Replace Enumerated States with a State Machine

Kyle Shevlin
InstructorKyle Shevlin

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Published 3 years ago
Updated 2 years ago

Enumerating the possible states of a function is a sound way to write a function, but it is imperative and could benefit from abstraction. We can do this with a state machine.

A state machine formalizes how we enumerate states and transitions. For the sake of clarity and to emphasize the core concepts, I will be overly verbose with my code in this lesson.

In this lesson, we'll replace the state enum with individual state objects. We'll also replace our events toggle() and break() with events TOGGLE and BREAK. I'll also demonstrate the usefulness of the Machine's initialState getter and the transition method. Lastly, I'll show what happens when we pass erroneous states or events into our machine.

Instructor: [0:00] We're going to replace this lightbulb functions that uses enumerated states with a state machine. We'll start by deleting the whole function. Next, instead of having the states enumerated in an object, we'll create individual objects for each possible state. We can combine these objects into another object that we'll call states.

[0:16] After that, we'll need to define an initial state, so I'll define a variable called initial, and I'm going to set it to the string "unlit." Now I can combine initial and states into an object we're going to call config. This will be the configuration object we'll pass the XState machine momentarily.

[0:32] Our config should also have an ID, and since this is a configuration for a lightbulb, I'll give it the string of "lightbulb." Now we've enumerated our states, but we've yet to enumerate the events and transitions between states in our machine configuration.

[0:45] With state machines, we trigger transitions through events, and we define which state nodes respond to which events. We'll do this by adding an on property to the state nodes that should respond to events. The lit and unlit states should both respond to a break event. Let's add those simultaneously.

[1:01] By convention, we capitalize the name of the event, and the event's value is the targeted state that we would like to transition to. Now, our lit and lit states also respond to a toggle event, but they transition to different targets.

[1:16] Notice that the broken state responds to no events, that is because it's a final of our machine. We can put the final touch on our config by setting the type of final to our broken state. Yes, that pun was intended.

[1:29] We can now import the machine factory function from the XState library. Since I'm merely using Node for my example, I will require it with common JS instead of importing as an ES6 module. I'll create my lightbulb machine by passing our config to the machine factory function.

[1:45] XState's machine function comes with a few useful getters and methods. Let's try some of them out on our lightbulb machine. We can get the initial state node by using the initial state getter, logging this out we will see the entirety of the unlit state returned by the machine. It's quite a lot of information.

[2:01] Next, the most useful method on a machine is the transition method. Transition is a pure function, it receives a state and an event argument, and returns the next state object. That object was so big it didn't even fit in my full terminal window, so from here on out, I'll only log out the value returned to us by the state object. That's more manageable.

[2:23] Let's try this out with some of the other states of our machine. If we set unlit to lit, it should toggle to unlit. If we set it to broken, it should stay broken. What happens if we pass it a state that the machine can't handle? It throws an error, and it tells us the child state foo does not exist on lightbulb.

[2:45] What about when we pass it an event that the machine doesn't handle? By default, the machine does nothing. It returns the state node that it started in, and doesn't take any transition.

[2:56] However, we can set our machine's config to strict true, and then calling an event that our machine can't handle throws an error. We get the error "machine lightbulb does not accept event foo."

Kevin Johnson
Kevin Johnson
~ 2 years ago

I think it could be less-confusing to some to comment-out your ellipses when hiding irrelevant code, otherwise it looks like you're trying to use the rest operator.