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

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

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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 component 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:

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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 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.

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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 ";".

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

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

Mixin Parameters

Mixins are allowed to have parameters, just like components.

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

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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"})
  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").

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