mobx-react

What is mobx-react?

The mobx-react package provides React bindings for MobX, a state management library. It allows you to connect your React components to MobX observables, making it easier to manage and react to state changes in your application.

What are mobx-react's main functionalities?

Observer Component

The observer function is used to create a reactive component that will automatically re-render when the observed state changes.

import React from 'react';
import { observer } from 'mobx-react';
import { observable } from 'mobx';

const appState = observable({
  count: 0
});

const Counter = observer(() => (
  <div>
    <button onClick={() => appState.count++}>Increment</button>
    <p>{appState.count}</p>
  </div>
));

export default Counter;

Injecting Stores

The inject function is used to inject stores into components, making it easier to access MobX stores within your React components.

import React from 'react';
import { Provider, inject, observer } from 'mobx-react';
import { observable } from 'mobx';

class Store {
  @observable count = 0;
}

const store = new Store();

const Counter = inject('store')(observer(({ store }) => (
  <div>
    <button onClick={() => store.count++}>Increment</button>
    <p>{store.count}</p>
  </div>
)));

const App = () => (
  <Provider store={store}>
    <Counter />
  </Provider>
);

export default App;

Using MobX with React Hooks

The useLocalObservable hook is used to create local observable state within a functional component, allowing you to use MobX with React hooks.

import React from 'react';
import { useLocalObservable, observer } from 'mobx-react';

const Counter = observer(() => {
  const state = useLocalObservable(() => ({
    count: 0,
    increment() {
      this.count++;
    }
  }));

  return (
    <div>
      <button onClick={state.increment}>Increment</button>
      <p>{state.count}</p>
    </div>
  );
});

export default Counter;

Other packages similar to mobx-react

redux

Redux is a popular state management library for JavaScript applications. Unlike MobX, which uses observables and decorators, Redux relies on a single immutable state tree and pure reducer functions to manage state. Redux is often used with the react-redux bindings to connect React components to the Redux store.

recoil

Recoil is a state management library for React that provides a more fine-grained approach to state management compared to Redux. It allows you to create atoms and selectors to manage state and derive state. Recoil is designed to work seamlessly with React's concurrent mode and hooks.

zustand

Zustand is a small, fast, and scalable state management library for React. It uses hooks to create and manage state, providing a simple and intuitive API. Unlike MobX, which uses observables, Zustand relies on React's built-in state management capabilities.

README

mobx-react

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

Installation

npm install mobx-react --save

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

import { observer } from "mobx-react"
// - or, for custom renderers without DOM: -
import { observer } from "mobx-react/custom"

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

If you are using React hooks with latest React 16.7 and you like living on the bleeding edge then have a look at the new mobx-react-lite.

Boilerplate projects that use mobx-react

• Minimal MobX, React, ES6, JSX, Hot reloading: MobX-React-Boilerplate
• TodoMVC MobX, React, ES6, JSX, Hot reloading: MobX-React-TodoMVC
• Minimal MobX, React, Typescript, TSX: MobX-React-Typescript-Boilerplate
• Minimal MobX, React, ES6(babel), JSPM with hot reloading modules: jspm-react
• React Native Counter: Mobx-React-Native-Counter
• React Native, TypeScript, React Navigation: Ignite Bowser

API documentation

observer(componentClass)

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

Apart from observables passed/injected in or defined inside an observer component, this.props and this.state are also observables themselves, so the component will react to all changes in props and state that are used by render.

See the MobX documentation for more details.

import { observer } from "mobx-react"

// ---- ES5 syntax ----

const TodoView = observer(
    React.createClass({
        displayName: "TodoView",
        render() {
            return <div>{this.props.todo.title}</div>
        }
    })
)

// ---- ES6 syntax ----

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

// ---- ESNext syntax with decorators ----

@observer
class TodoView extends React.Component {
    render() {
        return <div>{this.props.todo.title}</div>
    }
}

// ---- or just use a stateless component function: ----

const TodoView = observer(({ todo }) => <div>{todo.title}</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 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" })

React.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" })

React.render(<App person={person} />, document.body)
person.name = "Mike" // will cause the Observer region to re-render

Global error handler with onError

If a component throws an error, this logs to the console but does not 'crash' the app, so it might go unnoticed. For this reason it is possible to attach a global error handler using onError to intercept any error thrown in the render of an observer component. This can be used to hook up any client side error collection system.

import { onError } from "mobx-react"

onError(error => {
    console.log(error)
})

Server Side Rendering with useStaticRendering

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 useStaticRendering(true) when using server side rendering.

import { useStaticRendering } from "mobx-react"

useStaticRendering(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.

About shouldComponentUpdate

When using @observer on a component, don't implement shouldComponentUpdate, as it will override the default implementation that MobX provides. When using mobx-react, you should in general not need to write an sCU (in our entire Mendix code base we have none). If you really need to implement sCU, split the component into two, a reactive and non-reactive (with the sCU) part, or use <Observer> sections instead of observer on the entire component.

Similarly, PureComponent should not be combined with observer. As pure components are supposed to be dumb and never update themselves automatically, but only by getting passed in new props from the parent. observer is the opposite, it makes components smart and dependency aware, allowing them to update without the parents even needing to be aware of the change.

componentWillReact (lifecycle hook)

React components usually render on a fresh stack, so that makes it often hard to figure out what caused a component to re-render. When using mobx-react you can define a new life cycle hook, componentWillReact (pun intended) that will be triggered when a component is scheduled to be re-rendered because data it observes has changed. This makes it easy to trace renders back to the action that caused the rendering.

import { observer } from "mobx-react"

@observer
class TodoView extends React.Component {
    componentWillReact() {
        console.log("I will re-render, since the todo has changed!")
    }

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

• componentWillReact doesn't take arguments
• componentWillReact won't fire before the initial render (use componentDidMount or constructor instead)

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

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):

@inject("color")
@observer
class Button extends React.Component {
    render() {
        return <button style={{ background: this.props.color }}>{this.props.children}</button>
    }
}

class Message extends React.Component {
    render() {
        return (
            <div>
                {this.props.text} <Button>Delete</Button>
            </div>
        )
    }
}

class MessageList extends React.Component {
    render() {
        const children = this.props.messages.map(message => <Message text={message.text} />)
        return (
            <Provider color="red">
                <div>{children}</div>
            </Provider>
        )
    }
}

Notes:

• 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!
• If updates to an observable store are not triggering render(), make sure you are using Class methods for React lifecycle hooks such as componentWillMount() {}, using componentWillMount = () => {} will create a property on the instance and cause conflicts with mobx-react.
• Values provided through Provider should be final, to avoid issues like mentioned in React #2517 and React #3973, where optimizations might stop the propagation of new context. Instead, 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 warn you. To suppress that warning explicitly, you can use suppressChangedStoreWarning={true} as a prop at your own risk.
• 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.
• For mounted component instances, the wrapped component instance is available through the wrappedInstance property (except for stateless components).

Inject as function

The above example in ES5 would start like:

var Button = inject("color")(
    observer(
        React.createClass({
            /* ... etc ... */
        })
    )
)

A functional stateless component would look like:

var Button = inject("color")(
    observer(({ color }) => {
        /* ... etc ... */
    })
)

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:

const NameDisplayer = ({ name }) => <h1>{name}</h1>

const UserNameDisplayer = inject(stores => ({
    name: stores.userStore.name
}))(NameDisplayer)

const user = mobx.observable({
    name: "Noa"
})

const App = () => (
    <Provider userStore={user}>
        <UserNameDisplayer />
    </Provider>
)

ReactDOM.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:

const UserName = inject("userStore")(({ userStore, bold }) => someRendering())

UserName.propTypes = {
    bold: PropTypes.boolean.isRequired,
    userStore: PropTypes.object.isRequired // will always fail
}

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:

const UserName = inject("userStore")(({ userStore, bold }) => someRendering())

UserName.propTypes = {
    bold: PropTypes.boolean.isRequired // could be defined either here ...
}

UserName.wrappedComponent.propTypes = {
    // ... or here
    userStore: PropTypes.object.isRequired // correct
}

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.

import { IUserStore } from "myStore"

@inject(allStores => ({
    userStore: allStores.userStore as IUserStore
}))
class MyComponent extends React.Component<{ userStore?: IUserStore; otherProp: number }, {}> {
    /* etc */
}

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!
   // ...
}

With Flow

Currently, there is a community-discussion around the best way to use inject with Flow. Join the discussion at this gist.

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:

<Provider profile={profile}>
    <Person age={"30"} />
</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.

import { disposeOnUnmount } from "mobx-react"

class SomeComponent extends React.Component {
    // decorator version
    @disposeOnUnmount
    someReactionDisposer = reaction(...)

    // function version over properties
    someReactionDisposer = disposeOnUnmount(this, reaction(...))

    // function version inside methods
    componentDidMount() {
        // single function
        disposeOnUnmount(this, reaction(...))

        // or function array
        disposeOnUnmount(this, [
            reaction(...),
            reaction(...)
        ])
    }
}

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.

Internal DevTools Api

trackComponents()

Enables the tracking from components. Each rendered reactive component will be added to the componentByNodeRegistery and its renderings will be reported through the renderReporter event emitter.

renderReporter

Event emitter that reports render timings and component destructions. Only available after invoking trackComponents(). New listeners can be added through renderReporter.on(function(data) { /* */ }).

Data will have one of the following formats:

{
    event: 'render',
    renderTime: /* time spend in the .render function of a component, in ms. */,
    totalTime: /* time between starting a .render and flushing the changes to the DOM, in ms. */,
    component: /* component instance */,
    node: /* DOM node */
}

{
    event: 'destroy',
    component: /* component instance */,
    node: /* DOM Node */
}

componentByNodeRegistery

WeakMap. Its get function returns the associated reactive component of the given node. The node needs to be precisely the root node of the component. This map is only available after invoking trackComponents.

Debugging reactions with trace

Using Mobx.trace() inside a React render function will print out the observable that triggered the change. See the mobx trace docs for more information.