fix: track columns instead of cells

src/main/java/com/flowingcode/vaadin/addons/gridhelpers/HeaderFooterStylesHelper.java

@@ -29,15 +29,18 @@
 import com.vaadin.flow.dom.ClassList;
 import com.vaadin.flow.dom.Element;
 import java.io.Serializable;
+import java.lang.reflect.Method;
 import java.util.AbstractSet;
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import java.util.stream.Collectors;
 import lombok.Getter;
 import lombok.RequiredArgsConstructor;
+import lombok.SneakyThrows;
 import lombok.experimental.Delegate;
 import org.apache.commons.lang3.StringUtils;
 
@@ -121,24 +124,54 @@ protected String getSelectorTemplate() {
 
   }
 
+  private static final Method AbstractCell_getColumn;
+
+  static {
+    Method method = null;
+    try {
+      Class<?> AbstractCell =
+          Class.forName("com.vaadin.flow.component.grid.AbstractRow$AbstractCell");
+      method = AbstractCell.getDeclaredMethod("getColumn");
+      method.setAccessible(true);
+    } catch (ClassNotFoundException | NoSuchMethodException e) {
+      // Will use cell identity; keep field null.
+    }
+
+    AbstractCell_getColumn = method;
+  }
 
   private abstract class CellSelector<ROW, CELL> implements SelectorSupplier {
 
+    @Getter
+    // AbstractColumn (or CELL if reflection is not available)
+    private final Object column;
+
+    CellSelector(CELL cell) {
+      column = Objects.requireNonNull(getColumn(cell));
+    }
+
+    @SneakyThrows
+    protected final Object getColumn(CELL cell) {
+      if (AbstractCell_getColumn != null) {
+        return AbstractCell_getColumn.invoke(cell);
+      } else {
+        return cell;
+      }
+    }
+
     protected abstract RowSelector<ROW> getRowSelector();
 
     protected abstract CELL getCell(ROW row, Column<?> c);
 
-    protected abstract CELL getCell();
-
     private int getColumnIndex() {
       ROW row = getRowSelector().getRow();
       int j = -1;
 
-      CELL last = null;
-      CELL target = getCell();
+      Object last = null;
+      Object target = getColumn();
       for (Column<?> c : helper.getGrid().getColumns()) {
         if (c.isVisible()) {
-          CELL curr = getCell(row, c);
+          Object curr = getColumn(getCell(row, c));
           if (curr != last) {
             ++j;
             last = curr;
@@ -167,14 +200,11 @@ protected String getSelectorTemplate() {
 
   private final class HeaderCellSelector extends CellSelector<HeaderRow, HeaderCell> {
 
-    @Getter
-    final HeaderCell cell;
-
     @Getter
     final HeaderRowSelector rowSelector;
 
     public HeaderCellSelector(HeaderCell cell) {
-      this.cell = cell;
+      super(cell);
       for (HeaderRow row : helper.getGrid().getHeaderRows()) {
         if (row.getCells().contains(cell)) {
           rowSelector = new HeaderRowSelector(row);
@@ -194,14 +224,11 @@ protected HeaderCell getCell(HeaderRow row, Column<?> c) {
 
   private final class FooterCellSelector extends CellSelector<FooterRow, FooterCell> {
 
-    @Getter
-    final FooterCell cell;
-
     @Getter
     final FooterRowSelector rowSelector;
 
     public FooterCellSelector(FooterCell cell) {
-      this.cell = cell;
+      super(cell);
       for (FooterRow row : helper.getGrid().getFooterRows()) {
         if (row.getCells().contains(cell)) {
           rowSelector = new FooterRowSelector(row);

src/test/java/com/flowingcode/vaadin/addons/gridhelpers/it/HeaderFooterStylesCallables.java

@@ -34,4 +34,6 @@ public interface HeaderCellWrapper extends GridStylesHelper, RmiRemote { }
 
   HeaderRowWrapper getRow(int rowIndex);
 
+  void setColumnOrder(int... columnIndexes);
+
 }

src/test/java/com/flowingcode/vaadin/addons/gridhelpers/it/HeaderFooterStylesIT.java

@@ -20,6 +20,7 @@
 package com.flowingcode.vaadin.addons.gridhelpers.it;
 
 import static org.junit.Assert.assertEquals;
+import com.flowingcode.vaadin.addons.gridhelpers.it.HeaderFooterStylesCallables.HeaderCellWrapper;
 import com.flowingcode.vaadin.addons.gridhelpers.it.HeaderFooterStylesCallables.HeaderRowWrapper;
 import com.flowingcode.vaadin.testbench.rpc.HasRpcSupport;
 import com.vaadin.flow.component.grid.testbench.GridElement;
@@ -61,4 +62,18 @@ public void testHeaderClassesApplied() {
     }
   }
 
+  @Test
+  public void testHeaderCellMutability() {
+    // https://github.com/FlowingCode/GridHelpers/issues/134
+    // setColumnOrder and join reuse existing HeaderCell instances
+    HeaderRowWrapper row0 = $server.getRow(0);
+    HeaderCellWrapper header0 = row0.join(0, 1);
+    HeaderCellWrapper header1 = row0.join(2, 3);
+    $server.setColumnOrder(2, 3, 0, 1, 4);
+    header0.setClassName("row0-cell0");
+    header1.setClassName("row0-cell1");
+    assertEquals("row0-cell1", grid.getHeaderCell(0, 0).getAttribute("class"));
+    assertEquals("row0-cell0", grid.getHeaderCell(0, 1).getAttribute("class"));
+  }
+
 }

src/test/java/com/flowingcode/vaadin/addons/gridhelpers/it/HeaderFooterStylesView.java

@@ -30,6 +30,7 @@
 import com.vaadin.flow.router.Route;
 import elemental.json.JsonObject;
 import elemental.json.JsonValue;
+import java.util.List;
 import java.util.stream.Collectors;
 import java.util.stream.IntStream;
 import lombok.RequiredArgsConstructor;
@@ -50,7 +51,8 @@ public HeaderFooterStylesView() {
     getElement().getStyle().set("flex-grow", "1");
     grid = new Grid<>();
     for (int i = 0; i < 5; i++) {
-      grid.addColumn(x -> x).setHeader("col " + i);
+      String key = "col " + i;
+      grid.addColumn(x -> x).setHeader(key).setKey(key);
     }
     grid.prependHeaderRow();
     add(grid);
@@ -108,4 +110,11 @@ public HeaderRowWrapper getRow(int rowIndex) {
     return new HeaderRowWrapperImpl(row);
   }
 
+  @Override
+  public void setColumnOrder(int... columnIndexes) {
+    List<Column<Integer>> columns = grid.getColumns();
+    grid.setColumnOrder(
+        IntStream.of(columnIndexes).mapToObj(columns::get).collect(Collectors.toList()));
+  }
+
 }