keep playback going through undo and polish the queue drag

app/src/main/kotlin/com/subtracks/playback/Media3PlayerHandle.kt

@@ -46,6 +46,11 @@ class Media3PlayerHandle(
 
     override fun addLast(item: QueueItem) = controller.addMediaItems(controller.mediaItemCount, listOf(toMediaItem(item)))
 
+    override fun insertAt(
+        index: Int,
+        item: QueueItem,
+    ) = controller.addMediaItems(index, listOf(toMediaItem(item)))
+
     override fun removeFirst() = controller.removeMediaItem(0)
 
     override fun removeLast() = controller.removeMediaItem(controller.mediaItemCount - 1)

app/src/main/kotlin/com/subtracks/playback/PlaybackController.kt

@@ -46,10 +46,29 @@ data class PlaybackState(
     val hasPrevious: Boolean = false,
 )
 
-private data class QueueUndo(
-    val entries: List<QueueEntry>,
-    val cursor: Long,
-)
+private sealed interface QueueUndo {
+    val entries: List<QueueEntry>
+    val cursor: Long
+}
+
+private data class RemovedUndo(
+    override val entries: List<QueueEntry>,
+    override val cursor: Long,
+    val position: Long,
+    val item: QueueItem?,
+) : QueueUndo
+
+private data class MovedUndo(
+    override val entries: List<QueueEntry>,
+    override val cursor: Long,
+    val from: Long,
+    val to: Long,
+) : QueueUndo
+
+private data class ReloadUndo(
+    override val entries: List<QueueEntry>,
+    override val cursor: Long,
+) : QueueUndo
 
 class PlaybackController(
     private val sourceRepository: SourceRepository,
@@ -157,20 +176,27 @@ class PlaybackController(
             val snapshot = snapshot ?: return@withLock
             if (position !in 0 until snapshot.size) return@withLock
             val player = player ?: return@withLock
-            val undo = QueueUndo(snapshot.entries.map { it.entry }, queueRepository.cursor())
+            val cursor = queueRepository.cursor()
+            val entries = snapshot.entries.map { it.entry }
             if (snapshot.size == 1L) {
                 queueRepository.removeAt(snapshot, position)
                 stopLocked()
-                lastEdit = undo
+                lastEdit = ReloadUndo(entries, cursor)
                 return@withLock
             }
             val current = currentPosition() ?: return@withLock
             val target = (if (position < current) current - 1 else current).coerceIn(0, snapshot.size - 2)
             val playing = player.playWhenReady
+            val removed = if (position == current) null else queueRepository.itemAt(snapshot, position)?.toQueueItem()
             queueRepository.removeAt(snapshot, position)
             this.snapshot = queueRepository.snapshot()
             windowJob?.cancel()
-            lastEdit = undo
+            lastEdit =
+                if (removed == null) {
+                    ReloadUndo(entries, cursor)
+                } else {
+                    RemovedUndo(entries, cursor, position, removed)
+                }
             when {
                 position == current -> {
                     loadWindow(target, autoplay = playing, startPositionMs = 0)
@@ -207,13 +233,20 @@ class PlaybackController(
         if (from !in 0 until snapshot.size || to !in 0 until snapshot.size) return@withLock
         val player = player ?: return@withLock
         val current = currentPosition() ?: return@withLock
-        val undo = QueueUndo(snapshot.entries.map { it.entry }, queueRepository.cursor())
+        val cursor = queueRepository.cursor()
+        val entries = snapshot.entries.map { it.entry }
         if (!queueRepository.move(from, to)) return@withLock
         this.snapshot = queueRepository.snapshot()
         val target = movedCursor(current, from, to)
         windowJob?.cancel()
-        lastEdit = undo
-        if (from in windowStart..windowEnd && to in windowStart..windowEnd) {
+        val inWindow = from in windowStart..windowEnd && to in windowStart..windowEnd
+        lastEdit =
+            if (inWindow) {
+                MovedUndo(entries, cursor, from, to)
+            } else {
+                ReloadUndo(entries, cursor)
+            }
+        if (inWindow) {
             updating = true
             player.move((from - windowStart).toInt(), (to - windowStart).toInt())
             updating = false
@@ -230,7 +263,6 @@ class PlaybackController(
             val undo = lastEdit ?: return@withLock
             val player = player ?: return@withLock
             val playing = player.playWhenReady
-            val playingId = player.currentItem?.id
             val positionMs = player.currentPositionMs
             queueRepository.replace(undo.entries)
             val restored = queueRepository.snapshot()
@@ -241,9 +273,44 @@ class PlaybackController(
                 return@withLock
             }
             val target = undo.cursor.coerceIn(0, restored.size - 1)
+            val restoredId = queueRepository.itemAt(restored, target)?.song?.id
             windowJob?.cancel()
-            val same = queueRepository.itemAt(restored, target)?.song?.id == playingId
-            loadWindow(target, autoplay = playing, startPositionMs = if (same) positionMs else 0)
+            val canMirror = player.currentItem?.id != null && player.currentItem?.id == restoredId
+            val mirrored =
+                when {
+                    !canMirror -> {
+                        false
+                    }
+
+                    undo is RemovedUndo && undo.item != null -> {
+                        if (undo.position < windowStart) {
+                            windowStart++
+                            windowEnd++
+                        } else if (undo.position <= windowEnd) {
+                            updating = true
+                            player.insertAt((undo.position - windowStart).toInt(), undo.item)
+                            updating = false
+                            windowEnd++
+                        }
+                        true
+                    }
+
+                    undo is MovedUndo && undo.from in windowStart..windowEnd && undo.to in windowStart..windowEnd -> {
+                        updating = true
+                        player.move((undo.to - windowStart).toInt(), (undo.from - windowStart).toInt())
+                        updating = false
+                        true
+                    }
+
+                    else -> {
+                        false
+                    }
+                }
+            if (mirrored) {
+                shiftWindowLocked(target)
+            } else {
+                loadWindow(target, autoplay = playing, startPositionMs = if (canMirror) positionMs else 0)
+            }
             queueRepository.setCursor(target)
             refresh(target)
         }

app/src/main/kotlin/com/subtracks/playback/PlayerHandle.kt

@@ -22,6 +22,11 @@ interface PlayerHandle {
 
     fun addLast(item: QueueItem)
 
+    fun insertAt(
+        index: Int,
+        item: QueueItem,
+    )
+
     fun removeFirst()
 
     fun removeLast()

app/src/main/kotlin/com/subtracks/ui/playback/QueueScreen.kt

@@ -1,11 +1,16 @@
 package com.subtracks.ui.playback
 
+import androidx.compose.foundation.background
 import androidx.compose.foundation.clickable
 import androidx.compose.foundation.gestures.detectDragGestures
+import androidx.compose.foundation.gestures.scrollBy
 import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.PaddingValues
 import androidx.compose.foundation.layout.Row
 import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.height
 import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.layout.size
 import androidx.compose.foundation.lazy.LazyColumn
@@ -17,6 +22,7 @@ import androidx.compose.material.icons.rounded.DragHandle
 import androidx.compose.material3.ExperimentalMaterial3Api
 import androidx.compose.material3.Icon
 import androidx.compose.material3.IconButton
+import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.Scaffold
 import androidx.compose.material3.SnackbarDuration
 import androidx.compose.material3.SnackbarHost
@@ -34,9 +40,11 @@ import androidx.compose.runtime.rememberCoroutineScope
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.graphics.graphicsLayer
 import androidx.compose.ui.input.pointer.pointerInput
 import androidx.compose.ui.layout.onSizeChanged
+import androidx.compose.ui.platform.LocalDensity
 import androidx.compose.ui.unit.dp
 import androidx.compose.ui.zIndex
 import androidx.lifecycle.ViewModel
@@ -58,6 +66,7 @@ import com.subtracks.playback.PlaybackController
 import com.subtracks.ui.components.EmptyState
 import com.subtracks.ui.components.LoadingState
 import com.subtracks.ui.library.SongRow
+import kotlinx.coroutines.delay
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.launch
 import org.koin.compose.koinInject
@@ -127,6 +136,8 @@ fun QueueRoute(
     )
 }
 
+private enum class DropIndicator { Above, Below }
+
 @OptIn(ExperimentalMaterial3Api::class)
 @Composable
 fun QueueScreen(
@@ -143,9 +154,11 @@ fun QueueScreen(
     val listState = rememberLazyListState()
     val snackbarHostState = remember { SnackbarHostState() }
     val scope = rememberCoroutineScope()
+    val density = LocalDensity.current
     var rowHeight by remember { mutableFloatStateOf(0f) }
     var dragging by remember { mutableStateOf<Long?>(null) }
     var dragOffset by remember { mutableFloatStateOf(0f) }
+    var target by remember { mutableStateOf<Long?>(null) }
     var centered by remember { mutableStateOf(false) }
 
     val firstPosition = if (items.itemCount > 0) items.peek(0)?.position else null
@@ -159,6 +172,30 @@ fun QueueScreen(
         }
     }
 
+    LaunchedEffect(dragging) {
+        if (dragging == null) return@LaunchedEffect
+        val threshold = with(density) { 72.dp.toPx() }
+        val step = with(density) { 10.dp.toPx() }
+        while (true) {
+            val from = dragging ?: break
+            val info = listState.layoutInfo.visibleItemsInfo.firstOrNull { items.peek(it.index)?.position == from }
+            if (info != null) {
+                val edge =
+                    when {
+                        info.offset < listState.layoutInfo.viewportStartOffset + threshold -> -step
+                        info.offset + info.size > listState.layoutInfo.viewportEndOffset - threshold -> step
+                        else -> 0f
+                    }
+                if (edge != 0f) {
+                    val consumed = listState.scrollBy(edge)
+                    dragOffset += consumed
+                    target = dropTarget(items, from, dragOffset, rowHeight)
+                }
+            }
+            delay(16)
+        }
+    }
+
     val showUndo: (String) -> Unit = { message ->
         scope.launch {
             snackbarHostState.currentSnackbarData?.dismiss()
@@ -205,6 +242,7 @@ fun QueueScreen(
                                 coverArt = coverArt,
                                 dragging = dragging == item.position,
                                 dragOffset = if (dragging == item.position) dragOffset else 0f,
+                                indicator = indicatorFor(item.position, dragging, target),
                                 onClick = { onPlay(item.position) },
                                 onRemove = {
                                     onRemove(item.position)
@@ -214,23 +252,27 @@ fun QueueScreen(
                                 onDragStart = {
                                     dragging = item.position
                                     dragOffset = 0f
+                                    target = item.position
+                                },
+                                onDrag = { amount ->
+                                    dragOffset += amount
+                                    target = dropTarget(items, item.position, dragOffset, rowHeight)
                                 },
-                                onDrag = { dragOffset += it },
                                 onDragEnd = {
                                     val from = dragging
-                                    if (from != null && rowHeight > 0f) {
-                                        val to = (from + (dragOffset / rowHeight).roundToInt()).coerceAtLeast(0)
-                                        if (to != from) {
-                                            onMove(from, to)
-                                            showUndo("Queue reordered")
-                                        }
+                                    val to = target
+                                    if (from != null && to != null && to != from) {
+                                        onMove(from, to)
+                                        showUndo("Queue reordered")
                                     }
                                     dragging = null
                                     dragOffset = 0f
+                                    target = null
                                 },
                                 onDragCancel = {
                                     dragging = null
                                     dragOffset = 0f
+                                    target = null
                                 },
                             )
                         }
@@ -241,6 +283,30 @@ fun QueueScreen(
     }
 }
 
+private fun dropTarget(
+    items: LazyPagingItems<QueueWindowItem>,
+    from: Long,
+    offset: Float,
+    rowHeight: Float,
+): Long {
+    if (items.itemCount == 0 || rowHeight <= 0f) return from
+    val first = items.peek(0)?.position ?: from
+    val last = items.peek(items.itemCount - 1)?.position ?: from
+    return (from + (offset / rowHeight).roundToInt()).coerceIn(minOf(first, last), maxOf(first, last))
+}
+
+private fun indicatorFor(
+    position: Long,
+    dragging: Long?,
+    target: Long?,
+): DropIndicator? =
+    when {
+        dragging == null || target == null || position != target -> null
+        target > dragging -> DropIndicator.Below
+        target < dragging -> DropIndicator.Above
+        else -> null
+    }
+
 @Composable
 private fun QueueRow(
     item: QueueWindowItem,
@@ -248,6 +314,7 @@ private fun QueueRow(
     coverArt: (String?, Boolean) -> CoverArtRef?,
     dragging: Boolean,
     dragOffset: Float,
+    indicator: DropIndicator?,
     onClick: () -> Unit,
     onRemove: () -> Unit,
     onSize: (Int) -> Unit,
@@ -256,46 +323,70 @@ private fun QueueRow(
     onDragEnd: () -> Unit,
     onDragCancel: () -> Unit,
 ) {
-    SongRow(
-        song = item.item.song,
-        coverArtId = item.item.coverArt,
-        coverArt = coverArt,
-        isPlaying = isPlaying,
-        trailingContent = {
-            Row(verticalAlignment = Alignment.CenterVertically) {
-                IconButton(onClick = onRemove) {
-                    Icon(
-                        imageVector = Icons.Rounded.Close,
-                        contentDescription = "Remove from queue",
-                        modifier = Modifier.size(20.dp),
-                    )
-                }
-                Box(
-                    modifier =
-                        Modifier
-                            .size(40.dp)
-                            .pointerInput(item.position) {
-                                detectDragGestures(
-                                    onDragStart = { onDragStart() },
-                                    onDrag = { change, amount ->
-                                        change.consume()
-                                        onDrag(amount.y)
-                                    },
-                                    onDragEnd = onDragEnd,
-                                    onDragCancel = onDragCancel,
-                                )
-                            },
-                    contentAlignment = Alignment.Center,
-                ) {
-                    Icon(imageVector = Icons.Rounded.DragHandle, contentDescription = "Reorder")
-                }
-            }
-        },
+    Column(
         modifier =
             Modifier
                 .zIndex(if (dragging) 1f else 0f)
-                .graphicsLayer { translationY = dragOffset }
-                .onSizeChanged { size -> onSize(size.height) }
-                .clickable(onClick = onClick),
+                .graphicsLayer { translationY = dragOffset },
+    ) {
+        if (indicator == DropIndicator.Above) DropIndicatorLine()
+        SongRow(
+            song = item.item.song,
+            coverArtId = item.item.coverArt,
+            coverArt = coverArt,
+            isPlaying = isPlaying,
+            trailingContent = {
+                Row(verticalAlignment = Alignment.CenterVertically) {
+                    IconButton(onClick = onRemove) {
+                        Icon(
+                            imageVector = Icons.Rounded.Close,
+                            contentDescription = "Remove from queue",
+                            modifier = Modifier.size(20.dp),
+                        )
+                    }
+                    Box(
+                        modifier =
+                            Modifier
+                                .size(40.dp)
+                                .pointerInput(item.position) {
+                                    detectDragGestures(
+                                        onDragStart = { onDragStart() },
+                                        onDrag = { change, amount ->
+                                            change.consume()
+                                            onDrag(amount.y)
+                                        },
+                                        onDragEnd = onDragEnd,
+                                        onDragCancel = onDragCancel,
+                                    )
+                                },
+                        contentAlignment = Alignment.Center,
+                    ) {
+                        Icon(imageVector = Icons.Rounded.DragHandle, contentDescription = "Reorder")
+                    }
+                }
+            },
+            modifier =
+                Modifier
+                    .background(
+                        if (dragging) {
+                            MaterialTheme.colorScheme.surfaceContainerHighest.copy(alpha = 0.95f)
+                        } else {
+                            Color.Transparent
+                        },
+                    ).onSizeChanged { size -> onSize(size.height) }
+                    .clickable(onClick = onClick),
+        )
+        if (indicator == DropIndicator.Below) DropIndicatorLine()
+    }
+}
+
+@Composable
+private fun DropIndicatorLine() {
+    Box(
+        modifier =
+            Modifier
+                .fillMaxWidth()
+                .height(2.dp)
+                .background(MaterialTheme.colorScheme.primary),
     )
 }

app/src/test/kotlin/com/subtracks/playback/FakePlayer.kt

@@ -86,6 +86,15 @@ class FakePlayerHandle : PlayerHandle {
         items += item
     }
 
+    override fun insertAt(
+        index: Int,
+        item: QueueItem,
+    ) {
+        operations += "insertAt($index, ${item.id})"
+        items.add(index, item)
+        if (index <= this.index) this.index++
+    }
+
     override fun removeFirst() {
         operations += "removeFirst"
         items.removeAt(0)