[Carousel] Disallowing center aligned hero strategy with only 2 items…

… since it does not make any sense. With only 2 items there can only be a start state and end state with the hero strategy.

Resolves #3589

PiperOrigin-RevId: 568965460

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

@@ -32,4 +32,7 @@ interface Carousel {
 
   /** Gets the alignment of the carousel.  */
   @Alignment int getCarouselAlignment();
+
+  /** Gets the number of items in the carousel. */
+  int getItemCount();
 }

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

@@ -132,6 +132,8 @@ public class CarouselLayoutManager extends LayoutManager
   /** Aligns large items to the center of the carousel. */
   public static final int ALIGNMENT_CENTER = 1;
 
+  private int lastItemCount;
+
   /**
    * An estimation of the current focused position, determined by which item center is closest to
    * the first focal keyline. This is used when restoring item position after the carousel keylines
@@ -1419,6 +1421,29 @@ public void setOrientation(@RecyclerView.Orientation int orientation) {
     }
   }
 
+  @Override
+  public void onItemsAdded(@NonNull RecyclerView recyclerView, int positionStart, int itemCount) {
+    super.onItemsAdded(recyclerView, positionStart, itemCount);
+    updateItemCount();
+  }
+
+  @Override
+  public void onItemsRemoved(@NonNull RecyclerView recyclerView, int positionStart, int itemCount) {
+    super.onItemsRemoved(recyclerView, positionStart, itemCount);
+    updateItemCount();
+  }
+
+  private void updateItemCount() {
+    int newItemCount = getItemCount();
+    if (newItemCount == this.lastItemCount || keylineStateList == null) {
+      return;
+    }
+    if (carouselStrategy.shouldRefreshKeylineState(this, this.lastItemCount)) {
+      refreshKeylineState();
+    }
+    this.lastItemCount = newItemCount;
+  }
+
   /**
    * Enables features to help debug keylines and other internal layout manager logic.
    *

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

@@ -127,4 +127,16 @@ static int[] doubleCounts(int[] count) {
   boolean isContained() {
     return true;
   }
+
+  /**
+   * Whether or not the strategy keylines should be refreshed based on the old item count and the
+   * carousel's current parameters.
+   *
+   * @return true if the keylines should be refreshed.
+   */
+  boolean shouldRefreshKeylineState(Carousel carousel, int oldItemCount) {
+    // TODO: b/301332183 - Update existing strategies with logic on when to refresh keyline
+    // state based on item count.
+    return false;
+  }
 }

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

@@ -49,6 +49,7 @@ public class HeroCarouselStrategy extends CarouselStrategy {
 
   private static final int[] SMALL_COUNTS = new int[] {1};
   private static final int[] MEDIUM_COUNTS = new int[] {0, 1};
+  private static final int MIN_ITEMS_FOR_CENTER_ALIGNMENT = 2;
 
   @Override
   @NonNull
@@ -97,7 +98,8 @@ KeylineState onFirstChildMeasuredWithMargins(@NonNull Carousel carousel, @NonNul
       largeCounts[i] = largeCountMin + i;
     }
     boolean isCenterAligned =
-        carousel.getCarouselAlignment() == CarouselLayoutManager.ALIGNMENT_CENTER;
+        carousel.getCarouselAlignment() == CarouselLayoutManager.ALIGNMENT_CENTER
+            && carousel.getItemCount() > MIN_ITEMS_FOR_CENTER_ALIGNMENT;
     Arrangement arrangement =
         Arrangement.findLowestCostArrangement(
             availableSpace,
@@ -118,7 +120,16 @@ KeylineState onFirstChildMeasuredWithMargins(@NonNull Carousel carousel, @NonNul
         childMargins,
         availableSpace,
         arrangement,
-        carousel.getCarouselAlignment());
+        isCenterAligned
+            ? CarouselLayoutManager.ALIGNMENT_CENTER
+            : CarouselLayoutManager.ALIGNMENT_START);
     }
+
+  @Override
+  boolean shouldRefreshKeylineState(@NonNull Carousel carousel, int oldItemCount) {
+    return carousel.getCarouselAlignment() == CarouselLayoutManager.ALIGNMENT_CENTER
+        && (oldItemCount == MIN_ITEMS_FOR_CENTER_ALIGNMENT
+        || carousel.getItemCount() == MIN_ITEMS_FOR_CENTER_ALIGNMENT);
+  }
 }
 

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

@@ -39,6 +39,8 @@
 /** A helper class to facilitate Carousel tests */
 class CarouselHelper {
 
+  private static final int DEFAULT_ITEM_COUNT = 10;
+
   private CarouselHelper() {}
 
 
@@ -193,6 +195,11 @@ public boolean isHorizontal() {
       public int getCarouselAlignment() {
         return CarouselLayoutManager.ALIGNMENT_START;
       }
+
+      @Override
+      public int getItemCount() {
+        return DEFAULT_ITEM_COUNT;
+      }
     };
   }
 
@@ -217,6 +224,11 @@ public boolean isHorizontal() {
       public int getCarouselAlignment() {
         return CarouselLayoutManager.ALIGNMENT_CENTER;
       }
+
+      @Override
+      public int getItemCount() {
+        return DEFAULT_ITEM_COUNT;
+      }
     };
   }
 
@@ -246,6 +258,45 @@ public boolean isHorizontal() {
       public int getCarouselAlignment() {
         return alignment;
       }
+
+      @Override
+      public int getItemCount() {
+        return DEFAULT_ITEM_COUNT;
+      }
+    };
+  }
+
+  /**
+   * Creates a {@link Carousel} with a specified {@code size} for both width and height and the
+   * specified item count and alignment.
+   */
+  static Carousel createCarouselWithItemCount(int size, int alignment, int itemCount) {
+    return new Carousel() {
+
+      @Override
+      public int getContainerWidth() {
+        return size;
+      }
+
+      @Override
+      public int getContainerHeight() {
+        return size;
+      }
+
+      @Override
+      public boolean isHorizontal() {
+        return true;
+      }
+
+      @Override
+      public int getCarouselAlignment() {
+        return alignment;
+      }
+
+      @Override
+      public int getItemCount() {
+        return itemCount;
+      }
     };
   }
 

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

@@ -18,6 +18,7 @@
 import com.google.android.material.test.R;
 
 import static com.google.android.material.carousel.CarouselHelper.createCarousel;
+import static com.google.android.material.carousel.CarouselHelper.createCarouselWithItemCount;
 import static com.google.android.material.carousel.CarouselHelper.createCarouselWithWidth;
 import static com.google.android.material.carousel.CarouselHelper.createViewWithSize;
 import static com.google.common.truth.Truth.assertThat;
@@ -166,7 +167,7 @@ public void testKnownCenterAlignmentArrangement_correctlyCalculatesKeylineLocati
             ApplicationProvider.getApplicationContext(), (int) largeSize, (int) largeSize);
     int carouselSize = (int) (largeSize + smallSize * 2);
 
-    MultiBrowseCarouselStrategy strategy = new MultiBrowseCarouselStrategy();
+    HeroCarouselStrategy strategy = new HeroCarouselStrategy();
     List<Keyline> keylines =
         strategy.onFirstChildMeasuredWithMargins(
             createCarousel(
@@ -180,4 +181,35 @@ public void testKnownCenterAlignmentArrangement_correctlyCalculatesKeylineLocati
       assertThat(keylines.get(i).locOffset).isEqualTo(locOffsets[i]);
     }
   }
+
+  @Test
+  public void testCenterAlignment_isLeftAlignedWithMinItems() {
+    float largeSize = 40F * 3; // 120F
+    float smallSize = 40F;
+
+    View view =
+        createViewWithSize(
+            ApplicationProvider.getApplicationContext(), (int) largeSize, (int) largeSize);
+    int carouselSize = (int) (largeSize + smallSize * 2);
+
+    HeroCarouselStrategy strategy = new HeroCarouselStrategy();
+    List<Keyline> keylines =
+        strategy
+            .onFirstChildMeasuredWithMargins(
+                createCarouselWithItemCount(
+                    carouselSize, CarouselLayoutManager.ALIGNMENT_CENTER, 2),
+                view)
+            .getKeylines();
+
+    float minSmallItemSize =
+        view.getResources().getDimension(R.dimen.m3_carousel_small_item_size_min);
+
+    // keylines when there are only 2 items is {xsmall, large, small, xsmall}
+    float[] locOffsets =
+        new float[] {-.5F, (200 - minSmallItemSize) / 2F, 200 - minSmallItemSize / 2F, 200.5F};
+
+    for (int i = 0; i < keylines.size(); i++) {
+      assertThat(keylines.get(i).locOffset).isEqualTo(locOffsets[i]);
+    }
+  }
 }