Component Mixins

Tapestry 5 includes a radical feature, component mixins. Component mixins are a tricky concept; it basically allows a true component to be mixed together with special limited components called mixins. The component plus its mixins are represented as just a single tag in the component template, but all the behavior of all the elements.

The planned uses for this are to add validation to user input fields, or to add Ajax effects and behaviors to all sorts of components.

You can think of a mixin as a kind of mashup for a component; it combines the behavior of the component with the behavior of the mixin, and bundles it all in one place.

Mixins are used in two different scenarios: Instance mixins and Implementation mixins.

Mixin Classes

Mixin classes are stored in a mixins sub-package, below the application (or library) root package. This parallels where component and page classes are stored.

Other than that, mixin classes are exactly the same as any other component class.

Mixin Limitations

Currently, mixins are allowed to do anything a component can do, including parameters, render phase methods.

Mixins may not have a template. They integrate with the component strictly in terms of invoking render phase methods.

Mixins may have persistent fields, but currently, this is not implemented perfectly (there is a potential for a name clash between a mixin and the component or another mixin). Use persistent fields with mixins with care ... or better yet, delegate persistence to the container using parameters.

Mixins may not, themselves, have mixins.

Instance Mixins

An instance mixin is a mixin applied to a specific instance of a component. This can be done in the component template with the mixins attribute of the <comp> element. This is a comma-separated list of mixin names.

Alternately, when the Component annotation is used to define the component type, you may specify the mixins in two ways:

• The Mixins annotation allows a list of mixin names to be specified.
• The MixinClasses annotation allows a set of mixin class to be specified directly.
  The former is often less verbose, and allows core mixins to be overridden with application-specific mixins. The later format is more specific and more friendly in terms of refactoring (renaming a mixin class will rename the entry in the MixinClasses annotation as well).

Example:

  @Component(parameters=. . .) @Mixins({"Autocomplete", "DefaultFromCookie"})
  private TextField userId;

This example defines a component of type TextField and mixes in the hypothetical Autocomplete and DefaultFromCookie mixins.

Implementation Mixins

Implementation mixins, mixins which apply to all isntances of a component, are added using the Mixin annotation. This annotation defines a field that will containg the mixin instance.

public class AutocompleteField extendes TextField
{
  @Mixin
  private Autocomplete autocompleteMixin;
  
  . . .
}

Often, the type of the field is the exact mixin class to be instantiated.

In other cases, such as when the field's type is an interface or a base class, the value attribute of the annotation will be used to determine the mixin class name:

public class AutocompleteField extendes TextField
{
  @Mixin("Autocomplete")
  private Object autocompleteMixin;
  
  . . .
}

Mixin Parameters

Mixins are allowed to have parameters, just like components.

When binding parameters (either in the template, or using the parameters attribute of the Component annotation).

Tapestry will match each parameter name against the parameters defined by each class (which is to say, the component and each mixin).

If the component and a mix both define a parameter with the same name, then the component wins: the component's parameter will be bound, and the mixin's parameter will be unbound.

Alternately, you may prefix the name of the parameter with the unqualified name of the Mixin class; this eliminates the ambiguity. Example:

  @Component(parameters={"Autocomplete.id=auto", . . . }) @Mixins("Autocomplete", "DefaultFromCookie"})
  private TextField userId;

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.

Exception: Mixins whose class is annotated with MixinAfter are ordered after the component, not before.