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 behavior of the mixin with the behavior of the component, and bundles it all in one place. Mixins may be used to add validation to user input fields, or to add Ajax effects and behaviors to all sorts of 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 are stored in 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 the same as any other component class.
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.
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.
Mixins are used in two different scenarios: Instance mixins and Implementation 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 component tag. 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 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 refactor-safe (renaming a mixin class will rename the entry in the MixinClasses annotation as well).
This example defines a component of type TextField and mixes in the hypothetical Autocomplete and DefaultFromCookie mixins.
With @Mixins and @MixinClasses annotations, we can order the list of mixins, by adding a constraint.
You can specify many contraints for a mixin. You just need to separate them with a ";".
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.
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:
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 mixin 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.
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:
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:
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:
"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").
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: A mixins whose class is annotated with @MixinAfter 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.
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.
Tapestry includes the following mixins out-of-the-box.
|Autocomplete||modifies a text field to provide for auto-completion of text using values retrieved from the server as the user types. See instructions.|
|DiscardBody||discards a component's body. Returns false from the BeforeRenderBody phase, which prevents the rendering of the body.|
|FormFieldFocus||instruments the outer Form on which component the focus should be activated. Replaced by OverrideFieldFocus starting in Tapestry 5.4.|
|NotEmpty||attaches to any component that renders an element. At the end of the render, if the element is empty, then a non-breaking space ( ) is injected into the element.|
|OverrideFieldFocus||when attached to a form field, causes that field to gain focus. Starting in Tapestry 5.4, this supersedes FormFieldFocus.|
|RenderClientId||forces a client element to render its client id by ensuring that "getClientId" is called.|
|RenderDisabled||renders an "disabled" attribute if the containing component is disabled|
|RenderInformals||renders out all informal parameters, at the end of the BeginRender phase. This mixin can be used with components that render a single tag inside the BeginRender phase.|
|RenderNotification||triggers event notifications to identify when it enters the BeginRender and AfterRender render phases|
|TriggerFragment||when applied to a Checkbox or Radio component, links the input field and a FormFragment, making the field control the client-side visibility of the FormFragment|
|ZoneRefresh||periodically refreshes a Zone by triggering an event on the server using ajax requests.|
In addition, the following mixins are available from other sources:
|ClickOnce||From JumpStart, a mixin to apply to a submit button, ensuring it can't be double-clicked|
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.