mobx-react

Package with React component wrapper for combining React with MobX. Exports the observer decorator and other utilities. For documentation, see the MobX project. This package supports both React and React Native.

Compatibility matrix

Only the latest version is actively maintained. If you're missing a fix or a feature in older version, consider upgrading or using patch-package

NPM Version	Support MobX version	Supported React versions	Added support for:
v9	6.*	>16.8	Hooks, React 18.2 in strict mode
v7	6.*	>16.8 < 18.2	Hooks
v6	4.* / 5.*	>16.8 <17	Hooks
v5	4.* / 5.*	>0.13 <17	No, but it is possible to use <Observer> sections inside hook based components

mobx-react 6 / 7 is a repackage of the smaller mobx-react-lite package + following features from the mobx-react@5 package added:

• Support for class based components for observer and @observer
• Provider / inject to pass stores around (but consider to use React.createContext instead)
• PropTypes to describe observable based property checkers (but consider to use TypeScript instead)
• The disposeOnUnmount utility / decorator to easily clean up resources such as reactions created in your class based components.

Installation

npm install mobx-react --save

Or CDN: https://unpkg.com/mobx-react (UMD namespace: mobxReact)

1import { observer } from "mobx-react"

This package provides the bindings for MobX and React. See the official documentation for how to get started.

For greenfield projects you might want to consider to use mobx-react-lite, if you intend to only use function based components. React.createContext can be used to pass stores around.

API documentation

Please check mobx.js.org for the general documentation. The documentation below highlights some specifics.

observer(component)

Function (and decorator) that converts a React component definition, React component class, or stand-alone render function, into a reactive component. A converted component will track which observables are used by its effective render and automatically re-render the component when one of these values changes.

Functional Components

React.memo is automatically applied to functional components provided to observer. observer does not accept a functional component already wrapped in React.memo, or an observer, in order to avoid consequences that might arise as a result of wrapping it twice.

Class Components

shouldComponentUpdate is not supported. As such, it is recommended that class components extend React.PureComponent. The observer will automatically patch non-pure class components with an internal implementation of React.PureComponent if necessary.

Extending observer class components is not supported. Always apply observer only on the last class in the inheritance chain.

See the MobX documentation for more details.

1import { observer } from "mobx-react"
2
3// ---- ES6 syntax ----
4const TodoView = observer(
5    class TodoView extends React.Component {
6        render() {
7            return <div>{this.props.todo.title}</div>
8        }
9    }
10)
11
12// ---- ESNext syntax with decorator syntax enabled ----
13@observer
14class TodoView extends React.Component {
15    render() {
16        return <div>{this.props.todo.title}</div>
17    }
18}
19
20// ---- or just use function components: ----
21const TodoView = observer(({ todo }) => <div>{todo.title}</div>)

Note on using props and state in derivations

mobx-react version 6 and lower would automatically turn this.state and this.props into observables. This has the benefit that computed properties and reactions were able to observe those. However, since this pattern is fundamentally incompatible with StrictMode in React 18.2 and higher, this behavior has been removed in React 18.

As a result, we recommend to no longer mark properties as @computed in observer components if they depend on this.state or this.props.

1@observer
2class Doubler extends React.Component<{ counter: number }> {
3    @computed // BROKEN! <-- @computed should be removed in mobx-react > 7
4    get doubleValue() {
5        // Changes to this.props will no longer be detected properly, to fix it,
6        // remove the @computed annotation.
7        return this.props * 2
8    }
9
10    render() {
11        return <div>{this.doubleValue}</div>
12    }
13}

Similarly, reactions will no longer respond to this.state / this.props. This can be overcome by creating an observable copy:

1@observer
2class Alerter extends React.Component<{ counter: number }> {
3    @observable observableCounter: number
4    reactionDisposer
5
6    constructor(props) {
7        this.observableCounter = counter
8    }
9
10    componentDidMount() {
11        // set up a reaction, by observing the observable,
12        // rather than the prop which is non-reactive:
13        this.reactionDisposer = autorun(() => {
14            if (this.observableCounter > 10) {
15                alert("Reached 10!")
16            }
17        })
18    }
19
20    componentDidUpdate() {
21        // sync the observable from props
22        this.observableCounter = this.props.counter
23    }
24
25    componentWillUnmount() {
26        this.reactionDisposer()
27    }
28
29    render() {
30        return <div>{this.props.counter}</div>
31    }
32}

MobX-react will try to detect cases where this.props, this.state or this.context are used by any other derivation than the render method of the owning component and throw. This is to make sure that neither computed properties, nor reactions, nor other components accidentally rely on those fields to be reactive.

This includes cases where a render callback is passed to a child, that will read from the props or state of a parent component. As a result, passing a function that might later read a property of a parent in a reactive context will throw as well. Instead, when using a callback function that is being passed to an observer based child, the capture should be captured locally first:

1@observer
2class ChildWrapper extends React.Component<{ counter: number }> {
3    render() {
4        // Collapsible is an observer component that should respond to this.counter,
5        // if it is expanded
6
7        // BAD:
8        return <Collapsible onRenderContent={() => <h1>{this.props.counter}</h1>} />
9
10        // GOOD: (causes to pass down a fresh callback whenever counter changes,
11        // that doesn't depend on its parents props)
12        const counter = this.props.counter
13        return <Collapsible onRenderContent={() => <h1>{counter}</h1>} />
14    }
15}

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 decorator / function.

1class App extends React.Component {
2    render() {
3        return (
4            <div>
5                {this.props.person.name}
6                <Observer>{() => <div>{this.props.person.name}</div>}</Observer>
7            </div>
8        )
9    }
10}
11
12const person = observable({ name: "John" })
13
14ReactDOM.render(<App person={person} />, document.body)
15person.name = "Mike" // will cause the Observer region to re-render

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

1class App extends React.Component {
2    render() {
3        return (
4            <div>
5                {this.props.person.name}
6                <Observer render={() => <div>{this.props.person.name}</div>} />
7            </div>
8        )
9    }
10}
11
12const person = observable({ name: "John" })
13
14ReactDOM.render(<App person={person} />, document.body)
15person.name = "Mike" // will cause the Observer region to re-render

useLocalObservable hook

Local observable state can be introduced by using the useLocalObservable hook, that runs once to create an observable store. A quick example would be:

1import { useLocalObservable, Observer } from "mobx-react-lite"
2
3const Todo = () => {
4    const todo = useLocalObservable(() => ({
5        title: "Test",
6        done: true,
7        toggle() {
8            this.done = !this.done
9        }
10    }))
11
12    return (
13        <Observer>
14            {() => (
15                <h1 onClick={todo.toggle}>
16                    {todo.title} {todo.done ? "[DONE]" : "[TODO]"}
17                </h1>
18            )}
19        </Observer>
20    )
21}

When using useLocalObservable, all properties of the returned object will be made observable automatically, getters will be turned into computed properties, and methods will be bound to the store and apply mobx transactions automatically. If new class instances are returned from the initializer, they will be kept as is.

It is important to realize that the store is created only once! It is not possible to specify dependencies to force re-creation, nor should you directly be referring to props for the initializer function, as changes in those won't propagate.

Instead, if your store needs to refer to props (or useState based local state), the useLocalObservable should be combined with the useAsObservableSource hook, see below.

Note that in many cases it is possible to extract the initializer function to a function outside the component definition. Which makes it possible to test the store itself in a more straight-forward manner, and avoids creating the initializer closure on each re-render.

Note: using useLocalObservable is mostly beneficial for really complex local state, or to obtain more uniform code base. Note that using a local store might conflict with future React features like concurrent rendering.

Server Side Rendering with enableStaticRendering

When using server side rendering, normal lifecycle hooks of React components are not fired, as 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 enableStaticRendering(true) when using server side rendering.

1import { enableStaticRendering } from "mobx-react"
2
3enableStaticRendering(true)

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

Which components should be marked with observer?

The simple rule of thumb is: all components that render observable data. If you don't want to mark a component as observer, for example to reduce the dependencies of a generic component package, make sure you only pass it plain data.

Enabling decorators (optional)

Decorators are currently a stage-2 ESNext feature. How to enable them is documented here.

Should I still use smart and dumb components?

See this thread. TL;DR: the conceptual distinction makes a lot of sense when using MobX as well, but use observer on all components.

PropTypes

MobX-react provides the following additional PropTypes which can be used to validate against MobX structures:

• observableArray
• observableArrayOf(React.PropTypes.number)
• observableMap
• observableObject
• arrayOrObservableArray
• arrayOrObservableArrayOf(React.PropTypes.number)
• objectOrObservableObject

Use import { PropTypes } from "mobx-react" to import them, then use for example PropTypes.observableArray

Provider and inject

Note: usually there is no need anymore to use Provider / inject in new code bases; most of its features are now covered by React.createContext.

Provider is a component that can pass stores (or other stuff) using React's context mechanism to child components. This is useful if you have things that you don't want to pass through multiple layers of components explicitly.

inject can be used to pick up those stores. It is a higher order component that takes a list of strings and makes those stores available to the wrapped component.

Example (based on the official context docs):

1@inject("color")
2@observer
3class Button extends React.Component {
4    render() {
5        return <button style={{ background: this.props.color }}>{this.props.children}</button>
6    }
7}
8
9class Message extends React.Component {
10    render() {
11        return (
12            <div>
13                {this.props.text} <Button>Delete</Button>
14            </div>
15        )
16    }
17}
18
19class MessageList extends React.Component {
20    render() {
21        const children = this.props.messages.map(message => <Message text={message.text} />)
22        return (
23            <Provider color="red">
24                <div>{children}</div>
25            </Provider>
26        )
27    }
28}

Notes:

• It is possible to read the stores provided by Provider using React.useContext, by using the MobXProviderContext context that can be imported from mobx-react.
• If a component asks for a store and receives a store via a property with the same name, the property takes precedence. Use this to your advantage when testing!
• When using both @inject and @observer, make sure to apply them in the correct order: observer should be the inner decorator, inject the outer. There might be additional decorators in between.
• The original component wrapped by inject is available as the wrappedComponent property of the created higher order component.

"The set of provided stores has changed" error

Values provided through Provider should be final. Make sure that if you put things in context that might change over time, that they are @observable or provide some other means to listen to changes, like callbacks. However, if your stores will change over time, like an observable value of another store, MobX will throw an error. This restriction exists mainly for legacy reasons. If you have a scenario where you need to modify the set of stores, please leave a comment about it in this issue https://github.com/mobxjs/mobx-react/issues/745. Or a preferred way is to use React Context directly which does not have this restriction.

Inject as function

The above example in ES5 would start like:

1var Button = inject("color")(
2    observer(
3        class Button extends Component {
4            /* ... etc ... */
5        }
6    )
7)

A functional stateless component would look like:

1var Button = inject("color")(
2    observer(({ color }) => {
3        /* ... etc ... */
4    })
5)

Customizing inject

Instead of passing a list of store names, it is also possible to create a custom mapper function and pass it to inject. The mapper function receives all stores as argument, the properties with which the components are invoked and the context, and should produce a new set of properties, that are mapped into the original:

mapperFunction: (allStores, props, context) => additionalProps

Since version 4.0 the mapperFunction itself is tracked as well, so it is possible to do things like:

1const NameDisplayer = ({ name }) => <h1>{name}</h1>
2
3const UserNameDisplayer = inject(stores => ({
4    name: stores.userStore.name
5}))(NameDisplayer)
6
7const user = mobx.observable({
8    name: "Noa"
9})
10
11const App = () => (
12    <Provider userStore={user}>
13        <UserNameDisplayer />
14    </Provider>
15)
16
17ReactDOM.render(<App />, document.body)

N.B. note that in this specific case neither NameDisplayer nor UserNameDisplayer needs to be decorated with observer, since the observable dereferencing is done in the mapper function

Using PropTypes and defaultProps and other static properties in combination with inject

Inject wraps a new component around the component you pass into it. This means that assigning a static property to the resulting component, will be applied to the HoC, and not to the original component. So if you take the following example:

1const UserName = inject("userStore")(({ userStore, bold }) => someRendering())
2
3UserName.propTypes = {
4    bold: PropTypes.boolean.isRequired,
5    userStore: PropTypes.object.isRequired // will always fail
6}

The above propTypes are incorrect, bold needs to be provided by the caller of the UserName component and is checked by React. However, userStore does not need to be required! Although it is required for the original stateless function component, it is not required for the resulting inject component. After all, the whole point of that component is to provide that userStore itself.

So if you want to make assertions on the data that is being injected (either stores or data resulting from a mapper function), the propTypes should be defined on the wrapped component. Which is available through the static property wrappedComponent on the inject component:

1const UserName = inject("userStore")(({ userStore, bold }) => someRendering())
2
3UserName.propTypes = {
4    bold: PropTypes.boolean.isRequired // could be defined either here ...
5}
6
7UserName.wrappedComponent.propTypes = {
8    // ... or here
9    userStore: PropTypes.object.isRequired // correct
10}

The same principle applies to defaultProps and other static React properties. Note that it is not allowed to redefine contextTypes on inject components (but is possible to define it on wrappedComponent)

Finally, mobx-react will automatically move non React related static properties from wrappedComponent to the inject component so that all static fields are actually available to the outside world without needing .wrappedComponent.

Strongly typing inject

With TypeScript

inject also accepts a function ((allStores, nextProps, nextContext) => additionalProps) that can be used to pick all the desired stores from the available stores like this. The additionalProps will be merged into the original nextProps before being provided to the next component.

1import { IUserStore } from "myStore"
2
3@inject(allStores => ({
4    userStore: allStores.userStore as IUserStore
5}))
6class MyComponent extends React.Component<{ userStore?: IUserStore; otherProp: number }, {}> {
7    /* etc */
8}

Make sure to mark userStore as an optional property. It should not (necessarily) be passed in by parent components at all!

Note: If you have strict null checking enabled, you could muffle the nullable type by using the ! operator:

public render() {
   const {a, b} = this.store!
   // ...
}

Testing store injection

It is allowed to pass any declared store in directly as a property as well. This makes it easy to set up individual component tests without a provider.

So if you have in your app something like:

1<Provider profile={profile}>
2    <Person age={"30"} />
3</Provider>

In your test you can easily test the Person component by passing the necessary store as prop directly:

const profile = new Profile()
const mountedComponent = mount(
   <Person age={'30'} profile={profile} />
)

Bear in mind that using shallow rendering won't provide any useful results when testing injected components; only the injector will be rendered. To test with shallow rendering, instantiate the wrappedComponent instead: shallow(<Person.wrappedComponent />)

disposeOnUnmount(componentInstance, propertyKey | function | function[])

Function (and decorator) that makes sure a function (usually a disposer such as the ones returned by reaction, autorun, etc.) is automatically executed as part of the componentWillUnmount lifecycle event.

1import { disposeOnUnmount } from "mobx-react"
2
3class SomeComponent extends React.Component {
4    // decorator version
5    @disposeOnUnmount
6    someReactionDisposer = reaction(...)
7
8    // decorator version with arrays
9    @disposeOnUnmount
10    someReactionDisposers = [
11        reaction(...),
12        reaction(...)
13    ]
14
15
16    // function version over properties
17    someReactionDisposer = disposeOnUnmount(this, reaction(...))
18
19    // function version inside methods
20    componentDidMount() {
21        // single function
22        disposeOnUnmount(this, reaction(...))
23
24        // or function array
25        disposeOnUnmount(this, [
26            reaction(...),
27            reaction(...)
28        ])
29    }
30}

DevTools

mobx-react@6 and higher are no longer compatible with the mobx-react-devtools. That is, the MobX react devtools will no longer show render timings or dependency trees of the component. The reason is that the standard React devtools are also capable of highlighting re-rendering components. And the dependency tree of a component can now be inspected by the standard devtools as well, as shown in the image below:

FAQ

Should I use observer for each component?

You should use observer on every component that displays observable data. Even the small ones. observer allows components to render independently from their parent and in general this means that the more you use observer, the better the performance become. The overhead of observer itself is negligible. See also Do child components need @observer?

I see React warnings about forceUpdate / setState from React

The following warning will appear if you trigger a re-rendering between instantiating and rendering a component:

Warning: forceUpdate(...): Cannot update during an existing state transition (such as within `render`). Render methods should be a pure function of props and state.`

-- or --

Warning: setState(...): Cannot update during an existing state transition (such as within `render` or another component's constructor). Render methods should be a pure function of props and state; constructor side-effects are an anti-pattern, but can be moved to `componentWillMount`.

Usually this means that (another) component is trying to modify observables used by this components in their constructor or getInitialState methods. This violates the React Lifecycle, componentWillMount should be used instead if state needs to be modified before mounting.