[doc] Added documentation

docs/WritingNewViews.md

@@ -110,10 +110,45 @@ const MY_VIEW_DESCRIPTOR: definition.ViewDescriptor = {
 
 ### Priority evaluation
 
-The RDF dataset.
+When computing the priority of a view, the raw and complete RDF dataset is passed to the `priorityFor` method of the view implementation, along with the target of rendering, i.e. the primary entity to be rendered by this view.
+It is the responsability of the view implementation to determine its priority regarding the dataset and the target entity.
+For example, a view specific to books *should* try to detect the type of the target resource and *should* deactivate itself if it does not look like a book.
+In the case the view desires to deactivate itself, the `VIEW_PRIORITY_INAPPROPRIATE` constant can be returned.
 
-The target RDF resource.
+The strategy to be used to determine whether the view is appropriate and what is its priority is left to the view itself.
+A common strategy is to look for the triples with the `rdf:type` predicate on triples with the target entity as subject.
 
+For example:
+
+```ts
+priorityFor(store: $rdf.Formula, target: application.Resource): number {
+  // use an entity repository for easier manipulation of the RDF dataset
+  let repo = new rdfEntities.RdfEntityStore(store);
+  // get the entity identified as the target
+  let entity = repo.entityForUri(target.uri);
+
+  // determine whether the target is really is target, or a proxy of another entity with a focus predicate
+  // this step may not be applicable to all use cases
+  let focus = entity.getEntityForS("http://xmlns.com/foaf/0.1/focus");
+  if (focus !== null) entity = focus;
+
+  // list all the types of the entity (using rdf:type)
+  let entityTypes = entity.getEntitiesForS("http://www.w3.org/1999/02/22-rdf-syntax-ns#type");
+  if (entityTypes.length > 0) {
+    // look for a specific entity type
+    let x = entityTypes.find(
+      (entity: rdfEntities.RdfEntity) =>
+        entity.uris.indexOf("http://example.com/Book") >= 0
+    );
+    if (x) {
+      // The entity is a Book
+      return 10;
+    }
+  }
+  // could not find the types, the view may not be appropriate
+  return implementation.VIEW_PRIORITY_INAPPROPRIATE;
+}
+```
 
 ### Rendering