With a ViewDecorator you can wrap your Views in order to modify the behavior. ViewDecorators are useful for conditional aspects.

In the last section you learned how to enhance the generated HTML pages with debugging comments by simply setting a flag. Implementing these comments directly in the templates would be hard to maintain, hard to understand and distract from the actual functionality of the template. A ViewDecorator solves the problem much more effective. It can be switched on and off in the preview and live CAE, respectively, and it has no impact on template development.

Configuration

ViewDecorators are declared as Spring beans and appended to the viewDecorators list in the CAE's view-services. E.g. the configuration for the DebugViewDecorator looks like this:

<bean id="debugDecorator" class="com.coremedia.objectserver.view.DebugViewDecorator">
  <description>
    Decorates view fragments with debug comments
  </description>
</bean>

<customize:append id="addCAEDebugDecorator" bean="viewDecorators" enabled="${cae.view.debug-enabled}">
<description>
  Registers debug decorator
</description>
<list>
  <ref bean="debugDecorator"/>
</list>
</customize:append>

The activation of a ViewDecorator is controlled by the enabled flag of the customizer. For the DebugViewDecorator the cae.view.debug-enabled flag is by default set to true in the preview web application and to false in the live web application.

Implementation

The actual ViewDecorator interface consists of a single method

View decorate(View view)

While this interface is very flexible, it would be cumbersome to implement a decorating view from scratch. You would have to deal with ServletView, TextView and XmlView arguments and preserve the particular types for your decorating result view. In order to simplify this, the CAE provides the abstract ViewDecoratorBase which handles these type issues. If you extend the ViewDecoratorBase, you only have to implement getDecorator and return a custom Decorator. A Decorator consists of three decorate methods for the View interfaces ServletView, TextView and XmlView. The default implementations simply delegate to the render methods of the original views. Custom overriding can enhance or replace this behavior. For example, a decorate method for ServletViews might look like this:

@Override
public void decorate(ServletView view, Object self, String viewName, HttpServletRequest request, HttpServletResponse response) {
  try {
    Writer out = response.getWriter();
    out.write("<!-- Decoration before rendering -->");
    view.render(self, viewName, request, response);
    out.write("<!-- Decoration after rendering -->");
  } catch (IOException e) {
    throw new RuntimeException("Cannot decorate", e);
  }
}