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    Add Actions to Transitions to Fire Side Effects

    Kyle ShevlinKyle Shevlin
    xstateXState
    >=4.6.7
    javascriptJavaScript

    Actions are "fire-and-forget" side effects that are triggered by transitions. We set functions on a transition object's actions property to have them called when a transition is taken.

    javascript//...
    EVENT_NAME: {
      target: 'name-of-state-to-transition-to',
      actions: [(context, event) => { sideEffectToFire(context, event) }]
    }
    //...
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    Transcript

    Transcript

    Instructor: 0:00 We have a light bulb machine here, and I'd like to send an action when the light bulb breaks in the unlit state. Down here in unlit, I have a break event that leads us to broken. I'd like to fire off some kind of action, maybe a console.log that tells us that the build is broken.

    0:16 To start with, I'm going to replace this string with an object. This string is really a shorthand for this object of target broken. We're targeting the next state. Next, we're going to add an actions property. The value of actions can be a single function, or it can be an array of functions.

    0:35 Each function receives the context of the state machine and the event object that triggered the transition. In fact, from here, we can just log those out. We can update the machine, and we could see that the visualization changed, and it even shows the action that we'll take when we break.

    0:52 If I were to open the console, and I hit the break, we could see the context was currently undefined, and the event that was taken was the type break. Closing my console and resetting the machine, there's an alternative way to define this action.

    1:09 Machine takes a second argument known as the options object. On this object, we can define our actions as methods on the actions object. We'll create a method -- in this case, log broken -- and we'll simply log out, "Yo, I am broke."

    1:27 Then, where we want that action to be taken, we can simply write a shorthand of a string. We can update our machine and see that the visualization is updated and tells us which action name we're now calling.

    1:43 If I open the console again, we'll see that it still works. What's nice about having the context and the event sent to an action is that we can actually pass more information on our event to be used by an action.

    1:56 I'm going to add context and event back to this log broken, and on event, I'm going to expect some more information. Perhaps instead of just saying, "I'm broke," I can tell you what room I broke in. We'll update the machine, and then in the events panel, we can select break and add some information to it.

    2:14 We can add a location of office. When we send this, if we open our console, it says, "Yo, I am broke in the office." Now, if I want to add this to the break event on lit, it's as simple as repeating the steps. I update the machine, and we see that the actions are now available on both.

    2:38 Perhaps I toggle to lit, and then I log broken. I am in the broken state, which logs out undefined, because I forgot to define it on the event object.