Added refreshCells method

src/main/java/org/fxmisc/flowless/CellListManager.java

@@ -77,6 +77,27 @@ public C getCell(int itemIndex) {
         return cells.get(itemIndex);
     }
 
+    /**
+     * This is a noop on non visible items. For reusable Cells this will cause
+     * updateItem to be invoked on the next available pooled Cell. If a Cell is
+     * not available or reusable, a new Cell is created via the cell factory.
+     * @param fromItem - the start index, inclusive.
+     * @param toItem - the end index, exclusive.
+     */
+    public void refreshCells(int fromItem, int toItem) {
+        if (fromItem >= toItem) throw new IllegalArgumentException(
+            String.format("To must be greater than from. from=%s to=%s", fromItem, toItem)
+        );
+
+        IndexRange memorizedRange = cells.getMemoizedItemsRange();
+        fromItem = Math.max( fromItem, memorizedRange.getStart() );
+        toItem = Math.min( toItem, memorizedRange.getEnd() );
+
+        if (fromItem < toItem) {
+            cells.forget(fromItem, toItem);
+        }
+    }
+
     /**
      * Updates the list of cells to display
      *

src/main/java/org/fxmisc/flowless/VirtualFlow.java

@@ -221,6 +221,17 @@ public Optional<C> getCellIfVisible(int itemIndex) {
         return cellPositioner.getCellIfVisible(itemIndex);
     }
 
+    /**
+     * This is a noop on non visible items. For reusable Cells this will cause
+     * updateItem to be invoked on the next available pooled Cell. If a Cell is
+     * not available or reusable, a new Cell is created via the cell factory.
+     * @param fromIndex - the start index, inclusive.
+     * @param toIndex - the end index, exclusive.
+     */
+    public void refreshCells(int fromIndex, int toIndex) {
+        cellListManager.refreshCells(fromIndex, toIndex);
+    }
+
     /**
      * This method calls {@link #layout()} as a side-effect to insure
      * that the VirtualFlow is up-to-date in light of any changes

src/test/java/org/fxmisc/flowless/CellCreationAndLayoutEfficiencyTest.java

@@ -71,6 +71,21 @@ public void updating_an_item_outside_viewport_does_not_create_or_lay_out_cell()
         assertEquals(0, cellLayouts.getAndReset());
     }
 
+    @Test
+    public void refreshing_cells_in_viewport_creates_and_lays_them_out_once() {
+        // refresh cells in the viewport
+        interact(() -> flow.refreshCells(10,12));
+        assertEquals(2, cellCreations.getAndReset());
+        assertEquals(2, cellLayouts.getAndReset());
+    }
+    @Test
+    public void refreshing_cells_outside_viewport_does_not_create_or_lay_them_out() {
+        // refresh 10 cells, 5 in and 5 outside the viewport
+        interact(() -> flow.refreshCells(20,30));
+        assertEquals(5, cellCreations.getAndReset());
+        assertEquals(5, cellLayouts.getAndReset());
+    }
+
     @Test
     public void deleting_an_item_in_viewport_only_creates_and_lays_out_cell_once() {
         // delete an item in the middle of the viewport