[CONF] Apache Tapestry > Default Parameter

[confluence]

Default Parameter Page edited by Ulrich Stärk Changes (4) ... Let's say you have a component, {{OutputGadget}}, whose job is to output some information about an entity type, {{Gadget}}. {newcode:JAVA} public class OutputGadget { ... } } {newcode} This can now be used as {{<t:outputgadget t:id="currentGadget"/>}}, assuming {{currentGadget}} is a property of the container. ... Because this is such a common idiom, it has been made simpler for you. Rather than writing the code above, you can just use the autoconnect attribute of the Parameter annotation. This, effectively, creates the {{defaultGadget()}} method for you. In this case the code of component {{OutputGadget}} can be reduced to: {newcode:JAVA} public class OutputGadget { ... } {newcode} Full Content Default Parameter Many of the components provided with Tapestry share a common behavior: if the component's id matches a property of the container, then some parameter of the component (usually value) defaults to that property. This is desirable, in terms of not having to specify the component's id and then specify the same value as some other parameter. Making this work involves two concepts: default parameter methods (methods that can compute a default value for a parameter), and a service, ComponentDefaultProvider. Let's say you have a component, OutputGadget, whose job is to output some information about an entity type, Gadget. This can now be used as <t:outputgadget t:id="currentGadget"/>, assuming currentGadget is a property of the container. If there is no matching property, then the defaultGadget() method will return null, and a runtime exception will be thrown because the gadget parameter is required and not bound. The principal attribute on the Parameter annotation is not needed in the specific case; in some cases, a default for some other parameter may be based on the bound type of another parameter, the principal attribute forces the parameter to be resolved first. In many Tapestry form components, the value parameter is principal, so that the validate and translate parameters can computer defaults, based on the type and annotations bound to the value parameter. autoconnect attribute Because this is such a common idiom, it has been made simpler for you. Rather than writing the code above, you can just use the autoconnect attribute of the Parameter annotation. This, effectively, creates the defaultGadget() method for you. In this case the code of component OutputGadget can be reduced to: Change Notification Preferences View Online | View Changes