Trying out ScalaJS.

I'll try to port CycleJS to ScalaJS in order to learn both of it.

The egghead course looks like a good way to start 🤔 https://egghead.io/courses/cycle-js-fundamentals

If you want to use a library with Scala.js, you need to use one that is exported to <group>.<identifier>_sjs<version> (just like Scala _2.10 or _2.11). RxScala is currently not supporting this, according to their issue #161.

Therefore, I went with scala.rx. It is not really the same as RxScala, but you can have reactive variables with it. It has a different API, but may be working in a similar way.

Update: After fiddling around a lot with scala.rx, I've switched to another library. This results in an implementation that looks a lot more similar to the real CycleJS implementation shown in the videos.

As you cannot map with scala.rx but have Var and Rx{}, the code looks a bit different.

To separate logic and effects, the code is split into two functions. As this was done, the next step is to use the sink (the Rx[String] in our case) and hand it over to multiple effects: Writing into DOM and writing into console.

The main function is called logic() as main() is needed by js.App as main entry point.

In this case, the main function (called logic in our case) is split into separate sinks. This is done through a map in the video. It's not possible to return Obs directly and use it as the value of the observed Rx is not passed into the callback.

Weird behavior: When using i() = i() * 2 instead of i() = i() 2, the consoleLogEffect does not fire anymore. After fiddling around with this a bit more, I realized that i was set to Var(0) initially. Every update * 2 made it be 0 again, so it does not update internally and therefore does not fire again. To prevent that, one should use the .propagate() method on the Rx / Obs that listens on i.

When using scala.rx version 0.3.0 , it added Ownership context. This is used to prevent creating leaky Rx when nesting them. The documentation doesn't really say how to create a safe context, so in the current code, the line implicit private val ctx = Ctx.Owner.safe() is added to the App object as it doesn't seem to be possible to create this in the main() method itself.

This is more or less a matter of renaming things: effects are called drivers now and there is a map to show the respective drivers that are at work right now. If one is not used / needed, one can comment it out without having to touch multiple lines now.

In this video, the drivers start returning sources. The proxy logic is different to JavaScript as we are usually not passing around mutable state. It needs to be done in a more scala-like way.

The drivers have to use input and output parameters in order to use the same Variable as the proxySource. Using the .getOrElseUpdate() method on the proxySource Map, we can initialize the sources in the main method. This keeps the whole logic and variable initialization inside the main method.

Using the trigger method, we can actually get rid of the output parameter in the drivers method. This way, we have two separate variables, but this seems to be the way that Cycle does it itself.

To truly generalize the run method now, it's been pulled out into the Scycle object. Using type aliases makes it a bit easier to understand which types are used for what. It also shows that the logic method does a bit more right now than just taking the drivers output and giving some output to feed into the respective driver.

A small thing which is different to the implementation of the JavaScript is that the interval timer does not get reset every time the document is clicked. If we were starting a new interval timer on each click, we would need to introduce a variable that can be used to clear the old timer and remembers the new one. So it's possible to do but in order to keep the implementation of the core Cycle principle in focus, we will continue with the single timer.

The latest refactorings are done to abstract the driver and let it do a lot more for us. It receives a specific input (in case of the DomDriver it wants a dom.Element) and returns functions that can be used to receive events in the main function. Called selectEvents, it takes the currently available DOM elements and adds event listeners on them. In the current implementation, it cannot add listeners to the added elements yet, as the selectEvents call is done before the elements are added to the DOM.

The new implementation of the DomDriver takes all events that occur on the document and then filters out the ones that we created listeners for. This ensures to catch events of newly created DOM elements as well.

In this video, we see a small improvement on creating and handling the DOM elements. It uses Hyperscript, which is basically functions wrapping around the creation of DOM elements. Something similar is implemented with the Hyperscript trait and the HyperScriptElement classes. The Text case is somewhat special, as we need to wrap a text node into a span to let it count as an element. With an implicit conversion of String to Text (stringToTextNode) we can get rid of extra calls to Text().

When creating the DOM Driver, we now pass a selector to it, to select the container element. The container element is still the one we were using before (#app), so the result does not change. The makeXXXDriver functions now return a function expecting a LogicOutput and return a Driver. This way, we do not need to pass an explicit input and can just use / cast from the LogicOutput (that is what the input variable has been before anyways).

The drivers were moved into a separate package. During this refactoring, the ConsoleDriver received the input parameter just like the DomDriver.

Latest refactor: Move Hyperscript elements into dom package. Remove ConsoleDriver to focus on single dom driver and start with next lesson.

First, we create a few more helpers for the Hyperscript. A label, input and hr element helps to build the GUI. Doing a complete replacement of container.innerHTML with the outerHTML of input(), the GUI updates with a quirk: The input field looses focus whenever the view updates.

When using a virtual dom implementation that would not replace the whole input DOM element, we could mitigate this problem. But that means we need to implement a virtual dom before continuing.

Finally something we can test! Let's start by adding a scalatest dependency. A small test to check that the test setup works is a nice trick to be sure that you're not wasting time debugging your tests when in reality the build setup is not correct.

To be sure that the code can be tested correctly, we need to test within a browser instead of testing in Rhino environment. This can be done by either installing PhantomJS (npm i -g phantomjs) and disabling Rhino in sbt (by set scalaJSUseRhino in Global := false in the sbt REPL) or testing with a browser directly through the new HTML Test Runner feature in ScalaJS (use testHtmlFastOpt and open the path to the file presented in the output in the browser).

After building a simple virtual dom diffing algorithm, it turns out that adding "simple" optimizations may take some time to get them right. There may still be some bugs lurking in the current algorithm but we'll try to refocus on the real implementation of CycleJS again.

Being done with the virtual dom implementation for now, we can update our DOM driver to use it. It made sense to let the main function return a virtual dom representation as well instead of mapping from and to real HTML elements. This way, the driver only needs the virtual dom representation and the user code (our ScycleApp logic function) does not need to deal with the real DOM itself.

Before beginning the implementation of the next video, we are going to need a few more HTML elements: button and p. We are going to implement them just like all other of our Hyperscript elements for now.

After all the work with virtual dom was done, setting up the GUI is pretty straight forward. The two buttons, a paragraph and text label. In the video, we see how to merge and scan Rx.Observable streams. With the ScalaRx implementation we can use the triggerLater methods to update the result, which basically is our counter state.

To make it more in sync with the JavaScript implementation, we took the Rx context out of the Map. This way it will re-evaluate the Map with the calculations outside of the values we return for the drivers.

We start by making a new GUI for the desired result. The second step is to hook up the Rx variables of an HttpDriver to the app.

After struggling a lot trying to get the value of an Option, it looks like ScalaJS or rx.Scala do not like using Option.map or Option.get. The underlying problem seems to be that Rx[Something] does not like having null values assigned to it. We need to investigate this behavior further to see what we can do. Maybe having a Non-Request class would help.

Turns out that NonRequest doesn't really help. It's also not very clear to me why using response() in ScycleApp does not fire. A lot of debugging code exists in this commit now, but I don't think I'll come around to fix this. As it doesn't really look like the scala.rx library gets much love, I will push this on a branch and restart this project with another library: https://github.com/LukaJCB/rxscala-js

Using the rxscala-js library, the code will be much closer to the videos anyways. Let's see how far we get using that.

After trying out rxscala for the first few videos, we're going to restart implementing Cycle.js with rxscala-js. As it is based on Observables, we can implement it more easily following the videos.

To begin, we have to rewrite the main logic around observables. We can keep the VirtualDom implementation for now and see how far we get with the new library. First, we need to depend on the new library by changing the dependencies in our build.sbt file using "com.github.lukajcb" %%% "rxscala-js" % "0.4.0".

Another thing we need to do is add the js-deps script to our index-dev.html, as rxscala-js has a "real" JavaScript dependency. If we don't do that, we get errors in the browser later, even though everything compiles.

As a first step in the rewrite, we implement the core functions again, namely Scycle.run. The first working iteration is a simple Observable[_] => Observable[_]. This looks similar to what we had before, but to get there, we need to use sub functions to let the compiler be able inferring the correct types. The functions wireProxyToSink and createProxy do that.

This is not really the ideal scenario, especially since we still need to cast in the main (logic) function in our ScycleApp to get the correct Observable.

The first step to do less type casting is to make the apply method of the Driver cast itself to the correct types instead of having the user cast it.

During this video, we can see how to build a small application in Cycle.js that calculates a BMI. With a few range sliders (input fields of type range), we should get a calculated value. The first thing we do is to build structure of the resulting DOM tree.

So after fiddling around a lot with the new RxScala library, I've switched to just try to write tests and port over the CycleJS code to Scala by looking at the sources. I'm still not sure if that's a good idea or try to re-implement the core idea in ScalaJS. Especially as it uses quite a few any types and I'm unsure if that helps us at all here. On the other hand, I guess I'd need to reinvent an API that resonates more with Scala developers, but will not really resemble the current CycleJS implementation (through JavaScript Objects - or in our case Scala Maps).

Most probably I'll go on porting and then see if I can come up with a more Scala-ish API and rewrite it once I've learned enough on the way.

While porting the code, there was the need to add a small missing piece to the RxScala library. It did not fully support the dispose mechanism as CycleJS uses it. After fixing this, the library updated to the newest ScalaJS version and this port updated it as well. After updating, we need to install the jsdom NPM module through npm i jsdom, which seems to be new in order to get the tests for the still embedded virtual dom implementation working again.

After putting a lot of work into getting at least the adapter tests pass, it was time for a refactor. Refactoring the current code would mean to be able to cut a lot of unnecessary checks that CycleJS has due to JavaScript. While doing that, there are some type issues that are pretty tricky to fix, due to type erasure. I'm actually unsure if it is possible to fix that at all right now...

Thinking more about this, it might be a good idea to try and find a more scala-ish way. Using Maps feels a bit strange and, regarding the usage syntax in the tests, is not very user friendly. Maybe having a new type for Sinks and Sources could help. Got to investigate this idea further.

After fiddling around with lots of type issues, the main test for doing a complete cycle seems to work. Now the next step is to fix the DomDriver to emit the correct events. Right now this does not seem to be as trivial as expected even though the cycling test works.

Okay, so in the last few commits a small application could do the cycle with the DomDriver and a very simple HttpDriver. As the Scycle code is now a lot like the real CycleJS implementation and we can try to simplify the implementation a bit.

The next improvements were meant to get rid of some type issues and things that made the code overly complex. One thing to note is the DriverFunction[_, _]. It was used to make a driver always return an observable. If we look closely into the Cycle code, this concept does not even exist. By now, the code got rid of this structure and lets all drivers include some function to return an Observable[_]. This is more like the real implementation and works quite well now.

It seems like the current implementation is at least somewhat okay for now and works for easy apps (see ScycleApp). Maybe now is the time to test this against the last few videos and see if everything works as expected. We can also make some improvements to the creation to the drivers still: Cycle uses makeXXXDriver functions to construct a driver. The current code just relies on new XXXDriver instead.

With the newest commit, the makeXXXDriver was implemented. This factory method will create the correct driver as Cycle itself does it.

We continue with the videos again. After getting everything to run, we can finally do the last few videos again and see how using the API works now.

As mentioned before, we start by creating a small template with sliders. While doing that, we can see an issue with the current virtual dom implementation. Input tags did not allow min and max attributes yet and therefore had to be changed a bit in order to allow any kind of key/value pair. After fixing the tests, we can continue implementing everything.

One of the more hard to do things now was getting the DOM driver to be able to catch all events each time the DOM itself changes. In the current version, we simply unsubscribe and reattach all event listeners that were selected. So for the simple demo apps this may work, but at some point in the future, this could be optimized by having the selection of events and the virtual dom implementation work together. It's also not possible to unsubscribe an event right now, which should definitely be possible in a real application.

In this video, the main components were split up into three parts. The read effects, the state calculation and the write effects. There is not much to be done in the Scycle code, just the example application had to change according to the video: In essence, it's just refactoring to get the main logic function smaller and split out the various calculations and selectors into smaller functions.

There is a small thing that should be done before continuing. This thing here is developed in the open and at some point it should be available for others to consume. Right now you can only fork and build it yourself through sbt. It should be possible for anybody to just readily use it. Thus, we're going to release it to public now.

What does that mean and what do we have to do to achieve that?

First of all, we separate the examples from the core Scycle framework. Putting it into two directories helps a lot - using different src/main|examples|whatever folders does not seem to work for me. It's either all sources in one or trouble awaits.

So after splitting it into two sub-directories (scycle and scycle-examples), we have separate src/main/scala folders as well as src/test/scala. In each of our projects we will be able to have tests, dependence on other projects (examples depend on the Scycle core in our case here).

TODO

The video shows how to refactor a single slider into a component. It does it by putting a new kind of "driver" into the sources object, which just returns the properties of the slider.

A simple change is done in this video. The main function gets renamed to LabeledSlider and then a new main is written that uses this renamed function. In our case, the logic function is used for this as main is reserved for the ScalaJS export.

We moved LabeledSlider into its own class here, so the example ScycleApp uses it for a weight and height slider with different properties, using a simple SliderPropsDriver which provides a configuration for the sliders. To be in sync with the current implementation of the DomDriver, we had to change the select event. It is now returning another DomDriver instance which uses the same mutable Map for selectedEvents in order to synchronize all captured events.

The example app is now able to pass a preselected DomDriver to the two LabeledSlider instances.