swipe between tracks on the now playing screen

Left/right drags on the cover art skip to the next/previous track, mirroring the transport buttons; the cover follows the finger and settles back on release. Also drops the ripple on the title/artist block.

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

@@ -1,8 +1,13 @@
 package com.subtracks.ui.playback
 
+import androidx.compose.animation.core.animate
+import androidx.compose.animation.core.tween
 import androidx.compose.foundation.background
 import androidx.compose.foundation.basicMarquee
 import androidx.compose.foundation.clickable
+import androidx.compose.foundation.gestures.Orientation
+import androidx.compose.foundation.gestures.draggable
+import androidx.compose.foundation.gestures.rememberDraggableState
 import androidx.compose.foundation.interaction.MutableInteractionSource
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Box
@@ -53,6 +58,8 @@ 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.layout.onSizeChanged
 import androidx.compose.ui.platform.LocalDensity
 import androidx.compose.ui.platform.testTag
 import androidx.compose.ui.text.PlatformTextStyle
@@ -81,11 +88,16 @@ import com.subtracks.ui.theme.ArtworkSeedCache
 import com.subtracks.ui.theme.ArtworkTheme
 import com.subtracks.ui.theme.HeroGradient
 import com.subtracks.ui.theme.rememberArtworkColors
+import kotlinx.coroutines.Job
 import kotlinx.coroutines.launch
 import org.koin.compose.koinInject
 
 const val NOW_PLAYING_COVER_TAG = "now-playing-cover"
 
+private const val SWIPE_THRESHOLD_FRACTION = 0.25f
+private const val SWIPE_FLING_VELOCITY = 1000f
+private const val SWIPE_SETTLE_MS = 180
+
 @Composable
 fun NowPlayingRoute(
     onBack: () -> Unit,
@@ -200,6 +212,18 @@ fun NowPlayingScreen(
             platformStyle = PlatformTextStyle(includeFontPadding = false),
             lineHeightStyle = LineHeightStyle(LineHeightStyle.Alignment.Center, LineHeightStyle.Trim.Both),
         )
+    var swipeOffsetX by remember { mutableFloatStateOf(0f) }
+    var coverWidthPx by remember { mutableFloatStateOf(0f) }
+    var settleJob by remember { mutableStateOf<Job?>(null) }
+    val scope = rememberCoroutineScope()
+
+    fun settleSwipe() {
+        settleJob?.cancel()
+        settleJob =
+            scope.launch {
+                animate(swipeOffsetX, 0f, animationSpec = tween(SWIPE_SETTLE_MS)) { value, _ -> swipeOffsetX = value }
+            }
+    }
 
     CompositionLocalProvider(LocalContentColor provides MaterialTheme.colorScheme.onBackground) {
         ArtworkTheme(artwork) {
@@ -254,7 +278,21 @@ fun NowPlayingScreen(
                                     .weight(1f)
                                     .fillMaxWidth()
                                     .padding(top = 2.dp)
-                                    .clickable(
+                                    .onSizeChanged { coverWidthPx = it.width.toFloat() }
+                                    .graphicsLayer { translationX = swipeOffsetX }
+                                    .draggable(
+                                        orientation = Orientation.Horizontal,
+                                        state = rememberDraggableState { delta -> swipeOffsetX += delta },
+                                        onDragStarted = { settleJob?.cancel() },
+                                        onDragStopped = { velocity ->
+                                            val threshold = coverWidthPx * SWIPE_THRESHOLD_FRACTION
+                                            when {
+                                                swipeOffsetX <= -threshold || velocity <= -SWIPE_FLING_VELOCITY -> onNext()
+                                                swipeOffsetX >= threshold || velocity >= SWIPE_FLING_VELOCITY -> onPrevious()
+                                            }
+                                            settleSwipe()
+                                        },
+                                    ).clickable(
                                         interactionSource = remember { MutableInteractionSource() },
                                         indication = null,
                                         enabled = onAlbumClick != null,
@@ -299,6 +337,8 @@ fun NowPlayingScreen(
                                     Modifier
                                         .weight(1f)
                                         .clickable(
+                                            interactionSource = remember { MutableInteractionSource() },
+                                            indication = null,
                                             enabled = onArtistClick != null,
                                             onClickLabel = "Open artist",
                                         ) { onArtistClick?.invoke() },

app/src/test/kotlin/com/subtracks/ui/playback/NowPlayingTapsTest.kt

@@ -1,6 +1,7 @@
 package com.subtracks.ui.playback
 
 import androidx.activity.ComponentActivity
+import androidx.compose.ui.geometry.Offset
 import androidx.compose.ui.test.assertHeightIsAtLeast
 import androidx.compose.ui.test.assertHeightIsEqualTo
 import androidx.compose.ui.test.assertIsNotEnabled
@@ -10,6 +11,10 @@ import androidx.compose.ui.test.onNodeWithTag
 import androidx.compose.ui.test.onNodeWithText
 import androidx.compose.ui.test.onRoot
 import androidx.compose.ui.test.performClick
+import androidx.compose.ui.test.performTouchInput
+import androidx.compose.ui.test.swipe
+import androidx.compose.ui.test.swipeLeft
+import androidx.compose.ui.test.swipeRight
 import androidx.compose.ui.unit.dp
 import androidx.test.ext.junit.runners.AndroidJUnit4
 import com.subtracks.data.model.CoverArtRef
@@ -48,6 +53,8 @@ class NowPlayingTapsTest {
     private fun render(
         onAlbumClick: (() -> Unit)? = null,
         onArtistClick: (() -> Unit)? = null,
+        onNext: () -> Unit = {},
+        onPrevious: () -> Unit = {},
     ) {
         composeRule.setContent {
             SubtracksTheme {
@@ -60,8 +67,8 @@ class NowPlayingTapsTest {
                     onBack = {},
                     onQueue = {},
                     onPlayPause = {},
-                    onNext = {},
+                    onNext = onNext,
-                    onPrevious = {},
+                    onPrevious = onPrevious,
                     onAlbumClick = onAlbumClick,
                     onArtistClick = onArtistClick,
                     onSeek = {},
@@ -126,4 +133,52 @@ class NowPlayingTapsTest {
         composeRule.onNodeWithTag(NOW_PLAYING_COVER_TAG).assertIsNotEnabled()
         composeRule.onNodeWithText("Radiohead").assertIsNotEnabled()
     }
+
+    @Test
+    fun swipingLeftOnTheCoverSkipsToTheNextTrack() {
+        var next = false
+        var previous = false
+        render(onNext = { next = true }, onPrevious = { previous = true })
+
+        composeRule.onNodeWithTag(NOW_PLAYING_COVER_TAG).performTouchInput { swipeLeft() }
+
+        assertTrue("a left swipe should skip to the next track", next)
+        assertTrue("a left swipe should not go to the previous track", !previous)
+    }
+
+    @Test
+    fun swipingRightOnTheCoverGoesToThePreviousTrack() {
+        var next = false
+        var previous = false
+        render(onNext = { next = true }, onPrevious = { previous = true })
+
+        composeRule.onNodeWithTag(NOW_PLAYING_COVER_TAG).performTouchInput { swipeRight() }
+
+        assertTrue("a right swipe should go to the previous track", previous)
+        assertTrue("a right swipe should not skip to the next track", !next)
+    }
+
+    @Test
+    fun aShortSlowDragOnTheCoverDoesNotSkip() {
+        var next = false
+        var previous = false
+        render(onNext = { next = true }, onPrevious = { previous = true })
+
+        val cover = composeRule.onNodeWithTag(NOW_PLAYING_COVER_TAG)
+        val width =
+            cover
+                .fetchSemanticsNode()
+                .size.width
+                .toFloat()
+        cover.performTouchInput {
+            swipe(
+                start = Offset(width * 0.9f, centerY),
+                end = Offset(width * 0.7f, centerY),
+                durationMillis = 600,
+            )
+        }
+
+        assertTrue("a drag below the threshold should not skip", !next)
+        assertTrue("a drag below the threshold should not go to the previous track", !previous)
+    }
 }