Crossfade a block's time label (#68)

The block keeps its identity across a move and slides to the new slot, so
its label was the one thing left changing in a single frame.

app/src/main/java/de/jeanlucmakiola/calendula/ui/common/BlockPlacement.kt

@@ -1,16 +1,50 @@
 package de.jeanlucmakiola.calendula.ui.common
 
+import androidx.compose.animation.Crossfade
 import androidx.compose.animation.core.FiniteAnimationSpec
 import androidx.compose.animation.core.animateDpAsState
 import androidx.compose.animation.core.snap
 import androidx.compose.material3.ExperimentalMaterial3ExpressiveApi
 import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.Immutable
 import androidx.compose.runtime.getValue
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.text.style.TextOverflow
 import androidx.compose.ui.unit.Dp
 import de.jeanlucmakiola.floret.identity.rememberReduceMotion
 
+/**
+ * A timed block's own time label, crossfaded rather than replaced. The block
+ * keeps its identity across a move and slides to the new slot; the label is the
+ * one thing on it that would otherwise change in a single frame.
+ */
+@OptIn(ExperimentalMaterial3ExpressiveApi::class)
+@Composable
+fun BlockTimeLabel(label: String, color: Color, modifier: Modifier = Modifier) {
+    val spec: FiniteAnimationSpec<Float> = if (rememberReduceMotion()) {
+        snap()
+    } else {
+        MaterialTheme.motionScheme.fastEffectsSpec()
+    }
+    Crossfade(
+        targetState = label,
+        animationSpec = spec,
+        label = "block-time",
+        modifier = modifier,
+    ) { text ->
+        Text(
+            text = text,
+            style = MaterialTheme.typography.labelSmall,
+            maxLines = 1,
+            overflow = TextOverflow.Ellipsis,
+            color = color,
+        )
+    }
+}
+
 /** Where a timed block sits in its column, after tweening. */
 @Immutable
 data class BlockPlacement(val x: Dp, val y: Dp, val width: Dp, val height: Dp)

app/src/main/java/de/jeanlucmakiola/calendula/ui/day/DayScreen.kt

@@ -80,6 +80,7 @@ import de.jeanlucmakiola.calendula.ui.common.TodayAction
 import de.jeanlucmakiola.calendula.ui.common.CalendarFailure
 import de.jeanlucmakiola.calendula.ui.common.CalendarView
 import de.jeanlucmakiola.calendula.ui.common.IMPLEMENTED_VIEWS
+import de.jeanlucmakiola.calendula.ui.common.BlockTimeLabel
 import de.jeanlucmakiola.calendula.ui.common.animatedBlockPlacement
 import de.jeanlucmakiola.calendula.ui.common.ghostAlpha
 import de.jeanlucmakiola.calendula.ui.common.LocalEventMove
@@ -762,11 +763,8 @@ private fun EventBlock(
                 )
             }
             if (showTime) {
-                Text(
-                    text = timeLabel,
-                    style = MaterialTheme.typography.labelSmall,
-                    maxLines = 1,
-                    overflow = TextOverflow.Ellipsis,
+                BlockTimeLabel(
+                    label = timeLabel,
                     color = eventInk(fill, alpha = SECONDARY_INK_ALPHA),
                 )
             }

app/src/main/java/de/jeanlucmakiola/calendula/ui/week/WeekScreen.kt

@@ -89,6 +89,7 @@ import de.jeanlucmakiola.calendula.ui.common.CalendarFailure
 import de.jeanlucmakiola.calendula.ui.common.CalendarView
 import de.jeanlucmakiola.calendula.ui.common.IMPLEMENTED_VIEWS
 import de.jeanlucmakiola.calendula.ui.common.EventDimAlpha
+import de.jeanlucmakiola.calendula.ui.common.BlockTimeLabel
 import de.jeanlucmakiola.calendula.ui.common.animatedBlockPlacement
 import de.jeanlucmakiola.calendula.ui.common.ghostAlpha
 import de.jeanlucmakiola.calendula.ui.common.LocalEventMove
@@ -931,11 +932,8 @@ private fun EventBlock(
                 )
             }
             if (showTime) {
-                Text(
-                    text = timeLabel,
-                    style = MaterialTheme.typography.labelSmall,
-                    maxLines = 1,
-                    overflow = TextOverflow.Ellipsis,
+                BlockTimeLabel(
+                    label = timeLabel,
                     color = eventInk(fill, alpha = SECONDARY_INK_ALPHA),
                 )
             }