voltir / form.rx   1.1.2


Playing with forms in Scala.js

Scala versions: 2.11
Scala.js versions: 0.6


API to map case classes to/from a 'layout' of HTMLInput elements and Scala.Rx Vars. It is akin in purpose to the playframework form library, but entirely done in Scala.js... and a rather different approach in API.


Getting Started

 "com.stabletechs" %%% "formrx" % "1.1.0"

Form.rx is only compiled for Scala.js 0.6+

Quick Demo


Demo Source

Using Form.rx

In general two imports are required import formrx._ and import formrx.implicits.all._.

Form.rx Basics

Form.rx is a macro based tool, built on top of scala.rx, that binds a description of a user interface (referred to as a Layout) to an instance of a case class.

//Target case class
case class UserPass(firstname: String, pass : String)

//HTML Form representation
class UserPassLayout(implicit ctx: Ctx.Owner) {
  val firstname = input(`type`:="text").render
  val pass = input(`type`:="password").render

//Concrete form instance
val loginForm = FormRx[UserPass,UserPassLayout]

The FormRx macro is used to construct the two-way data binding between the UserPass case class and its UserPassLayout form representation, which in this case is two HTML input fields.

This mapping is typesafe and refactor friendly. For instance, if in UserPass we rename firstname to just name, we get the following compile time error:

[error] /home/nick/tests/formidable-demo/src/main/scala/example/ScalaJSExample.scala:23: The layout is not fully defined: Missing fields are:
[error] --- name
[error]   val form = FormRx[UserPass,UserPassLayout]
[error]                    ^
[error] one error found
[error] (compile:compile) Compilation failed

With the loginForm created, it can be used, for example, with scalatags in the following way:

 import scalatags.JsDom.all._
 val loginTag: HtmlTag = 
     onsubmit := {() =>
     loginForm.current.now match {
       case Success(usrPass) => println(s"$usrPass")
       case Failure(err) => println(s"FAILED!: ${err.getMessage}")

In this example, loginForm.firstname and loginForm.pass are the HTML input elements as defined in UserPassLayout. loginForm.current has type Rx[Try[UserPass]] and is updated every time either input field is modified. In this case though, we are ignoring scala.rxs data binding functionality and using .now to get out only the latest value when the user hits Submit. The resulting loginTag is just a normal HtmlTag and can be used in any scalatag based project.

In addition to .current, FormRx also provides by default a .set(...) and .reset() functions. .set takes an instance of the target case class and populates the form with the data from that case class, for example:


.reset() on the other hand takes no arguments and simply resets the form to a default state, in this case the empty string is used as the default value and the form is effectively cleared:


The combination of .set and .reset simplifies the task of "editing" existing data, which makes FormRx forms ideal for being used for "upsert" like tasks when form data must both be created and edited.

Form.rx Nesting

Form.rx allows for mapping tree like ADT structures by allowing layouts to nest inside of each other. For example:

  case class Inner(foo: String, bar: Int)
  case class Nested(top: String, inner: Inner, other: Inner)

To create a Layout for Nested we can start with Layout for Inner:

  class InnerLayout(implicit ctx: Ctx.Owner) {
    val foo = input(`type`:="text").render
    val bar = SelectionRx[Int]()(
      Opt(42)(value:="Life","What is?"),

Here Inner.foo is bound to a simple HTML input element and Inner.bar is bound to a HTML selection drop down with 4 possible values.

We can then use InnerLayout to define NestedLayout:

  class NestedLayout(implicit ctx: Ctx.Owner) {
    val top = input(`type`:="text").render
    val inner = FormRx[Inner,InnerLayout]
    val other = FormRx[Inner,InnerLayout]

Which can then be used, for example, in the following way:

val nestedForm = FormRx[Nested,NestedLayout]

Using rx.Vars

In addition to HTML input elements, rx.Vars can be used as a mechanism to bind specific fields, allowing for a great deal of flexibility in building a user interface.

For example, InnerLayout could be redefined to the following:

  class InnerLayout(implicit ctx: Ctx.Owner) {
    val foo = input(`type`:="text").render
    val bar = Var(0)

Where bar could then be manipulated and used like any other rx.Var[Int], for example (from the demo):

def buttons(inp: Var[Int]): Rx[HtmlTag] = Rx {
    label("Current Value: " + inp()),
      li(a(cls:="button", onclick:={ () => inp() = inp.now + 1 })("Inc")),
      li(a(cls:="button", onclick:={ () => inp() = inp.now - 1 })("Dec"))

Lists and Sets

One problem when building a user interface is deciding how to represent data structures like Lists and Sets. When building a user interface, there are a myriad of ways one might want to use to present the data, all of which are equally valid. Form.rx includes some utilities to help with common data structures such as Lists and Sets, and is also highly extensible such that other data structures and/or custom layouts can be freely mixed in.

InputRx.list(...) and InputRx.set(...) defines such a helper for fields that can be considered "tag like" (see example 5 in the demo).

For example, say we want to define a form for the following data:

  sealed trait SkillLevel
  case object Average extends SkillLevel
  case object Intermediate extends SkillLevel
  case object Expert extends SkillLevel

  case class Skill(name: String, level: SkillLevel)

  case class Profile(foo: String, bar: Int, skills: List[Skill])

InputRx.list(...) could be used to define a form element for the skills field:

  class SkillLayout(implicit ctx: Ctx.Owner) {
    val name = input(`type`:="text").render
    val level = SelectionRx[SkillLevel]()(

  class ProfileLayout(implicit ctx: Ctx.Owner) {
    val foo = input(`type`:="text").render
    val bar = Var(-1)
    def newSkill(txt: String): Skill = Skill(txt,Average)

    val skills = InputRx
      .list(input(`type`:="text", placeholder:="New Skill*"))(newSkill)(() => FormRx[Skill,SkillLayout])

At the heart of InputRx.list(..) and InputRx.set(..) is some constructor of type String => T. The idea is to construct new instances of type T given whatever the user types in. However, these helpers also take a constructor for a FormRx[T], allowing each constructed T to also be individually edited with some arbitrary form of its own.

Custom FormRx Definitions