How to set new values to whole MobX observable array

Question

In MobX how can I set new values to a whole observable array without re-setting every value?

First thought would be:

let arr = observable([]);
autorun(() => {
  console.log('my array changed!');
});
arr = ['foo', 'bar'];

But that would not fire the autorun, I would just erased my observable array and replaced it with a new value/array.

So, what is the correct way to do this?

My solution to this was to use another variable with a setter, and inside the setter function change the observable array index by index, replacing, adding and deleting indexes. Like this:

(jsFiddle here)

const {observable, computed, extendObservable} = mobx;
const {observer} = mobxReact;
const {Component} = React;
const {render} = ReactDOM
const {autorun} = mobx

class Store {
    @observable entries = [1,2,3,4,5];
    set rows(arr) {
        // add new values
        for (let i = 0, l = arr.length; i < l; i++) {
            this.entries[i] = arr[i];
        }
        // remove rest of entries
        let diff = this.entries.length - arr.length;
        if (diff > 0) {
            while (diff > 0) {
                this.entries.pop();
                diff--;
            }
        }
    }
}
const store = new Store();

@observer
class App extends Component {
    updateRows(){
        return this.props.entries.map(
            (row, i) => <p key={i}>{row}</p>
        );
    }
    render() {
        const rows = this.updateRows();
        return <div>{rows}</div>;
    }
}

setTimeout(() => {
    store.rows = ['foo', 'bar'];
    document.body.innerHTML += 'Timeout fired...';
}, 1000);


render(
    <App entries={store.entries} />,
    document.getElementById('mount')
);

Is this the correct way?
or is there a way to just use the same variable to re-assign a value to the whole array?

Answer 1

TLDR; use store.entries.replace(['foo', 'bar']);

I found a method .replace() that will replace the content of the whole array and trigger the render. I found it after a suggestion about the clear() method and from there looking more carefully into the docs

.replace(newItems);

Replaces all existing entries in the array with new ones.

The code would then look like this:

(jsFiddle)

const {observable, autorun} = mobx;

class Store {
     @observable arr = [1,2,3,4,5];
}
const store = new Store();
autorun(() => {
  console.log('my array changed!', store.arr.slice());
});
setTimeout(() => {
   store.arr.replace(['foo', 'bar']);
}, 1000);

The whole code would be:

(jsFiddle)

const {observable, computed, extendObservable} = mobx;
const {observer} = mobxReact;
const {Component} = React;
const {render} = ReactDOM
const {autorun} = mobx

class Store {
     @observable entries = [1,2,3,4,5];
}
const store = new Store();

@observer
class App extends Component {
    updateRows(){
        return this.props.entries.map(
            (row, i) => <p key={i}>{row}</p>
        );
    }
  render() {
  const rows = this.updateRows();
    return <div>{rows}</div>;
  }
}

setTimeout(() => {
    store.entries.replace(['foo', 'bar']);
}, 1000);


render(
  <App entries={store.entries} />,
  document.getElementById('mount')
);