A Component Mixin is a way to supplement an existing Tapestry component with additional behavior.

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.

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You can think of a mixin as a kind of mashup for a component; it combines the new behavior of the component mixin with the existing behavior of the mixincomponent, and bundles it all in one place. Mixins are used in two different scenarios: Instance mixins and Implementation mixins.may be used to add specialized validation to user input fields, dynamically modify the HTML output of a component, or to add Ajax effects and behaviors of all sorts to components.

Tapestry comes with several mixins, such as the Autocomplete mixin which adds autocomplete behavior to an ordinary TextField Component. In addition, you can easily create your own.

Mixin Classes

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

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

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

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

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Mixins may not, themselves, have mixins.

Using Mixins

Mixins are used in two different scenarios: Instance mixins and Implementation 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> elementcomponent tag. This is a comma-separated list of mixin names.

<t:textfield t:id="accountName" t:mixins="Autocomplete,DefaultFromCookie" />

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 classes 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

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refactor-safe (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.

Ordering the Mixins

With @Mixins and @MixinClasses annotations, we can order the list of mixins, by adding a constraint.

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

  @Component(parameters=. . .) 
  @MixinClasses(value={Autocomplete.class, DefaultFromCookie.class}, order={"","before:AutoComplete"})
  private TextField userId;

You can specify many contraints for a mixin. You just need to separate them with a ";".

Implementation Mixins

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

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

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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 extendesextends TextField
{
  @Mixin("Autocomplete")
  private Object autocompleteMixin;
  
  . . .
}

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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).

Here we provide a value of ".5" seconds for the Autocomplete mixin's "frequency" parameter:

<t:container xmlns:t="http://tapestry.apache.org/schema/tapestry_5_4.xsd">
...
<t:textfield t:id="accountName" t:mixins="Autocomplete" t:Autocomplete.frequency=".5" />

The parameter name should be prefixed with the name of the mixin class ("Autocomplete." above).

You can do the same thing with the @Component annotation:

@Component(parameters={"Autocomplete.frequency=.5", . . . })
@Mixins("Autocomplete", "DefaultFromCookie"})
private TextField userId;

When using the Tapestry 5.3 and earlier XSDs, you may omit the class name prefix (e.g. t:frequency=".5" instead of t:Autocomplete.frequency=".5"), but then Tapestry has to resolve conflicts. If a component and a 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:

If the component and a mixin both supports informal parameters, the mixin will receive the all the unqualified informal parameters. If more than one Mixin supports informal parameters the results are undefined.

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 some 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 each is 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

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

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

Withing a given phase and class (@MixinAfter vs. mixin before), mixin ordering is determined by the ordering constraints specified in the mixin definitions. The constraint definitions follow the same conventions as ordered service configurations. How you specify the constraints depends on how the mixin is specified.

  @Mixin("Autocomplete",order={"before:DiscardBody","after:RenderDisabled"}
  private TextField userId;

  <input t:id="myfield" t:mixins="autocomplete::before:discardbody;after:renderdisabled,
          defaultfromcookie::before:autocomplete"/>

  @Component(...)
  @Mixins("Autocomplete::before:discardbody;after:renderdisabled","DefaultFromCookie::before:autocomplete"))
  private TextField userId;

  @Component(...)
  @MixinClasses(value={Autocomplete.class,DefaultFromCookie.class},
                order={"before:discardbody;after:renderdisabled","before:autocomplete")

The ordering is always specified in terms of the order of the "forward" rendering process (setupRender, beginRender, etc.). When the "reverse" rendering phases (afterRender, etc.) occur, the mixin order is exactly reversed. Mixins which have no associated ordering constraints will be ordered in a manner which is consistent with the specified constraints for all other mixins, but is otherwise unspecified.

Available Mixins

In addition, the following mixins are available from other sources:

Additional Tools

Tapestry-MixinsXpath is a third-part Tapestry module that allows XPath traversal of the Tapestry (server-side) DOM, which can be extremely useful in certain mixins.