Extreme Java

After some CRUD experiments with LiftWeb, I’m really in love with the “view first” paradigm. Specially because the XHTML bindings are very webdesigner-friendly.

One big limitation is the <lift:msgs> snippet - it produces a mandatory “Error” title. This obligatory text is unacceptable when developing enterprise applications. Hopefully, there’s an workaround: you can create a snippet named “Msgs” with a “render” method. Your snippet will “override” the builtin (without overriding the class), like this:

package mypkg.snippets
class Msgs {
  def msgs(cls : String, ms : List[NodeSeq]) = ms match {
    case Nil => Nil
    case x => <div class={ cls }>{ ms.flatMap(m => <p>{ m }</p>) }</ul>
  }
  def render(xml : NodeSeq) : NodeSeq =
    <div id={ LiftRules.noticesContainerId }>
      { msgs("msgError", noIdMessages(errors)) }
      { msgs("msgWarning", noIdMessages(warnings)) }
      { msgs("msgNotice", noIdMessages(notices)) }
    </div>
}

This little snippet will emit messages using <div> and <p> instead of <ul> and <li> (with that hardcoded title block). And, since you have the control over the snippet, you can put stub code inside <lift:msgs/> and allow your webdesigner use their favorite WYSIWYG editor:

<lift:msgs>
  <div class="msgError">
    <p>name must have 3 charactes</p>
  </div>
</lift:msgs>

Since Java and Scala are complementary, I will post Scala-related stuff here, too (without creating a new blog).

Today’s tip refers to Lift (Scala’s JavaEE/Rails/Grails/Django). Suppose your webdesigner gives you this XHTML:

<input type="text" class="x" style="width: 100px;"/>

When you bind this using Lift, one of possible solutions is surround it with a tag:

<mybind:myfield><input type="text" class="x" style="width: 100px;"/></mybind:myfield>

Please notice I put it without spaces or newlines between “mybind:myfield” and “input” - this will be important later.

I prefer this way because this XHTML can be opened in other applications, like Firefox and Dreamweaver, making life easier for the webdesigner.

Then, if I bind it using old mama’s recipe:

bind("mybind", form, "myfield" -> myfield.toForm)

Lift will remove my “input” tag and replace it - destroying “class” and “style”. To solve this, I merge “toForm” tag with original attributes:

def merge(form : => Box[NodeSeq])(input : NodeSeq) : NodeSeq = {
  var in = input.first
  var attrs = form.open_!.first.attributes

  new Elem(in.prefix, in.label,
      in.attributes append attrs,
      in.scope, Group(in.child))
}
bind("mybind", form,
    FuncBindParam("myfield", merge(myfield.toForm)))

Function “merge” takes the form and returns a function that receives the original XHTML and translates it into XHTML with Lift’s attributes (”id”, “name”, “lift:gc”, etc). I guess I can improve it somehow, but works great. And, since I’m keeping only the first child of source XHTML, you need to keep “bind” and “input” together (as I said before).

I know I can use a map function to ignore whitespaces, but I’ll leave it as an later exercise (for you and for me, too).

I’m testing if Scala can be used to increase productivity here. Pretty nice language: as powerful as Ruby and JVM-compatible. To improve my skills, I decided to implement Mandelbrot on Scala, using Swing’s BufferedImage to show the image. Very simple, indeed. The code is:

import java.awt.image.BufferedImage
import javax.swing.ImageIcon
import javax.swing.JFrame
import javax.swing.JLabel
object Mandelbrot extends Application {
  case class Complex(r : Double, i : Double) {
    def +(b : Complex) = Complex(r + b.r, i + b.i)
    def *(b : Complex) = Complex(r * b.r - i * b.i, r * b.i + i * b.r)
    def insideM = (r * r + i * i) < (2 * 2)
  }

  implicit def start = Complex(0, 0)

  def pc(z : Complex, c : Complex) : Complex = z * z + c

  def iter(qtd : Int, c : Complex)(implicit z : Complex) : Int = {
    if (qtd == 0) 0
    else if (!z.insideM) qtd
    else iter(qtd - 1, c)(pc(z, c))
  }

  val scale = 1.0
  def pixToComplex(x : Double, y : Double) =
    Complex(((x / 640.0) * 3.0 - 2.0) / scale,
        ((y / 480.0) * 2.0 - 1.0) / scale)

  def raster(f : (Int, Int, Int) => Unit) = {
    for (y <- 0 until 480; x <- 0 until 640) {
      f(x, y, iter(1024, pixToComplex(x, y)))
    }
  }

  def qtdToColor(c : Int) = List(c / 4, c / 4, c / 4).toArray

  val frame = new JFrame("Mandelbrot")
  frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE)

  val lbl = new JLabel
  frame.add(lbl)
  frame.setSize(640, 480)
  frame.setVisible(true)

  val img = new BufferedImage(640, 480, BufferedImage.TYPE_3BYTE_BGR)
  raster((x, y, c) => img.getRaster.setPixel(x, y, qtdToColor(c)))
  lbl.setIcon(new ImageIcon(img))
}

Since I’m new to Scala, I guess it’s not the best solution. Nevertheless, I have an “Yes-we-can” feeling, because this code have some Scala features, like higher-order functions and (possibly) tail recursion. And, of course, I felt productive implementing Mandelbrot’s recursive algorithm (I got a lot of errors misunderstanding the algorithm, BTW).