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Computeds with arguments {🚀}

The computed annotation can only be used on getters, which don't take arguments. What about computations that do take arguments? Take the below example of a React component that renders a specific Item, and the application supports multi-selection.

How can we implement a derivation like store.isSelected(item.id)?

import * as React from 'react'
import { observer } from 'mobx-react-lite'

const Item = observer(({ item, store }) => (
    <div className={store.isSelected(item.id) ? "selected" : ""}>
        {item.title}
    </div>
))

There are four ways in which we can approach this. You can try the solutions below in this CodeSandbox.

1. Derivations don't need to be computed

A function doesn't need to be marked as computed in order for MobX to track it. The above example would already work completely fine out of the box. It is important to realize that computed values are only caching points. If the derivations are pure (and they should be), having a getter or function without computed doesn't change the behavior, it is just slightly less efficient.

The above example works fine despite isSelected not being a computed. The observer component will detect and subscribe to any observables that were read by isSelected because the function executes as part of rendering that is tracked.

It is good to realize that all Item components, in this case, will respond to future selection changes, as they all subscribe directly to the observables that capture the selection. This is a worst-case example. In general, it is completely fine to have unmarked functions that derive information, and this is a good default strategy until numbers prove anything else should be done.

2. Close over the arguments

This is a more efficient implementation compared to the original.

import * as React from 'react'
import { computed } from 'mobx'
import { observer } from 'mobx-react-lite'

const Item = observer(({ item, store }) => {
    const isSelected = computed(() => store.isSelected(item.id)).get()
    return (
        <div className={isSelected ? "selected" : ""}>
            {item.title}
        </div>
    )
})

We create a fresh computed value in the middle of a reaction. This works fine and does introduce that additional caching point, avoiding all components having to directly respond to every selection change. The advantage of this approach is that the component itself will only re-render if the isSelected state toggles, in which case we indeed have to re-render to swap the className.

The fact that we create a new computed in a next render is fine, this one will now become the caching point and the previous one will be cleaned up nicely. This is a great and advanced optimization technique.

3. Move the state

In this specific case the selection could also be stored as an isSelected observable on the Item. The selection in the store could then be expressed as a computed rather than an observable: get selection() { return this.items.filter(item => item.isSelected) }, and we don't need isSelected anymore.

4. Use computedFn {🚀}

Finally, computedFn from mobx-utils can be used in the definition of todoStore.selected to automatically memoize isSelected. It creates a function that memoizes the output for every combination of input arguments.

We recommend to not resort to this one too quickly. It is typical for memoization, that you will need to think about how many different arguments the function is going to be called with, before you can reason about the memory consumption. It does however automatically clean up entries if their results aren't observed by any reaction, so it won't leak memory in normal circumstances.

Again, check out the linked CodeSandbox to try this one out.