Improve LayoutHelper usage

Author: kizitonwose

view/api/view.api

@@ -74,12 +74,9 @@ public final class com/kizitonwose/calendar/view/DaySize : java/lang/Enum {
 	public static fun values ()[Lcom/kizitonwose/calendar/view/DaySize;
 }
 
-public abstract interface class com/kizitonwose/calendar/view/LayoutHelper {
-	public abstract fun calculateExtraLayoutSpace (Landroidx/recyclerview/widget/RecyclerView$State;[I)V
-}
-
-public final class com/kizitonwose/calendar/view/LayoutHelper$DefaultImpls {
-	public static fun calculateExtraLayoutSpace (Lcom/kizitonwose/calendar/view/LayoutHelper;Landroidx/recyclerview/widget/RecyclerView$State;[I)V
+public abstract class com/kizitonwose/calendar/view/LayoutHelper {
+	public fun <init> ()V
+	public fun getCalculateExtraLayoutSpace ()Lkotlin/jvm/functions/Function2;
 }
 
 public final class com/kizitonwose/calendar/view/MarginValues {

view/src/main/java/com/kizitonwose/calendar/view/CalendarView.kt

@@ -184,7 +184,7 @@ public open class CalendarView : RecyclerView {
         }
 
     /**
-     * Interface with methods that can be overridden
+     * Helper class with methods that can be overridden
      * in the internal layout manager.
      */
     public var layoutHelper: LayoutHelper? = null

view/src/main/java/com/kizitonwose/calendar/view/LayoutHelper.kt

@@ -5,10 +5,13 @@ import androidx.recyclerview.widget.RecyclerView
 import com.kizitonwose.calendar.view.internal.CalendarLayoutManager
 
 /**
- * An interface with methods that can be overridden
- * in the internal [LinearLayoutManager].
+ * Helper class with properties that match the methods that can
+ * be overridden in the internal [LinearLayoutManager]. This is
+ * an abstract class instead of an interface so we can have
+ * default values for properties as we need to call `super`
+ * for properties that are not provided (null).
  */
-public interface LayoutHelper {
+public abstract class LayoutHelper {
     /**
      * Calculates the amount of extra space (in pixels) that should be laid out by
      * [CalendarLayoutManager] and stores the result in [extraLayoutSpace].
@@ -18,5 +21,5 @@ public interface LayoutHelper {
      *
      * @see [LinearLayoutManager.calculateExtraLayoutSpace]
      */
-    public fun calculateExtraLayoutSpace(state: RecyclerView.State, extraLayoutSpace: IntArray) {}
+    public open val calculateExtraLayoutSpace: ((state: RecyclerView.State, extraLayoutSpace: IntArray) -> Unit)? = null
 }

view/src/main/java/com/kizitonwose/calendar/view/WeekCalendarView.kt

@@ -151,7 +151,7 @@ public open class WeekCalendarView : RecyclerView {
         }
 
     /**
-     * Interface with methods that can be overridden
+     * Helper class with methods that can be overridden
      * in the internal layout manager.
      */
     public var layoutHelper: LayoutHelper? = null

view/src/main/java/com/kizitonwose/calendar/view/YearCalendarView.kt

@@ -327,7 +327,7 @@ public open class YearCalendarView : RecyclerView {
         }
 
     /**
-     * Interface with methods that can be overridden
+     * Helper class with methods that can be overridden
      * in the internal layout manager.
      */
     public var layoutHelper: LayoutHelper? = null

view/src/main/java/com/kizitonwose/calendar/view/internal/CalendarLayoutManager.kt

@@ -73,13 +73,9 @@ internal abstract class CalendarLayoutManager<IndexData, DayData>(
     }
 
     override fun calculateExtraLayoutSpace(state: RecyclerView.State, extraLayoutSpace: IntArray) {
-        val layoutHelper = getLayoutHelper()
-        if (layoutHelper != null) {
-            layoutHelper.calculateExtraLayoutSpace(state, extraLayoutSpace)
-            // If the interface is provided but the method is not overridden.
-            if (extraLayoutSpace.isEmpty()) {
-                super.calculateExtraLayoutSpace(state, extraLayoutSpace)
-            }
+        val calculateExtraLayoutSpace = getLayoutHelper()?.calculateExtraLayoutSpace
+        if (calculateExtraLayoutSpace != null) {
+            calculateExtraLayoutSpace(state, extraLayoutSpace)
         } else {
             super.calculateExtraLayoutSpace(state, extraLayoutSpace)
         }