Supporting Informal Parameters

Error Page Recipe

Cookbook

Component Libraries

Any component that closely emulates a particular HTML element should support informal parameters. You'll find that many of the built-in Tapestry components, such as Form, Label and TextField, do exactly that.

To support informal parameters, a component class should use either the @SupportsInformalParameters annotation or the RenderInformals mixin. Otherwise, providing informal parameters to a component will do nothing: any additional parameters will be ignored.

Approach 1: @SupportsInformalParameters

In the example below we create an Img component, a custom replacement for the <img> tag. Its src parameter will be an asset. We'll use the @SupportsInformalParameters annotation to tell Tapestry that the component should support informal parameters.

@SupportsInformalParameters
public class Img
{
    @Parameter(required=true, allowNull=false, defaultPrefix=BindingConstants.ASSET)
    private Asset src;

    @Inject
    private ComponentResources resources;

    boolean beginRender(MarkupWriter writer)
    {
         writer.element("img", "src", src);
         resources.renderInformalParameters(writer);
         writer.end();
         return false;
    }
}

The call to renderInformalParameters() is what converts and outputs the informal parameters. It should occur after your code has rendered attributes into the element (earlier written attributes will not be overwritten by later written attributes).

Returning false from beginRender() ensures that the body of the component is not rendered, which makes sense for an <img> tag, which has no body.

Approach 2: RenderInformals

Another, equivalent, approach is to use the RenderInformals mixin:

public class Img
{
    @Parameter(required=true, allowNull=false, defaultPrefix=BindingConstants.ASSET)
    private Asset src;

    @Mixin
    private RenderInformals renderInformals;

    void beginRender(MarkupWriter writer)
    {
        writer.element("img", "src", src);
    }

    boolean beforeRenderBody(MarkupWriter writer)
    {
        writer.end();
        return false;
    }
}

This variation splits the rendering of the tag in two pieces, so that the RenderInformals mixin can operate (after beginRender() and before beforeRenderBody()).