[1695] Add a precondition expression on node descriptions in view DSL

Also add some new variables:
- diagramEvent to know which event is occurring during a diagram
rendering
- previousDiagram to know the previous states of the graphical elements
during a diagram rendering

Bug: #1695
Signed-off-by: Axel RICHARD <axel.richard@obeo.fr>

packages/diagrams/backend/sirius-components-diagrams/src/main/java/org/eclipse/sirius/components/diagrams/components/DiagramComponent.java

@@ -45,6 +45,7 @@ public Element render() {
         VariableManager variableManager = this.props.getVariableManager();
         DiagramDescription diagramDescription = this.props.getDiagramDescription();
         var optionalPreviousDiagram = this.props.getPreviousDiagram();
+        variableManager.put(DiagramComponentProps.PREVIOUS_DIAGRAM, optionalPreviousDiagram.orElse(null));
 
         String label = diagramDescription.getLabelProvider().apply(variableManager);
 

packages/diagrams/backend/sirius-components-diagrams/src/main/java/org/eclipse/sirius/components/diagrams/components/DiagramComponentProps.java

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2019, 2022 Obeo and others.
+ * Copyright (c) 2019, 2023 Obeo and others.
  * 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
@@ -32,6 +32,9 @@
  */
 @Immutable
 public final class DiagramComponentProps implements IProps {
+
+    public static final String PREVIOUS_DIAGRAM = "previousDiagram";
+
     private VariableManager variableManager;
 
     private DiagramDescription diagramDescription;

packages/diagrams/backend/sirius-components-diagrams/src/main/java/org/eclipse/sirius/components/diagrams/components/NodeComponent.java

@@ -96,6 +96,7 @@ public Element render() {
         for (Object semanticElement : semanticElements) {
             VariableManager nodeVariableManager = variableManager.createChild();
             nodeVariableManager.put(VariableManager.SELF, semanticElement);
+            nodeVariableManager.put(IDiagramEvent.DIAGRAM_EVENT, optionalDiagramEvent.orElse(null));
 
             String targetObjectId = nodeDescription.getTargetObjectIdProvider().apply(nodeVariableManager);
             var optionalPreviousNode = nodesRequestor.getByTargetObjectId(targetObjectId);

packages/diagrams/backend/sirius-components-diagrams/src/main/java/org/eclipse/sirius/components/diagrams/events/IDiagramEvent.java

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2021 THALES GLOBAL SERVICES.
+ * Copyright (c) 2021, 2023 THALES GLOBAL SERVICES.
  * 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
@@ -17,6 +17,11 @@
  *
  * @author fbarbin
  */
+@SuppressWarnings("checkstyle:InterfaceIsType")
 public interface IDiagramEvent {
 
+    /**
+     * The name of the variable used to store and retrieve the diagram event from a variable manager.
+     */
+    String DIAGRAM_EVENT = "diagramEvent";
 }