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Note that when you define an implementation mixin, and the mixin has parameters, there's no way to bind those parameters as part of the implementation. They simply become available when the composite component (including the mixin) is introduced into a page.

Binding the parameter of the core component

It is sometimes desirable to access the current value of a parameter defined in the component associated with the mixin. For example: normally, when the textfield component is marked disabled, it renders a text field with a disabled attribute, but you want it to output the (plaintext) value when disabled. A mixin for this purpose would need access to at least the disabled, and value parameters, and possibly the translate parameter (for a client-side representation). You can access the disabled parameter via @InjectContainer and checking isDisabled on the field, but textfield currently provides no access to value or translate. In this case, you can bind the core-component parameter using the @BindParameter annotation:

  public class MyMixin
  {
    @BindParameter
    private boolean disabled;

    @BindParameter
    private FieldTranslator translate;

    @BindParameter
    private Object value;

    Boolean beginRender(MarkupWriter writer)
    {
        ...
        if (disabled)
        {
           ...
           String stringValue = translate.toClient(value));
           ...
        }
        ...
    }
    ....

Tapestry will "link" the disabled, translate, and value fields above to parameters of the same name on the associated component. The fields are not parameters to the mixin, but local copies of the component parameter. They are cached only if the associated component parameter is cached. They are read-write, and Tapestry handles synchronizing the value between mixins and the associated component such that even with a cached parameter, components and mixins will share the same value for a given parameter/bound-parameter during render. Only declared parameters of the associated components may be bound.

By default, Tapestry will bind the parameter with the same name as the field. You can explicitly declare the parameter to bind via the value attribute:

  @BindParameter("translate")
  private FieldTranslator translator;

In same cases, a mixin will be used on different components using different names for a similar parameter type. For instance, BeanEditor has an "object" parameter; most form fields have a "value" parameter, and Grid has a "source" parameter. These parameters have different names but share the feature of being the "principle" parameter on which the components are acting. A mixin useable by all three components can specify multiple potential parameter values to bind. The first value that matches a declared parameter of the associated component will be used:

public class MyMixin
{
  ...
  @BindParameter({"value","object","source"})
  private Object principalObject;
  ...
}

"MyMixin" can be used on a textfield (principalObject is bound to "value"), on BeanEditor or BeanDisplay (principalObject is bound to "object"), or on Grid or Loop (principalObject is bound to "source").

Render Phase Ordering

All mixins for a component execute their render phase methods before the component's render phase methods for most phases. However, in the later phases (AfterRender, CleanupRender) the order of executing is reversed.

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