[2032] Add the selection variable to form

Bug: eclipse-sirius#2032
Signed-off-by: Florian Rouëné <florian.rouene@obeosoft.com>

packages/forms/backend/sirius-components-collaborative-forms/src/main/java/org/eclipse/sirius/components/collaborative/forms/FormDescriptionAggregator.java

@@ -17,6 +17,7 @@
 import java.util.Optional;
 import java.util.function.Function;
 
+import org.eclipse.sirius.components.collaborative.forms.variables.FormVariableProvider;
 import org.eclipse.sirius.components.core.api.IObjectService;
 import org.eclipse.sirius.components.forms.description.FormDescription;
 import org.eclipse.sirius.components.forms.description.PageDescription;
@@ -39,6 +40,7 @@ public Optional<FormDescription> aggregate(List<PageDescription> pageDescription
         for (Object object : objects) {
             VariableManager pageVariableManager = new VariableManager();
             pageVariableManager.put(VariableManager.SELF, object);
+            pageVariableManager.put(FormVariableProvider.SELECTION.name(), objects);
 
             eligiblePageDescriptions.addAll(pageDescriptions.stream()
                     .filter(pageDescription -> pageDescription.getCanCreatePredicate().test(pageVariableManager))

packages/forms/backend/sirius-components-collaborative-forms/src/main/java/org/eclipse/sirius/components/collaborative/forms/variables/FormVariableProvider.java

@@ -0,0 +1,35 @@
+/*******************************************************************************
+ * Copyright (c) 2023 Obeo.
+ * This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License v2.0
+ * which accompanies this distribution, and is available at
+ * https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ *     Obeo - initial API and implementation
+ *******************************************************************************/
+package org.eclipse.sirius.components.collaborative.forms.variables;
+
+import java.util.List;
+
+import org.eclipse.sirius.components.core.api.variables.IVariableProvider;
+import org.eclipse.sirius.components.core.api.variables.Variable;
+import org.springframework.stereotype.Service;
+
+/**
+ * Used to provide the variables available for all form operations.
+ *
+ * @author frouene
+ */
+@Service
+public class FormVariableProvider implements IVariableProvider {
+
+    public static final Variable SELECTION = new Variable("selection", List.of(Object.class), "The selection of the workbench");
+
+    @Override
+    public List<Variable> getVariables(String operation) {
+        return List.of();
+    }
+}