doc/6.3.x/developer/later/extensions-provide_custom_extender.txt

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Sirius - Model Extension
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Writing your own Extender
-------------------------
You may want to write your own Extender if you want to define a new Metamodel extension mechanism or if you want to change the way 
the model accesses are done on EMF objects.
 
To provide your own Extender you should use the ::
 
  org.eclipse.sirius.ecore.extender.ExtenderProvider 

extension point and provides your own::

  org.eclipse.sirius.ecore.extender.business.api.accessor.IExtenderProvider

implementation.
 
And then describe it through plugin.xml::

  <extension
         point="org.eclipse.sirius.ecore.extender.ExtenderProvider">
      <extenderprovider
            priority="highest"
            providerClass="com.example.MyExtenderProvider">
      </extenderprovider>
   </extension>
   
And your provider will be asked to tell if yes or no the extender should be provided considering the resourceset.

The priority field will be used to organize the order of the extender calls. 
If you set the priority to highest, then you are sure every model access request will first go through
your extender. If it set to lowest, then you know your extender will only be called if no other extenders 
fullfilled the request.

Dynamic update of the Extender
------------------------------

Your extender might need to update it's state depending on the selected viewpoints. A new API is available for that need. You can provide a MetamodelDescriptorProviders (org.eclipse.sirius.api.extender.MetamodelDescriptorProvider)  which will be asked
for each change in the viewpoint selection, whether it provides MetamodelDescriptors or not. Then these descriptors will be passed by the ModelAccessor API to every instance of IMetamodelExtender so 
that it can change it's internal state. For instance the EcoreIntrinsicExtender provides MetamodelDescriptors ::

    DiagramDescription diag = (DiagramDescription) desc;
                for (EPackage pak : diag.getMetamodel()) {
                    String nsURI = pak.getNsURI();
                    EPackage registeredPackage = EPackage.Registry.INSTANCE.getEPackage(nsURI);
                    if (registeredPackage != null) {
                        result.add(new EcoreMetamodelDescriptor(registeredPackage));
                    } else {

                        result.add(new EcoreMetamodelDescriptor(pak));
                    }
                }

And then the extender reuse them in the updateMetamodels(..) Method ::

	public void updateMetamodels(final Collection metamodelDescriptors) {
	        final Iterator it = metamodelDescriptors.iterator();
	        while (it.hasNext()) {
	            final Object obj = it.next();
	            if (obj instanceof EcoreMetamodelDescriptor) {
	                final EPackage pak = ((EcoreMetamodelDescriptor) obj).resolve();
	                if (pak != null)
	                    addEClassFromEPackage(pak);
	            }
	
	        }
	
	    }