[1146] Add an ADR on EditingContext actions

Bug: eclipse-sirius#1146
Signed-off-by: Raphaël Pagé <raphael.page@obeo.fr>

doc/adrs/051_add_support_for_editing_context_action.adoc

@@ -0,0 +1,125 @@
+= ADR-051 - Add Support for EditingContext Action
+
+== Context
+
+Stereotypes descriptions are hard-coded and limited.
+The main thing that they can do is to copy JSON into the resource set.
+We want to do more things, for example defining when an action is available based on the editing context.
+Replacing stereotypes descriptions would also reduce the dependency to the concept of Document.
+
+== Decision
+
+In order to allow for more flexibility, Stereotypes descriptions will be transformed in EditingContext Actions.
+There won't be any visible changes for the end user.
+They should be supported by both the programmatic API and the GraphQL API.
+Every action will not be available all the time. Their availability will be computed and will depend on the editing context.
+The actions will be able to manipulate the editing context or contact a remote service.
+
+=== Backend
+
+The actions will be represented by the following POJO which will be sent back to the frontend to be displayed in the user interface :
+
+```java
+public class EditingContextAction {
+  private final String id;
+  private final String label;
+}
+```
+
+The following interfaces will need to be implemented in order to contribute new actions :
+
+```java
+public interface IEditingContextActionProvider {
+  List<IEditingContextAction> getEditingContextAction(IEditingContext editingContext);
+}
+```
+
+```java
+public interface IEditingContextAction {
+  boolean isApplicable(IEditingContext editingContext);
+
+  void apply(IEditingContext editingContext);
+}
+```
+
+An InvokeEditingContextActionEventHandler will be called by the GraphQL data fetcher in order to execute an action selected by the end user :
+
+```java
+public class InvokeEditingContextActionEventHandler implements IEditingContextEventHandler {
+
+  private final List<IEditingContextActionHandler> editingContextActionHandlers;
+
+  public boolean canHandle(IEditingContext editingContext, IInput input) {
+    return input instanceof InvokeEditingContextActionInput;
+  }
+
+  public void handle(One<IPayload> payloadSink, Many<ChangeDescription> changeDescriptionSink, IEditingContext editingContext, IInput input) {
+    // Use the relevant editingContextActionHandler to handle the input
+  }
+}
+```
+
+This event handler will be used to call the appropriate action handler which will be provided by the same person who has contributed the action :
+
+```java
+public interface IEditingContextActionHandler {
+  boolean canHandle(IEditingContext editingContext, InvokeEditingContextActionInput);
+
+  IPayload handle(IEditingContext editingContext, InvokeEditingContextActionInput);
+}
+```
+
+```java
+public final class InvokeEditingContextActionInput implements IInput {
+  private UUID id;
+  private String actionId;
+}
+```
+
+```java
+public class InvokeEditingContextActionSuccessPayload implements IPayload {
+  private UUID id;
+}
+```
+
+=== GraphQL API
+
+The GraphQL API will look like this :
+```
+type EditingContext {
+  actions: EditingContextEditingContextActionsConnection!
+}
+
+type EditingContextEditingContextActionsConnection {
+  edges: [EditingContextEditingContextActionsEdge!]!
+  pageInfo: PageInfo!
+}
+
+type EditingContextEditingContextActionsEdge {
+  node: EditingContextAction!
+}
+
+type EditingContextAction {
+  id: ID!
+  label: String!
+}
+
+type Mutation {
+  invokeEditingContextAction(input: InvokeEditingContextActionInput!): InvokeEditingContextActionPayload!
+}
+
+input InvokeEditingContextActionInput {
+  id: ID!
+  actionId: String!
+}
+
+union InvokeEditingContextActionPayload = ErrorPayload | InvokeEditingContextActionSuccessPayload
+
+type InvokeEditingContextActionSuccessPayload {
+  id: ID!
+}
+``` 
+
+== Status
+
+WIP