automagical

Automagical is a light layer over Mobx that automatically turns classes into Mobx stores. As of Mobx 5 a lot of automagical was basically implemented in Mobx core under the decorate function and so it was simplified.

What automagical does give you is the ability to really easily do reactions. Before in Mobx you'd have to do quite a bit to set up a well-structured reaction. Here's a contrived example:

class MyStore {
  val = 0
  val2 = 0

  constructor() {
    const dispose = Mobx.reaction(
      () => this.val,
      val => {
        this.val2 = val + 1
        // and if you want async stuff that cancels on next reaction, good luck...
      },
      {
        // have to set this to get a nice log
        name: `MyStore.reactToVal`,
      },
    )
    // have to handle dispose manually each time
    this.subscriptions.add({ dispose })
  }
}

With automagical + react, you can do this:

class MyStore {
  val = 0

  // this will also log as MyStore.val2 automatically
  val2 = react(
    () => this.val,
    // react() stores this value to the class property
    val => val + 1,
  )
}

This is nice, but pretty simple. It saves a lot of code though in the long run. But the real value comes with some of the features we've put together over time using react.

Asynchronous reactions

The second you want to do something asynchronous with Mobx you're forced to put together a variety of reptitive and brittle logic to track what's going on. With react you have a lot less to think about.

First, you can just use async functions and it handles when they re-run automatically:

class MyStore {
  val = react(
    () => Mobx.now(2000),
    async () => {
      // if this takes longer than 2000, no worries, it will cancel and use the next fetch!
      return await fetch('/something')
    },
  )
}

What if you want to sleep a bit during typing?

class MyStore {
  query = ''
  answer = react(
    () => this.query,
    async (query, { sleep }) => {
      // really nice debouncing
      await sleep(100)
      return await fetch(`/search?q=${query}`)
    },
  )
}

Or wait for some things to be true?

class MyStore {
  query = ''
  serverResults = react(() => this.query, query => ({ query, answer: await fetch(query) }))
  localResults = react(() => this.query, query => ({ query, answer: await someLocalQuery(query) }))
  allResults = react(
    () => this.query,
    async (query, { when, sleep }) => {
      await when(() => this.localResults.query === query)
      await when(() => this.serverResults.query === query, 200)
      await sleep(100)
      return [...this.localResults.answer, ...this.serverResults.answer]
    },
  )
}

Internally these helper functions throw a special value that is caught and handled properly. When the reaction re-runs it also ignores and throws after the last async function.

You can also do your own cancels. This will prevent the reaction from logging as a success, so you can debug just "valid" reactions, which is helpful as your app grows:

class MyStore {
  val = react(
    () => this.something,
    () => {
      ensure('something', somethingIsTrue)
      return Math.random()
    },
  )
}

There is also a whenChanged helper which is like when but continues once the value changes at all.

Multiple updates

Finally, you can multiple-updates, where you set a value multiple times over the course of a reaction:

class MyStore {
  val = react(
    () => this.someEvent,
    async (_, { sleep, setValue }) => {
      setValue('loading')
      await sleep(100)
      setValue('done loading')
    },
  )
}

Note: you can't mix setValue and using return to return a value. Automagical will throw an error if it detects this.

Similarly, getValue exists which helps out when abstracting react functions. Otherwise you can get the current value usually by just referencing the variable name within the reaction.