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

mobx-react is a wrapper around mobx-react-lite for applications that also need class component support:

• Support for function components through mobx-react-lite
• Support for class based components for observer and @observer

Installation

npm install mobx-react

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

import { observer } from "mobx-react"

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

Use React.createContext 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.

import { observer } from "mobx-react"

// ---- ES6 syntax ----
const TodoView = observer(
    class TodoView extends React.Component {
        render() {
            return <div>{this.props.todo.title}</div>
        }
    }
)

// ---- ESNext syntax with decorator syntax enabled ----
@observer
class TodoView extends React.Component {
    render() {
        return <div>{this.props.todo.title}</div>
    }
}

// ---- or just use function components: ----
const 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.

@observer
class Doubler extends React.Component<{ counter: number }> {
    @computed // BROKEN! <-- @computed should be removed in mobx-react > 7
    get doubleValue() {
        // Changes to this.props will no longer be detected properly, to fix it,
        // remove the @computed annotation.
        return this.props * 2
    }

    render() {
        return <div>{this.doubleValue}</div>
    }
}

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

@observer
class Alerter extends React.Component<{ counter: number }> {
    @observable observableCounter: number
    reactionDisposer

    constructor(props) {
        this.observableCounter = counter
    }

    componentDidMount() {
        // set up a reaction, by observing the observable,
        // rather than the prop which is non-reactive:
        this.reactionDisposer = autorun(() => {
            if (this.observableCounter > 10) {
                alert("Reached 10!")
            }
        })
    }

    componentDidUpdate() {
        // sync the observable from props
        this.observableCounter = this.props.counter
    }

    componentWillUnmount() {
        this.reactionDisposer()
    }

    render() {
        return <div>{this.props.counter}</div>
    }
}

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:

@observer
class ChildWrapper extends React.Component<{ counter: number }> {
    render() {
        // Collapsible is an observer component that should respond to this.counter,
        // if it is expanded

        // BAD:
        return <Collapsible onRenderContent={() => <h1>{this.props.counter}</h1>} />

        // GOOD: (causes to pass down a fresh callback whenever counter changes,
        // that doesn't depend on its parents props)
        const counter = this.props.counter
        return <Collapsible onRenderContent={() => <h1>{counter}</h1>} />
    }
}

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.

class App extends React.Component {
    render() {
        return (
            <div>
                {this.props.person.name}
                <Observer>{() => <div>{this.props.person.name}</div>}</Observer>
            </div>
        )
    }
}

const person = observable({ name: "John" })

ReactDOM.render(<App person={person} />, document.body)
person.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

class App extends React.Component {
    render() {
        return (
            <div>
                {this.props.person.name}
                <Observer render={() => <div>{this.props.person.name}</div>} />
            </div>
        )
    }
}

const person = observable({ name: "John" })

ReactDOM.render(<App person={person} />, document.body)
person.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:

import { useLocalObservable, Observer } from "mobx-react"

const Todo = () => {
    const todo = useLocalObservable(() => ({
        title: "Test",
        done: true,
        toggle() {
            this.done = !this.done
        }
    }))

    return (
        <Observer>
            {() => (
                <h1 onClick={todo.toggle}>
                    {todo.title} {todo.done ? "[DONE]" : "[OPEN]"}
                </h1>
            )}
        </Observer>
    )
}

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), sync those values into the store with useEffect.

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.

import { enableStaticRendering } from "mobx-react"

enableStaticRendering(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.

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.