Using the MobX @action decorator with async functions and .then

Question

I'm using MobX 2.2.2 to try to mutate state inside an async action. I have MobX's useStrict set to true.

@action someAsyncFunction(args) {
  fetch(`http://localhost:8080/some_url`, {
    method: 'POST',
    body: {
      args
    }
  })
  .then(res => res.json())
  .then(json => this.someStateProperty = json)
  .catch(error => {
    throw new Error(error)
  });
}

I get:

Error: Error: [mobx] Invariant failed: It is not allowed to create or change state outside an `action` when MobX is in strict mode. Wrap the current method in `action` if this state change is intended

Do I need to supply the @action decorator to the second .then statement? Any help would be appreciated.

Answer 1

Do I need to supply the @action decorator to the second .then statement? Any help would be appreciated.

This is pretty close to the actual solution.

.then(json => this.someStateProperty = json)

should be

.then(action(json => this.someStateProperty = json))

Keep in mind action can be called in many ways that aren't exclusive to @action. From the docs on action:

• action(fn)
• action(name, fn)
• @action classMethod
• @action(name) classMethod
• @action boundClassMethod = (args) => { body }
• @action(name) boundClassMethod = (args) => { body }

are all valid ways to mark a function as an action.

Here's a bin demonstrating the solution: http://jsbin.com/peyayiwowu/1/edit?js,output

mobx.useStrict(true);
const x = mobx.observable(1);

// Do async stuff
function asyncStuff() {
  fetch('http://jsonplaceholder.typicode.com/posts')
    .then((response) => response.json())
    // .then((objects) => x.set(objects[0])) BREAKS
    .then(mobx.action((objects) => x.set(objects[0])))
}

asyncStuff()

As for why your error actually happens I'm guessing that the top level @action doesn't recursively decorate any functions as actions inside the function it's decorating, meaning your anonymous function passed into your promise wasn't really an action.

Answer 2

To complement the above answer; indeed, action only works on the function you pass to it. The functions in the then are run on a separate stack and should therefor be recognizable as separate actions.

Note that you can also give the actions a name as well so that you easily recognize them in the devtools if you use those:

then(action("update objects after fetch", json => this.someStateProperty = json))