Monday, June 04, 2007

Tapestry Components in Scala

Recently, I became interested in Scala: Multiple inheritance, good support for functional programming, a nice syntax, to mention the greatest highlights.

As a first exercise, I tried to write some Tapestry-4 components in Scala to gain some real-world experience, and to see whether Tapestry's pretty extensive use of bytecode-generation would somehow break Scala's Java-compatibility. I was pretty pleased with the results:

The following is a trait to add authorisation support to arbitrary (form-)components. It controls whether to render the component into which it is mixed in and binds its disabled parameter.

package ch.marcus.components;

import org.apache.tapestry._

trait AccessControlled extends AbstractComponent {
object binding extends IBinding {
def getObject ="readOnly")) // TODO: delegate to ac-service
def getObject(c: Class) = getObject
def getDescription = "AccessControl disabled-binding"
def setObject(o: Object) = {}
def isInvariant = false;
def getLocation = null;

override def finishLoad = {
setBinding("disabled", binding )

/**Derived components delegate to the desired renderComponent method here: */
def renderAccessControlled ( w: IMarkupWriter, c: IRequestCycle )

override def renderComponent( w: IMarkupWriter, c: IRequestCycle ) {
if ( ! getIdPath.contains("invisible") ) // TODO: delegate to ac-service

Here is how to derive an access-controlled version of the standard TextField component by inheriting from TextField and the trait we just defined:

package ch.marcus.components;

import org.apache.tapestry._
import org.apache.tapestry.form._

abstract class AuthorisedTextField extends TextField with AccessControlled {
//duplicate from scala.Object to make Tap's class-enhancer happy
@remote override def $tag(): Int = 0

override def renderAccessControlled( w:IMarkupWriter, c: IRequestCycle )
= super[TextField].renderComponent(w,c);


The only gotcha is the override of $tag which seems to be necessary due to a glitch in Tapestries class-enhancer. Oh, and for components with a specification(.jwc)-file this must be copied for the derived component. This is slightly annoying, but unnecessary for component that consequently use annotations (specless components).

This is how a Tapestry page looks like in Scala:

package ch.marcus.pages
import org.apache.tapestry.annotations._
import scala.reflect._

abstract class Home extends ScalaPage {
val text = "Hello, this is Scala."

def getMbr : String
def setMbr( m:String )

def onSubmit = {
setMbr (getMbr + "x")

Note, how clean the code looks without all the syntactic noise you'd have to add in Java and how nice the Tapestry annotations (Persist) work with Scala.