About using event listeners to extend the navigation cartridges

You can use the Assembler eventing framework as an extension point for navigation cartridges in cases where extending an existing cartridge handler is insufficient.

If you are making modifications to the navigation cartridges, you can trigger processing logic based on Assembler events instead of subclassing the core cartridge handlers.

Using an event listener instead of extending a cartridge handler introduces the following considerations:

Related links

The Assembler provides an empty implementation of the AssemblerEventListener, AssemblerEventAdapter. You can extend this implementation to create a listener that triggers on an Assembler event.

To create an event listener:

  1. Create a new Java class that extends the AssemblerEventAdapter.

    For example: 

    public class ResultsListListener extends AssemblerEventAdapter {
}

  2. Override the methods that correspond to the events for which you wish to trigger custom processing logic: 

    public class ResultsListListener extends AssemblerEventAdapter {
    @Override
    public void cartridgePreprocessStarting(AssemblerEvent event){
    ...
    }

    @Override
    public void cartridgeProcessComplete(AssemblerEvent event){
    ...
    }
}

    For a list of Assembler events, see the Assembler event framework reference or refer to the Assembler API Reference (Javadoc).

  3. Add conditional logic to restrict processing to a specific cartridge handler: 

    public class ResultsListListener extends AssemblerEventAdapter {
    ...

    @Override
    public void cartridgeProcessComplete(AssemblerEvent event){
        if(event.getContentItem() != null && "ResultsList".equals(event.getContentItem().getType()){

            ...
        }
    }
}

  4. Add processing logic.

    The example below prefixes the max_price property on a record with a dollar sign: 

    public class ResultsListListener extends AssemblerEventAdapter {

    ...

    @Override
    public void cartridgeProcessComplete(AssemblerEvent event){
        if(event.getContentItem() != null && "ResultsList".equals(event.getContentItem().getType()){
            ResultsList resultsList = (ResultsList) event.getContentItem();
            for(Record record : resultsList.getRecords()){
                Attribute price = record.getAttributes().get("product.max_price");
                if(price != null){
                    for(int i = 0 ; i < price.size(); i++){
                        price.set(i, "$" + price.get(i).toString());
                    }
                }
            }
        }
    }
}

After creating a new listener, you must register it by including it in the list of listeners for the assemblerFactory object.

You must register all event listeners with the AssemblerFactory object.

The AssemblerFactory takes event listeners as constructor arguments. These listeners are instantiated with each Assembler object created by the factory class.

Optionally, you may also choose to use the Assembler.addAssemblerEventListener() method to add a listener for a single assembly request.

Example 5. Example

The example below uses the ResultsListListener defined in the previous topic, registered in the Discover Electronics reference application.

The reference application uses the Assembler context file to configure global application properties. The configuration bean for the AssemblerFactory includes a list of listeners as constructor arguments: 

<bean id="assemblerFactory" class="com.endeca.infront.assembler.spring.SpringAssemblerFactory"
    scope="singleton">
    <constructor-arg>
        ...
    </constructor-arg>
    <constructor-arg>
        <!-- List of listeners registered in the assembler -->
        <list>
            <bean class="com.endeca.infront.ResultsListListener" />
            <bean class="com.endeca.infront.logger.SLF4JAssemblerEventLogger" />
            <bean class="com.endeca.infront.assembler.event.request.ContentItemAugmentAdapter">
                <constructor-arg ref="springUtility"/>
            </bean>
            ...
        </list>
    </constructor-arg>
</bean>


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