Fix background parallax

Jetsnack/app/src/main/java/com/example/jetsnack/ui/components/Gradient.kt

@@ -21,12 +21,14 @@ import androidx.compose.foundation.background
 import androidx.compose.foundation.border
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.composed
+import androidx.compose.ui.draw.drawBehind
 import androidx.compose.ui.draw.drawWithContent
 import androidx.compose.ui.graphics.BlendMode
 import androidx.compose.ui.graphics.Brush
 import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.graphics.Shape
 import androidx.compose.ui.graphics.TileMode
+import androidx.compose.ui.unit.Density
 import androidx.compose.ui.unit.Dp
 import androidx.compose.ui.unit.dp
 
@@ -47,13 +49,30 @@ fun Modifier.offsetGradientBackground(
     offset: Float = 0f
 ) = background(
     Brush.horizontalGradient(
-        colors,
+        colors = colors,
         startX = -offset,
         endX = width - offset,
         tileMode = TileMode.Mirror
     )
 )
 
+fun Modifier.offsetGradientBackground(
+    colors: List<Color>,
+    width: Density.() -> Float,
+    offset: Density.() -> Float = { 0f }
+) = drawBehind {
+    val actualOffset = offset()
+
+    drawRect(
+        Brush.horizontalGradient(
+            colors = colors,
+            startX = -actualOffset,
+            endX = width() - actualOffset,
+            tileMode = TileMode.Mirror
+        )
+    )
+}
+
 fun Modifier.diagonalGradientBorder(
     colors: List<Color>,
     borderSize: Dp = 2.dp,

Jetsnack/app/src/main/java/com/example/jetsnack/ui/components/Snacks.kt

@@ -34,7 +34,7 @@ import androidx.compose.foundation.layout.wrapContentWidth
 import androidx.compose.foundation.lazy.LazyRow
 import androidx.compose.foundation.lazy.items
 import androidx.compose.foundation.lazy.itemsIndexed
-import androidx.compose.foundation.rememberScrollState
+import androidx.compose.foundation.lazy.rememberLazyListState
 import androidx.compose.foundation.shape.CircleShape
 import androidx.compose.material.Icon
 import androidx.compose.material.IconButton
@@ -53,6 +53,7 @@ import androidx.compose.ui.platform.LocalDensity
 import androidx.compose.ui.res.painterResource
 import androidx.compose.ui.text.style.TextOverflow
 import androidx.compose.ui.tooling.preview.Preview
+import androidx.compose.ui.unit.Density
 import androidx.compose.ui.unit.Dp
 import androidx.compose.ui.unit.dp
 import coil.compose.AsyncImage
@@ -67,13 +68,8 @@ import com.example.jetsnack.ui.utils.mirroringIcon
 
 private val HighlightCardWidth = 170.dp
 private val HighlightCardPadding = 16.dp
-
-// The Cards show a gradient which spans 3 cards and scrolls with parallax.
-private val gradientWidth
-    @Composable
-    get() = with(LocalDensity.current) {
-        (3 * (HighlightCardWidth + HighlightCardPadding).toPx())
-    }
+private val Density.cardWidthWithPaddingPx
+    get() = (HighlightCardWidth + HighlightCardPadding).toPx()
 
 @Composable
 fun SnackCollection(
@@ -129,28 +125,33 @@ private fun HighlightedSnacks(
     onSnackClick: (Long) -> Unit,
     modifier: Modifier = Modifier
 ) {
-    val scroll = rememberScrollState(0)
+    val rowState = rememberLazyListState()
+    val cardWidthWithPaddingPx = with(LocalDensity.current) { cardWidthWithPaddingPx }
+
+    val scrollProvider = {
+        // Simple calculation of scroll distance for homogenous item types with the same width.
+        val offsetFromStart = cardWidthWithPaddingPx * rowState.firstVisibleItemIndex
+        offsetFromStart + rowState.firstVisibleItemScrollOffset
+    }
+
     val gradient = when ((index / 2) % 2) {
         0 -> JetsnackTheme.colors.gradient6_1
         else -> JetsnackTheme.colors.gradient6_2
     }
-    // The Cards show a gradient which spans 3 cards and scrolls with parallax.
-    val gradientWidth = with(LocalDensity.current) {
-        (6 * (HighlightCardWidth + HighlightCardPadding).toPx())
-    }
+
     LazyRow(
+        state = rowState,
         modifier = modifier,
         horizontalArrangement = Arrangement.spacedBy(16.dp),
         contentPadding = PaddingValues(start = 24.dp, end = 24.dp)
     ) {
         itemsIndexed(snacks) { index, snack ->
             HighlightSnackItem(
-                snack,
-                onSnackClick,
-                index,
-                gradient,
-                gradientWidth,
-                scroll.value
+                snack = snack,
+                onSnackClick = onSnackClick,
+                index = index,
+                gradient = gradient,
+                scrollProvider = scrollProvider
             )
         }
     }
@@ -214,17 +215,13 @@ private fun HighlightSnackItem(
     onSnackClick: (Long) -> Unit,
     index: Int,
     gradient: List<Color>,
-    gradientWidth: Float,
-    scroll: Int,
+    scrollProvider: () -> Float,
     modifier: Modifier = Modifier
 ) {
-    val left = index * with(LocalDensity.current) {
-        (HighlightCardWidth + HighlightCardPadding).toPx()
-    }
     JetsnackCard(
         modifier = modifier
             .size(
-                width = 170.dp,
+                width = HighlightCardWidth,
                 height = 250.dp
             )
             .padding(bottom = 16.dp)
@@ -239,12 +236,22 @@ private fun HighlightSnackItem(
                     .height(160.dp)
                     .fillMaxWidth()
             ) {
-                val gradientOffset = left - (scroll / 3f)
                 Box(
                     modifier = Modifier
                         .height(100.dp)
                         .fillMaxWidth()
-                        .offsetGradientBackground(gradient, gradientWidth, gradientOffset)
+                        .offsetGradientBackground(
+                            colors = gradient,
+                            width = {
+                                // The Cards show a gradient which spans 6 cards and scrolls with parallax.
+                                6 * cardWidthWithPaddingPx
+                            },
+                            offset = {
+                                val left = index * cardWidthWithPaddingPx
+                                val gradientOffset = left - (scrollProvider() / 3f)
+                                gradientOffset
+                            }
+                        )
                 )
                 SnackImage(
                     imageUrl = snack.imageUrl,
@@ -312,8 +319,7 @@ fun SnackCardPreview() {
             onSnackClick = { },
             index = 0,
             gradient = JetsnackTheme.colors.gradient6_1,
-            gradientWidth = gradientWidth,
-            scroll = 0
+            scrollProvider = { 0f }
         )
     }
 }