[Carousel] Prevent scrolling if there's less items than focal keylines

PiperOrigin-RevId: 625466841

Author: imhappi

lib/java/com/google/android/material/carousel/CarouselLayoutManager.java

@@ -784,6 +784,10 @@ private static KeylineRange getSurroundingKeylineRange(
         keylines.get(startMinDistanceIndex), keylines.get(endMinDistanceIndex));
   }
 
+  private KeylineState getKeylineStartingState(KeylineStateList keylineStateList) {
+    return isLayoutRtl() ? keylineStateList.getEndState() : keylineStateList.getStartState();
+  }
+
   /**
    * Update the current keyline state by shifting it in response to any change in scroll offset.
    *
@@ -794,8 +798,7 @@ private void updateCurrentKeylineStateForScrollOffset(
     if (maxScroll <= minScroll) {
       // We don't have enough items in the list to scroll and we should use the keyline state
       // that aligns items to the start of the container.
-      this.currentKeylineState =
-          isLayoutRtl() ? keylineStateList.getEndState() : keylineStateList.getStartState();
+      this.currentKeylineState = getKeylineStartingState(keylineStateList);
     } else {
       this.currentKeylineState =
           keylineStateList.getShiftedState(scrollOffset, minScroll, maxScroll);
@@ -1454,6 +1457,12 @@ private int scrollBy(int distance, Recycler recycler, State state) {
       recalculateKeylineStateList(recycler);
     }
 
+    // If the number of items is equal or less than the number of focal items, we should not be able
+    // to scroll.
+    if (getItemCount() <= getKeylineStartingState(keylineStateList).getTotalVisibleFocalItems()) {
+      return 0;
+    }
+
     // Calculate how much the carousel should scroll and update the scroll offset.
     int scrolledBy = calculateShouldScrollBy(distance, scrollOffset, minScroll, maxScroll);
     scrollOffset += scrolledBy;

lib/java/com/google/android/material/carousel/KeylineState.java

@@ -52,6 +52,7 @@
 final class KeylineState {
 
   private final float itemSize;
+  private int totalVisibleFocalItems;
   private final List<Keyline> keylines;
   private final int firstFocalKeylineIndex;
   private final int lastFocalKeylineIndex;
@@ -65,6 +66,11 @@ private KeylineState(
     this.keylines = Collections.unmodifiableList(keylines);
     this.firstFocalKeylineIndex = firstFocalKeylineIndex;
     this.lastFocalKeylineIndex = lastFocalKeylineIndex;
+    for (int i = firstFocalKeylineIndex; i <= lastFocalKeylineIndex; i++) {
+      if (keylines.get(i).cutoff == 0) {
+        this.totalVisibleFocalItems += 1;
+      }
+    }
   }
 
   /**
@@ -81,6 +87,11 @@ List<Keyline> getKeylines() {
     return keylines;
   }
 
+  /** Returns the number of focal items in the keyline state. */
+  int getTotalVisibleFocalItems() {
+    return totalVisibleFocalItems;
+  }
+
   /** Returns the first focal keyline in the list. */
   Keyline getFirstFocalKeyline() {
     return keylines.get(firstFocalKeylineIndex);

lib/javatests/com/google/android/material/carousel/CarouselLayoutManagerTest.java

@@ -614,6 +614,18 @@ public void testRequestChildRectangleOnScreen_doesntScrollIfChildIsFocal() throw
     assertThat(layoutManager.scrollOffset).isEqualTo(0);
   }
 
+  @Test
+  public void testSingleItem_shouldNotScrollWithPadding() throws Throwable {
+    recyclerView.setPadding(50, 0, 50, 0);
+    recyclerView.setClipToPadding(false);
+    setAdapterItems(recyclerView, layoutManager, adapter, createDataSetWithSize(1));
+    int originalLeft = recyclerView.getChildAt(0).getLeft();
+
+    scrollHorizontallyBy(recyclerView, layoutManager, 100);
+
+    assertThat(recyclerView.getChildAt(0).getLeft()).isEqualTo(originalLeft);
+  }
+
   /**
    * Assigns explicit sizes to fixtures being used to construct the testing environment.
    *