MobX - Observable value promised in a store constructor using fromPromise stays null when accessed in another store? - mobx

Question

So I have 2 stores, an AuthorStore:
class AuthorStore {
constructor() {
// has author.name and is always present in storage
AsyncStorage.getItem('author').then(action((data) => {
this.author = JSON.parse(data);
}));
}
#observable author = null;
}
and a BookStore:
import AuthorStore from 'authorStore';
class BookStore {
#observable book = {
authorName: AuthorStore.author.name,
bookTitle: null
}
}
I keep getting an error in BookStore that it cannot get property of null, as if the AuthorStore.author.name is null. So it's reading the default author value from the AuthorStore without the constructor running first to assign it the value.
I came across the new mobx-utils fromPromise which I think would get the author value if it exists in local storage, and wait for AsyncStorage to assign it to the author observable, so it can be called from another store without being null.
I tried using fromPromise first in the AuthorStore to log the author value, but it shows as Got undefined in console, and the usual null error in the BookStore when it comes to the AuthorStore.author part.
UPDATE:
class AuthorStore {
#observable author = null;
#computed get theAuthor() {
authorPromise = fromPromise(AsyncStorage.getItem('author').then(data => JSON.parse(data)));
// combine with when..
when(
() => authorPromise.state !== "pending",
() => {
console.log("Got Author", authorPromise.reason || authorPromise.value) // This runs, and returns author
console.log("Got Name", authorPromise.reason || authorPromise.value.name) // This runs, and returns name
return authorPromise.value; // This doesn't get returned in BookStore when calling this computed
}
);
}
}
class BookStore {
#observable book = {
authorName: AuthorStore.theAuthor.name, // doesn't get computed returned value from promise
bookTitle: null
}
}
How can I get the fromPromise value assigned by the AuthorStore computed function theAuthor to return the promised authorPromise value into BookStore under authorName?

Answer 1

FromPromise creates a new object wrapping the original promise. So your authorFromStorage is just a normal promise in your example, not having a state property at all. So you should change your code to:
authorPromise = fromPromise(AsyncStorage.getItem('author').then(data => JSON.parse(data)))
And then when(() => authorPromise.state !== "pending") etc..
** UPDATE **
class AuthorStore {
#observable author = null;
constructor() {
AsyncStorage.getItem('author').then(data => { this.author = JSON.parse(data) });
}
}
class BookStore {
#observable book = {
authorName: function() { // a function in an observable creates a computed prop
return AuthorStore.author && AuthorStore.author.name
},
bookTitle: null
}
}