Allow overriding calculateExtraLayoutSpace in the layout manager

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

@@ -183,6 +183,12 @@ public open class CalendarView : RecyclerView {
             }
         }
 
+    /**
+     * Interface with methods that can be overridden
+     * in the internal layout manager.
+     */
+    public var layoutHelper: LayoutHelper? = null
+
     private val scrollListenerInternal = object : OnScrollListener() {
         override fun onScrolled(recyclerView: RecyclerView, dx: Int, dy: Int) {}
         override fun onScrollStateChanged(recyclerView: RecyclerView, newState: Int) {

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

@@ -0,0 +1,22 @@
+package com.kizitonwose.calendar.view
+
+import androidx.recyclerview.widget.LinearLayoutManager
+import androidx.recyclerview.widget.RecyclerView
+import com.kizitonwose.calendar.view.internal.CalendarLayoutManager
+
+/**
+ * An interface with methods that can be overridden
+ * in the internal [LinearLayoutManager].
+ */
+public interface LayoutHelper {
+    /**
+     * Calculates the amount of extra space (in pixels) that should be laid out by
+     * [CalendarLayoutManager] and stores the result in [extraLayoutSpace].
+     * [extraLayoutSpace[0]] should be used for the extra space at the top in a vertical
+     * calendar or left in a horizontal calendar, and extraLayoutSpace[1] should be used for
+     * the extra space at the bottom in a vertical calendar or right in a horizontal calendar.
+     *
+     * @see [LinearLayoutManager.calculateExtraLayoutSpace]
+     */
+    public fun calculateExtraLayoutSpace(state: RecyclerView.State, extraLayoutSpace: IntArray) {}
+}

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

@@ -150,6 +150,12 @@ public open class WeekCalendarView : RecyclerView {
             }
         }
 
+    /**
+     * Interface with methods that can be overridden
+     * in the internal layout manager.
+     */
+    public var layoutHelper: LayoutHelper? = null
+
     private val scrollListenerInternal = object : OnScrollListener() {
         override fun onScrolled(recyclerView: RecyclerView, dx: Int, dy: Int) {}
         override fun onScrollStateChanged(recyclerView: RecyclerView, newState: Int) {

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

@@ -326,6 +326,12 @@ public open class YearCalendarView : RecyclerView {
             }
         }
 
+    /**
+     * Interface with methods that can be overridden
+     * in the internal layout manager.
+     */
+    public var layoutHelper: LayoutHelper? = null
+
     private val scrollListenerInternal = object : OnScrollListener() {
         override fun onScrolled(recyclerView: RecyclerView, dx: Int, dy: Int) {}
         override fun onScrollStateChanged(recyclerView: RecyclerView, newState: Int) {

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

@@ -6,6 +6,7 @@ import android.view.ViewGroup
 import androidx.recyclerview.widget.LinearLayoutManager
 import androidx.recyclerview.widget.LinearSmoothScroller
 import androidx.recyclerview.widget.RecyclerView
+import com.kizitonwose.calendar.view.LayoutHelper
 import com.kizitonwose.calendar.view.MarginValues
 
 internal abstract class CalendarLayoutManager<IndexData, DayData>(
@@ -18,6 +19,7 @@ internal abstract class CalendarLayoutManager<IndexData, DayData>(
     abstract fun getItemMargins(): MarginValues
     abstract fun scrollPaged(): Boolean
     abstract fun notifyScrollListenerIfNeeded()
+    abstract fun getLayoutHelper(): LayoutHelper?
 
     fun scrollToIndex(indexData: IndexData) {
         val position = getaItemAdapterPosition(indexData)
@@ -70,6 +72,19 @@ 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)
+            }
+        } else {
+            super.calculateExtraLayoutSpace(state, extraLayoutSpace)
+        }
+    }
+
     private inner class CalendarSmoothScroller(position: Int, val day: DayData?) :
         LinearSmoothScroller(calView.context) {
         init {

view/src/main/java/com/kizitonwose/calendar/view/internal/monthcalendar/MonthCalendarLayoutManager.kt

@@ -2,6 +2,7 @@ package com.kizitonwose.calendar.view.internal.monthcalendar
 
 import com.kizitonwose.calendar.core.CalendarDay
 import com.kizitonwose.calendar.view.CalendarView
+import com.kizitonwose.calendar.view.LayoutHelper
 import com.kizitonwose.calendar.view.MarginValues
 import com.kizitonwose.calendar.view.internal.CalendarLayoutManager
 import com.kizitonwose.calendar.view.internal.dayTag
@@ -18,4 +19,5 @@ internal class MonthCalendarLayoutManager(private val calView: CalendarView) :
     override fun getItemMargins(): MarginValues = calView.monthMargins
     override fun scrollPaged(): Boolean = calView.scrollPaged
     override fun notifyScrollListenerIfNeeded() = adapter.notifyMonthScrollListenerIfNeeded()
+    override fun getLayoutHelper(): LayoutHelper? = calView.layoutHelper
 }

view/src/main/java/com/kizitonwose/calendar/view/internal/weekcalendar/WeekCalendarLayoutManager.kt

@@ -1,5 +1,6 @@
 package com.kizitonwose.calendar.view.internal.weekcalendar
 
+import com.kizitonwose.calendar.view.LayoutHelper
 import com.kizitonwose.calendar.view.MarginValues
 import com.kizitonwose.calendar.view.WeekCalendarView
 import com.kizitonwose.calendar.view.internal.CalendarLayoutManager
@@ -17,4 +18,5 @@ internal class WeekCalendarLayoutManager(private val calView: WeekCalendarView)
     override fun getItemMargins(): MarginValues = calView.weekMargins
     override fun scrollPaged(): Boolean = calView.scrollPaged
     override fun notifyScrollListenerIfNeeded() = adapter.notifyWeekScrollListenerIfNeeded()
+    override fun getLayoutHelper(): LayoutHelper? = calView.layoutHelper
 }

view/src/main/java/com/kizitonwose/calendar/view/internal/yearcalendar/YearCalendarLayoutManager.kt

@@ -5,6 +5,7 @@ import android.view.View
 import android.view.ViewGroup
 import androidx.recyclerview.widget.LinearSmoothScroller
 import com.kizitonwose.calendar.core.CalendarDay
+import com.kizitonwose.calendar.view.LayoutHelper
 import com.kizitonwose.calendar.view.MarginValues
 import com.kizitonwose.calendar.view.YearCalendarView
 import com.kizitonwose.calendar.view.internal.CalendarLayoutManager
@@ -24,6 +25,8 @@ internal class YearCalendarLayoutManager(private val calView: YearCalendarView)
     override fun getItemMargins(): MarginValues = calView.yearMargins
     override fun scrollPaged(): Boolean = calView.scrollPaged
     override fun notifyScrollListenerIfNeeded() = adapter.notifyYearScrollListenerIfNeeded()
+    override fun getLayoutHelper(): LayoutHelper? = calView.layoutHelper
+
     fun smoothScrollToMonth(month: YearMonth) {
         val indexPosition = adapter.getAdapterPosition(month)
         if (indexPosition == NO_INDEX) return
@@ -42,18 +45,18 @@ internal class YearCalendarLayoutManager(private val calView: YearCalendarView)
             calView.post {
                 val itemView = calView.findViewHolderForAdapterPosition(indexPosition)?.itemView
                     ?: return@post
-                val offset = calculateDayViewOffsetInParent(month, itemView)
+                val offset = calculateMonthViewOffsetInParent(month, itemView)
                 scrollToPositionWithOffset(indexPosition, -offset)
                 calView.post { notifyScrollListenerIfNeeded() }
             }
         }
     }
 
-    private fun calculateDayViewOffsetInParent(month: YearMonth, itemView: View): Int {
-        val dayView = itemView.findViewWithTag<View>(monthTag(month)) ?: return 0
+    private fun calculateMonthViewOffsetInParent(month: YearMonth, itemView: View): Int {
+        val monthView = itemView.findViewWithTag<View>(monthTag(month)) ?: return 0
         val rect = Rect()
-        dayView.getDrawingRect(rect)
-        (itemView as ViewGroup).offsetDescendantRectToMyCoords(dayView, rect)
+        monthView.getDrawingRect(rect)
+        (itemView as ViewGroup).offsetDescendantRectToMyCoords(monthView, rect)
         return if (orientation == VERTICAL) rect.top else rect.left
     }
 
@@ -72,7 +75,7 @@ internal class YearCalendarLayoutManager(private val calView: YearCalendarView)
             if (month == null) {
                 return dy
             }
-            val offset = calculateDayViewOffsetInParent(month, view)
+            val offset = calculateMonthViewOffsetInParent(month, view)
             return dy - offset
         }
 
@@ -81,7 +84,7 @@ internal class YearCalendarLayoutManager(private val calView: YearCalendarView)
             if (month == null) {
                 return dx
             }
-            val offset = calculateDayViewOffsetInParent(month, view)
+            val offset = calculateMonthViewOffsetInParent(month, view)
             return dx - offset
         }
     }