A Guided Example

The normal use case for Tyrian is to build a Single Page App/Application (SPA). You can also use Tyrian for Server-side Rendering (SSR), but this page will focus on SPAs.

A guided example

Let’s walk through an example to see what goes into a Tyrian App.

The ‘counter’ is a very common example you’ll come across for many frameworks, it’s a handy Rosetta stone for when you need to orientate yourself in a new framework quickly.

The example is comprised of two buttons, + and -, and some text that shows a count that goes up and dow when you press the buttons.

Counter Code

The version of this in the examples is already quite lean, but the version below has been stripped back to the minimum.

import tyrian.Html.*
import tyrian.*
import cats.effect.IO

import scala.scalajs.js.annotation.*

// @JSExportTopLevel("TyrianApp") // Commented out to appease mdoc..
object Main extends TyrianApp[Msg, Model]:

  def init(flags: Map[String, String]): (Model, Cmd[IO, Msg]) =
    (0, Cmd.None)

  def update(model: Model): Msg => (Model, Cmd[IO, Msg]) =
    case Msg.Increment => (model + 1, Cmd.None)
    case Msg.Decrement => (model - 1, Cmd.None)

  def view(model: Model): Html[Msg] =
    div()(
      button(onClick(Msg.Decrement))("-"),
      div()(model.toString),
      button(onClick(Msg.Increment))("+")
    )

  def subscriptions(model: Model): Sub[IO, Msg] =
    Sub.None
    
type Model = Int

enum Msg:
  case Increment, Decrement

Lets go through it…

TyrianApp

Starting at the top, we have the most common imports that bring in all the basics you’ll need to build your SPA.

All Tyrian SPAs must extend TyrianApp which is parameterized by a message type and a model type. These types can be anything you like, but typically Msg is an enum or ADT, and Model is probably a case class (in our case we’re just using an Int, but we’ll come back to that).

Extending TyrianApp[Msg, Model] will produce helpful compile errors that will tell you all the functions you need to implement, i.e. init, update, view and subscriptions.

The other thing you must do is export the app using Scala.js’s @JSExportTopLevel("TyrianApp"). You can call it anything you like, but all the examples expect the name “TyrianApp”.

The model

type Model = Int

Our app is a counter, so we need a number we can increment and decrement. In this super simple example, an Int is all that we need for our whole model. Normally you’d probably have a case class or something instead. To make it fit nicely, we’ve allocated our Int to a Model type alias.

The version in the examples uses an opaque type, but here we’ve reduced it to a type alias.

To use our model, we’re going to have to initialize it!

import tyrian.*
import tyrian.Html.*
import cats.effect.IO

type Model = Int
  def init(flags: Map[String, String]): (Model, Cmd[IO, Msg]) =
    (0, Cmd.None)

There’s a few things going on here, the only bit we really care about here is the 0 because that is going to be the starting value of our ‘model’.

Some of the other things you can see here:

  • flags - Flags can be passed into the app at launch time, think of them like command line arguments.
  • Cmd[IO, Msg] - Commands aren’t used in the example, but they allow you to capture and run side effects and emit resulting events. They are a requirement for the function signature, and here we satisfy that with Cmd.None.

Rendering the page

Let’s draw the page. All the functions in Tyrian are encouraged to be pure, which means they operate solely on their arguments to produce a value.

The view takes the latest immutable (read-only) model, and produces some HTML in the form of Html[Msg].

  def view(model: Model): Html[Msg] =
    div(
      button("-"),
      div(model.toString),
      button("+")
    )

Here we make a div, add a - button, the another div containing the count (i.e. the model) as plain text, and finally another + button. If you’re familiar with HTML this should all look pretty familiar.

If you wanted to add an id attribute to the div, you would do so like this:

  def view(model: Model): Html[Msg] =
    div(id := "my container")(
      button("-"),
      div(model.toString),
      button("+")
    )

Of course a button isn’t much use unless it does something, and what we can do is emit an event, called a message, when the button is clicked. For that we need to declare our message type which we’ll do as a simple enum that represents the two actions we want to perform:

enum Msg:
  case Increment, Decrement

…and add our click events:

  def view(model: Model): Html[Msg] =
    div(
      button(onClick(Msg.Decrement))("-"),
      div(model.toString),
      button(onClick(Msg.Increment))("+")
    )

Note the return type of view is Html[Msg]. This is because unlike normal JavaScript, the onClick is not directly instigating a normal callback, the HTML elements are mapped through and produce messages as values that are passed back to Tyrian.

Updating the counter’s value

The final thing we need to do is react to the messages the view is sending, as follows:

  def update(model: Model): Msg => (Model, Cmd[IO, Msg]) =
    case Msg.Increment => (model + 1, Cmd.None)
    case Msg.Decrement => (model - 1, Cmd.None)

Recall that our ‘model’ is just a type alias for an Int, so all we do is match on the Msg enum type, and either increment or decrement the model - done!

Subscriptions

Subscriptions are part of the standard requirements, but this example doesn’t use them for anything. They allow you to “subscribe” to processes that emit events over time.