ssured/mobx-react-lite

Lightweight React bindings for MobX based on experimental React hooks

mobx-react-lite

This is a next iteration of mobx-react coming from introducing React hooks which simplifies a lot of internal workings of this package. Class based components are not supported except using <Observer> directly in its render method.

You need React version 16.7.0-alpha.0 which is highly experimental and not recommended for a production.

Project is written in TypeScript and provides type safety out of the box. No Flow Type support is planned at this moment, but feel free to contribute.

API documentation

observer(componentClass)

Function that converts a function component into a reactive component, which tracks which observables are used automatically re-renders the component when one of these values changes. Observables can be passed through props, accessed from context or created locally with useObservable.

import { observer, useObservable } from "mobx-react-lite"

const FriendlyComponent = observer(() => {
    const friendNameRef = React.useRef()
    const data = useObservable({
        friends: [] as string[],
        addFriend(favorite: boolean = false) {
            if (favorite === true) {
                data.friends.unshift(friendNameRef.current.value + " * ")
            } else {
                data.friends.push(friendNameRef.current.value)
            }
            friendNameRef.current.value = ""
        },
        get friendsCount() {
            return data.friends.length
        }
    })

    return (
        <div>
            <b>Count of friends: {data.friendsCount} </b>
            {data.friends.map(friend => (
                <div>{friend}</div>
            ))}
            <hr />
            <input ref={friendNameRef} />
            <button onClick={data.addFriend}>Add friend </button>
            <button onClick={() => data.addFriend(true)}>Add favorite friend</button>
        </div>
    )
})

Observer

Observer is a React component, which applies observer to an anonymous region in your component. It takes as children a single, argumentless function which should return exactly one React component. The rendering in the function will be tracked and automatically re-rendered when needed. This can come in handy when needing to pass render function to external components (for example the React Native listview), or if you dislike the observer function.

import { Observer } from "mobx-react-lite"

function ObservePerson(props) {
    const person = useObservable({ name: "John" })
    return (
        <div>
            {person.name}
            <Observer>{() => <div>{person.name}</div>}</Observer>
            <button onClick={() => (person.name = "Mike")}>No! I am Mike</button>
        </div>
    )
}

In case you are a fan of render props, you can use that instead of children. Be advised, that you cannot use both approaches at once, children have a precedence. Example

import { Observer } from "mobx-react-lite"

function ObservePerson(props) {
    const person = useObservable({ name: "John" })
    return (
        <div>
            {person.name}
            <Observer render={() => <div>{person.name}</div>} />
            <button onClick={() => (person.name = "Mike")}>No! I am Mike</button>
        </div>
    )
}

useObservable

React hook that allows creating observable object within a component body and keeps track of it over renders. Gets all the benefits from observable objects including computed properties and methods. You can also use arrays and Map which are useful to track dynamic list/table of information. The Set is not supported (see mobxjs/mobx#69).

Warning: With current implementation you also need to wrap your component to observer otherwise the rerender on update won't happen.

import { observer, useObservable } from "mobx-react-lite"

const TodoList = observer(() => {
    const todos = useObservable(new Map<string, boolean>())
    const todoRef = React.useRef()
    const addTodo = React.useCallback(() => {
        todos.set(todoRef.current.value, false)
        todoRef.current.value = ""
    }, [])
    const toggleTodo = React.useCallback((todo: string) => {
        todos.set(todo, !todos.get(todo))
    }, [])

    return (
        <div>
            {Array.from(todos).map(([todo, done]) => (
                <div onClick={() => toggleTodo(todo)} key={todo}>
                    {todo}
                    {done ? " ✔" : " ⏲"}
                </div>
            ))}
            <input ref={todoRef} />
            <button onClick={addTodo}>Add todo</button>
        </div>
    )
})

Note that if you want to track a single scalar value (string, number, boolean), you would need a boxed value which is not recognized by useObservable. However, we recommend to just useState instead which gives you almost same result (with slightly different API).

useComputed

Another React hook that simplifies computational logic. It's just a tiny wrapper around MobX computed function that runs computation whenever observable values change. In conjuction with observer the component will rerender based on such a change.

const Calculator = observer(({ hasExploded }: { hasExploded: boolean }) => {
    const inputRef = React.useRef()
    const inputs = useObservable([1, 3, 5])
    const result = useComputed(
        () => (hasExploded ? "💣" : inputs.reduce(multiply, 1) * Number(!hasExploded)),
        [hasExploded]
    )

    return (
        <div>
            <input ref={inputRef} />
            <button onClick={() => inputs.push(parseInt(inputRef.current.value) | 1)}>
                Multiply
            </button>
            <div>
                {inputs.join(" * ")} = {result}
            </div>
        </div>
    )
})

Notice that since the computation depends on non-observable value, it has to be passed as a second argument to useComputed. There is React useMemo behind the scenes and all rules applies here as well except you don't need to specify dependency on observable values.

Server Side Rendering with useStaticRendering

When using server side rendering, the components are rendered only once. Since components are never unmounted, observer components would in this case leak memory when being rendered server side. To avoid leaking memory, call useStaticRendering(true) when using server side rendering which essentially disables observer.

import { useStaticRendering } from "mobx-react-lite"

useStaticRendering(true)

This makes sure the component won't try to react to any future data changes.

Why no Provider/inject?

Historically the Provider was useful because a lot of boilerplate was required due to experimental (but widely used) context. By introducing new Context API in React 16.3 it's fairly easy to do this.

const StoreContext = React.createContext(createStore())

// a file with a component
function ConnectedComponent() {
    // replacement for inject
    const store = useContext(StoreContext)
}

If you need to create a store sometimes later, you can just render StoreContext.Provider somewhere in tree.

const StoreContext = React.createContext()

function App({ children }) {
    return <StoreContext.Provider value={createStore()}>{children}</StoreContext.Provider>
}

What about smart/dumb components?

The React hooks don't force anyone to suddenly have a state inside a dumb component that is supposed to only render stuff. You can separate your concerns in a similar fashion.

import { createSelector } from 'react-selector-hooks'

const userSelector = createSelector(({ user ) => ({
    name: user.name,
    age: user.age
}))

function UiComponent({ name, age }) {
    return (
        <div>
            <div>Name: {name}</div>
            <div>Age: {age}</div>
        </div>
    )
}

export default () => {
    // you may extract these two lines into a custom hook
    const store = useContext(StoreContext)
    const data = userSelector(store)
    return UiComponent({...data})
    // perhaps wrap it inside observer in here?
    return observer(UiComponent({...data}))
}

It may look a bit more verbose than a classic inject, but there is nothing stopping you to make your own inject HOC which is so much easier since everything is just a funciton.

// make universal HOC

function inject(useSelector, baseComponent) {
    const store = useContext(StoreContext)
    const selected = useSelector(store)
    // optional memo essentially making a pure component
    return React.memo(props => baseComponent({ ...store, ...props }))
}

// use the HOC with a selector

export default inject(userSelector, UiComponent)