[Mobx] Use MobX actions to change and guard state

This lesson explains how actions can be used to control and modify the state of your application. They help you to structure your code base and integrate well with the MobX React Devtools. Actions automatically create transactions, which group changes together.

const {observable, computed} = mobx;
const {observer} = mobxReact;
const {Component} = React;
const DevTools = mobxDevtools.default;

const t = new class Temperature {
  @observable unit = "C";
  @observable temperatureCelsius = 25;

  @computed get temperatureKelvin() {
    console.log("calculating Kelvin")
    return this.temperatureCelsius * (9/5) + 32
  }

  @computed get temperatureFahrenheit() {
    console.log("calculating Fahrenheit")
    return this.temperatureCelsius + 273.15
  }

  @computed get temperature() {
    console.log("calculating temperature")
    switch(this.unit) {
      case "K": return this.temperatureKelvin + "ºK"
      case "F": return this.temperatureFahrenheit + "ºF"
      case "C": return this.temperatureCelsius + "ºC"
    }
  }
}

const App = observer(({ temperature }) => (
  <div>
    {temperature.temperature}
    <DevTools />
  </div>
))

ReactDOM.render(
  <App temperature={t} />,
  document.getElementById("app")
)

We have @Observable and @Computed defined, once we change any @Observable value, we can get new value in the @Computed.

But currently, the way we changing the value is though directly object mutation, such as:

t.unit = "F"

Of course, it is not good enough, what we can do is using @action to change the value:

const {observable, computed, action, transaction, useStrict} = mobx;
const {observer} = mobxReact;
const {Component} = React;
const DevTools = mobxDevtools.default;

useStrict(true);

const t = new class Temperature {
  @observable unit = "C";
  @observable temperatureCelsius = 25;

  @computed get temperatureKelvin() {
    console.log("calculating Kelvin")
    return this.temperatureCelsius * (9/5) + 32
  }

  @computed get temperatureFahrenheit() {
    console.log("calculating Fahrenheit")
    return this.temperatureCelsius + 273.15
  }

  @computed get temperature() {
    console.log("calculating temperature")
    switch(this.unit) {
      case "K": return this.temperatureKelvin + "ºK"
      case "F": return this.temperatureFahrenheit + "ºF"
      case "C": return this.temperatureCelsius + "ºC"
    }
  }

  @action setUnit(newUnit) {
    this.unit = newUnit;
  }

  @action setCelsius(degrees) {
    this.temperatureCelsius = degrees;
  }

  @action("update temperature and unit")
  setTemperatureAndUnit(degrees, unit) {
    this.setCelsius(degrees);
    this.setUnit(unit);
  }
}

const App = observer(({ temperature }) => (
  <div>
    {temperature.temperature}
    <DevTools />
  </div>
))

ReactDOM.render(
  <App temperature={t} />,
  document.getElementById("app")
)

Action can be anynomous action or named action:

@action setCelsius(degrees)
@action("update temperature and unit") // named