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.
+     */
+    public void refreshCells(int fromItem, int toItem) {
+    	int start = Math.min( fromItem, toItem );
+    	int end = Math.max( fromItem, toItem );
+
+    	IndexRange memorizedRange = cells.getMemoizedItemsRange();
+    	int min = memorizedRange.getStart();
+    	int max = memorizedRange.getEnd();
+
+    	start = Math.max( start, min );
+    	end = Math.min( end, max );
+
+    	if ( start < max && end > min ) {
+    		cells.forget(start, end);
+    	}
+    }
+
     /**
      * Updates the list of cells to display
      *

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

@@ -221,6 +221,15 @@ 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.
+     */
+    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 +71,14 @@ 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 deleting_an_item_in_viewport_only_creates_and_lays_out_cell_once() {
         // delete an item in the middle of the viewport