[DatePicker] Make the month buttons navigate through months, not pages

Author: pubiqq

lib/java/com/google/android/material/datepicker/CalendarConstraints.java

@@ -88,6 +88,10 @@ boolean isWithinBounds(long date) {
     return start.getDay(1) <= date && date <= end.getDay(end.daysInMonth);
   }
 
+  boolean isWithinBounds(Month month) {
+    return start.compareTo(month) <= 0 && end.compareTo(month) >= 0;
+  }
+
   /**
    * Returns the {@link DateValidator} that determines whether a date can be clicked and selected.
    */

lib/java/com/google/android/material/datepicker/MaterialCalendar.java

@@ -463,26 +463,20 @@ public void onClick(View view) {
           }
         });
 
-    monthNext.setOnClickListener(
-        new OnClickListener() {
-          @Override
-          public void onClick(View view) {
-            int currentItem = getLayoutManager().findFirstVisibleItemPosition();
-            if (currentItem + 1 < recyclerView.getAdapter().getItemCount()) {
-              setCurrentMonth(monthsPagerAdapter.getPageMonth(currentItem + 1));
-            }
-          }
-        });
-    monthPrev.setOnClickListener(
-        new OnClickListener() {
-          @Override
-          public void onClick(View view) {
-            int currentItem = getLayoutManager().findLastVisibleItemPosition();
-            if (currentItem - 1 >= 0) {
-              setCurrentMonth(monthsPagerAdapter.getPageMonth(currentItem - 1));
-            }
-          }
-        });
+    monthNext.setOnClickListener(view -> {
+      Month currentMonth = getCurrentMonth();
+      Month nextMonth = currentMonth.monthsLater(1);
+      if (calendarConstraints.isWithinBounds(nextMonth)) {
+        setCurrentMonth(nextMonth);
+      }
+    });
+    monthPrev.setOnClickListener(view -> {
+      Month currentMonth = getCurrentMonth();
+      Month prevMonth = currentMonth.monthsLater(-1);
+      if (calendarConstraints.isWithinBounds(prevMonth)) {
+        setCurrentMonth(prevMonth);
+      }
+    });
   }
 
   private void postSmoothRecyclerViewScroll(final int position) {