(@)computed

Egghead.io lesson 3: computed values

Computed values are values that can be derived from the existing state or other computed values. Conceptually, they are very similar to formulas in spreadsheets. Computed values can't be underestimated, as they help you to make your actual modifiable state as small as possible. Besides that they are highly optimized, so use them wherever possible.

Don't confuse computed with autorun. They are both reactively invoked expressions, but use @computed if you want to reactively produce a value that can be used by other observers and autorun if you don't want to produce a new value but rather want to achieve an effect. For example imperative side effects like logging, making network requests etc.

Computed values are automatically derived from your state if any value that affects them changes. Computed values can be optimized away in many cases by MobX as they are assumed to be pure. For example, a computed property won't re-run if none of the data used in the previous computation changed. Nor will a computed property re-run if is not in use by some other computed property or reaction. In such cases it will be suspended.

This automatic suspension is very convenient. If a computed value is no longer observed, for example the UI in which it was used no longer exists, MobX can automatically garbage collect it. This differs from autorun's values where you must dispose of them yourself. It sometimes confuses people new to MobX, that if you create a computed property but don't use it anywhere in a reaction, it will not cache its value and recompute more often than seems necessary. However, in real life situations this is by far the best default, and you can always forcefully keep a computed value awake if you need to, by using either observe or keepAlive.

Note that computed properties are not enumerable. Nor can they be overwritten in an inheritance chain.

@computed

If you have decorators enabled you can use the @computed decorator on any getter of a class property to declaratively created computed properties.

import {observable, computed} from "mobx";

class OrderLine {
    @observable price = 0;
    @observable amount = 1;

    constructor(price) {
        this.price = price;
    }

    @computed get total() {
        return this.price * this.amount;
    }
}

computed modifier

If your environment doesn't support decorators, use the computed(expression) modifier in combination with extendObservable / observable to introduce new computed properties.

@computed get propertyName() { } is basically sugar for extendObservable(this, { propertyName: get func() { } }) in the constructor call.

import {extendObservable, computed} from "mobx";

class OrderLine {
    constructor(price) {
        extendObservable(this, {
            price: price,
            amount: 1,
            // valid:
            get total() {
                return this.price * this.amount
            },
            // also valid:
            total: computed(function() {
                return this.price * this.amount
            })
        })
    }
}

Setters for computed values

It is possible to define a setter for computed values as well. Note that these setters cannot be used to alter the value of the computed property directly, but they can be used as 'inverse' of the derivation. For example:

const box = observable({
    length: 2,
    get squared() {
        return this.length * this.length;
    },
    set squared(value) {
        this.length = Math.sqrt(value);
    }
});

And similarly

class Foo {
    @observable length = 2;
    @computed get squared() {
        return this.length * this.length;
    }
    set squared(value) { //this is automatically an action, no annotation necessary
        this.length = Math.sqrt(value);
    }
}

Note: always define the setter after the getter, some TypeScript versions are known to declare two properties with the same name otherwise.

Note: setters require MobX 2.5.1 or higher

computed(expression) as function

computed can also be invoked directly as function. Just like observable.box(primitive value) creates a stand-alone observable. Use .get() on the returned object to get the current value of the computation, or .observe(callback) to observe its changes. This form of computed is not used very often, but in some cases where you need to pass a "boxed" computed value around it might prove useful.

Example:

import {observable, computed} from "mobx";
var name = observable("John");

var upperCaseName = computed(() =>
    name.get().toUpperCase()
);

var disposer = upperCaseName.observe(change => console.log(change.newValue));

name.set("Dave");
// prints: 'DAVE'

Options for computed

When using computed as modifier or as box, it accepts a second options argument with the following optional arguments:

• name: String, the debug name used in spy and the MobX devtools
• context: The this that should be used in the provided expression
• setter: The setter function to be used. Without setter it is not possible to assign new values to a computed value. If the second argument passed to computed is a function, this is assumed to be a setter.
• compareStructural: By default false. When true, the output of the expression is structurally compared with the previous value before any observer is notified about a change. This makes sure that observers of the computation don't re-evaluate if new structures are returned that are structurally equal to the original ones. This is very useful when working with point, vector or color structures for example. The same behaviour can be achieved by specifying the equals option with comparer.structural.
• equals: By default comparer.default. This acts as a comparison function for comparing the previous value with the next value. If this function considers the previous and next values to be equal, then observers will not be re-evaluated. This is useful when working with structural data, and types from other libraries. For example, a computed moment instance could use (a, b) => a.isSame(b). If specified, this will override compareStructural.

@computed.struct for structural comparison

The @computed decorator does not take arguments. If you want to to create a computed property which does structural comparison, use @computed.struct.

@computed.equals for custom comparison

If you want to to create a computed property which does custom comparison, use @computed.equals(comparer).

Built-in comparers

MobX provides three built-in comparers which should cover most needs:

• comparer.identity: Uses the identity (===) operator to determine if two values are the same.
• comparer.default: The same as comparer.identity, but also considers NaN to be equal to NaN.
• comparer.structural: Performs deep structural comparison to determine if two values are the same.

Note on error handling

If a computed value throws an exception during its computation, this exception will be caught and rethrown any time its value is read. It is strongly recommended to always throw Error's, so that the original stack trace is preserved. E.g.: throw new Error("Uhoh") instead of throw "Uhoh". Throwing exceptions doesn't break tracking, so it is possible for computed values to recover from exceptions.

Example:

const x = observable(3)
const y = observable(1)
const divided = computed(() => {
    if (y.get() === 0)
        throw new Error("Division by zero")
    return x.get() / y.get()
})

divided.get() // returns 3

y.set(0) // OK
divided.get() // Throws: Division by zero
divided.get() // Throws: Division by zero

y.set(2)
divided.get() // Recovered; Returns 1.5