reduceMotion) snap() else fadeSpec,
         label = "split-day-selection",
     )
+    // Background and content are separate layers, mirroring how the paged row is
+    // built — and so the pill can be tagged for the morph on its own. Tagged as
+    // one piece with its contents inside, the dots would be nested shared
+    // elements within a shared element and travel twice.
     Box(
         modifier = modifier
             .padding(horizontal = CELL_GAP, vertical = 1.dp)
+            .selectable(selected = isSelected, onClick = onClick),
+        contentAlignment = Alignment.TopCenter,
+    ) {
+        Box(
+            Modifier
+                .fillMaxSize()
+                .morphElement(MonthMorphKey.Cell(date))
                 .clip(CELL_SHAPE)
                 .background(background)
                 .border(
                     width = 1.5.dp,
                     color = MaterialTheme.colorScheme.primary.copy(alpha = selection),
                     shape = CELL_SHAPE,
+                ),
         )
-            .selectable(selected = isSelected, onClick = onClick),
-        contentAlignment = Alignment.TopCenter,
-    ) {
         Column(
             modifier = Modifier.fillMaxSize().padding(top = 4.dp),
             horizontalAlignment = Alignment.CenterHorizontally,
@@ -1255,10 +1314,10 @@ private fun SplitDayCell(
                 date = date,
                 isToday = isToday,
                 inMonth = inMonth,
-                modifier = Modifier.fillMaxWidth(),
+                modifier = Modifier.fillMaxWidth().morphElement(MonthMorphKey.DayNumber(date)),
             )
             Spacer(Modifier.height(2.dp))
-            SplitDots(events = events, hidden = hidden, dark = dark)
+            SplitDots(date = date, events = events, hidden = hidden, dark = dark)
         }
     }
 }
@@ -1272,7 +1331,7 @@ private fun SplitDayCell(
  * bar with no dot to grow out of.
  */
 @Composable
-private fun SplitDots(events: List<EventInstance>, hidden: Int, dark: Boolean) {
+private fun SplitDots(date: LocalDate, events: List<EventInstance>, hidden: Int, dark: Boolean) {
     if (events.isEmpty()) return
     val soften = LocalSoftenColors.current
     Row(
@@ -1280,9 +1339,13 @@ private fun SplitDots(events: List<EventInstance>, hidden: Int, dark: Boolean) {
         verticalAlignment = Alignment.CenterVertically,
     ) {
         events.forEach { event ->
+            // The dot keeps its own circle and the bar its 4dp corners; they
+            // cross-fade inside shared bounds, and at the dot's size the two
+            // radii are a fraction of a pixel apart.
             Box(
                 modifier = Modifier
                     .size(SPLIT_DOT_SIZE)
+                    .morphBounds(MonthMorphKey.Event(date, event.instanceId))
                     .background(eventFill(event.color, dark, soften), CircleShape),
             )
         }
@@ -1540,6 +1603,7 @@ private fun MonthWeekRow(
                             .weight(1f)
                             .fillMaxHeight()
                             .padding(horizontal = CELL_GAP, vertical = 1.dp)
+                            .morphElement(MonthMorphKey.Cell(d))
                             .background(
                                 color = when {
                                     inMonth(d) -> MaterialTheme.colorScheme.surfaceContainer
@@ -1586,7 +1650,9 @@ private fun MonthWeekRow(
                                 } else {
                                     null
                                 },
-                                modifier = Modifier.weight(1f),
+                                modifier = Modifier
+                                    .weight(1f)
+                                    .morphElement(MonthMorphKey.DayNumber(d)),
                             )
                         }
                     }
@@ -1615,7 +1681,17 @@ private fun MonthWeekRow(
                                 )
                                 .width(colW * cols)
                                 .height(EVENT_ROW_HEIGHT)
-                                .padding(horizontal = CELL_GAP + 1.dp, vertical = 1.dp),
+                                .padding(horizontal = CELL_GAP + 1.dp, vertical = 1.dp)
+                                // Anchored on the day the bar starts *in this row*,
+                                // which is where its dot sat. A bar carried in from
+                                // the previous week starts at column 0, and column
+                                // 0's dot is the one that grows into it.
+                                .morphBounds(
+                                    MonthMorphKey.Event(
+                                        date = week.days[span.startCol],
+                                        instanceId = span.event.instanceId,
+                                    ),
+                                ),
                         )
                     }
                     // Single-day timed pills + overflow, per column. Pills fill the
@@ -1643,7 +1719,8 @@ private fun MonthWeekRow(
                                     )
                                     .width(colW)
                                     .height(EVENT_ROW_HEIGHT)
-                                    .padding(horizontal = CELL_GAP + 1.dp, vertical = 1.dp),
+                                    .padding(horizontal = CELL_GAP + 1.dp, vertical = 1.dp)
+                                    .morphBounds(MonthMorphKey.Event(d, ev.instanceId)),
                             )
                         }
                         val hidden = (week.countByDay[d] ?: 0) - occupied.size - pillsShown.size