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<!DOCTYPE HTML>

<head>

<title>(@)observer | MobX</title>

</head>

<body>

</div>

Introduction

</a>

</li>

1. Introduction to MobX

1.1.

The Gist of MobX

</a>

</li>

1.2.

Concepts & Principles

</a>

</li>

</ul>

</li>

2.

API overview

</a>

</li>

3. Making things observable

3.1.

observable

</a>

</li>

3.2.

@observable

</a>

</li>

3.3.

objects

</a>

</li>

3.4.

arrays

</a>

</li>

3.5.

maps

</a>

</li>

3.6.

boxed values

</a>

</li>

3.7.

decorators

</a>

</li>

</ul>

</li>

4. Reacting to observables

4.1.

(@)computed

</a>

</li>

4.2.

autorun

</a>

</li>

4.3.

when

</a>

</li>

4.4.

reaction

</a>

</li>

4.5.

(@)observer

</a>

</li>

4.6.

Understanding what MobX reacts to

</a>

</li>

</ul>

</li>

5. Changing observables

5.1.

action

</a>

</li>

5.2.

async actions & flows

</a>

</li>

5.3.

Object api

</a>

</li>

</ul>

</li>

6. Utility functions

6.1.

toJS

</a>

</li>

6.2.

extendObservable

</a>

</li>

6.3.

createAtom

</a>

</li>

6.4.

intercept & observe

</a>

</li>

6.5.

mobxUtils.fromPromise

</a>

</li>

6.6.

mobxUtis.fromResource

</a>

</li>

6.7.

mobxUtils.toStream

</a>

</li>

6.8.

mobxUtils.fromStream

</a>

</li>

6.9.

mobxUtils.now

</a>

</li>

6.10.

mobxUtils.keepAlive

</a>

</li>

6.11.

mobxUtils.createTransformer

</a>

</li>

6.12.

mobxUtils.expr

</a>

</li>

</ul>

</li>

7.

Blogs, videos, related projects

</a>

</li>

8. Tips & Tricks

8.1.

Frequently Asked Questions

</a>

</li>

8.2.

Common Pitfalls & Best Practices

</a>

</li>

8.3.

How to (not) use decorator syntax

</a>

</li>

8.4.

Using trace for debugging

</a>

</li>

8.5.

Defining data stores

</a>

</li>

8.6.

Optimizing React components

</a>

</li>

8.7.

DevTools

</a>

</li>

8.8.

spy

</a>

</li>

</ul>

</li>

9.

Donate

</a>

</li>

<li>

Published with GitBook

</a>

</li>

</ul>

</nav>

</div>

</a>

</div>

</div>

<button type="button" data-font="0" class="button">Serif</button>

</div>

<button type="button" id="color-theme-preview-0" class="button size-3" data-theme="0">White</button>

<button type="button" id="color-theme-preview-1" class="button size-3" data-theme="1">Sepia</button>

<button type="button" id="color-theme-preview-2" class="button size-3" data-theme="2">Night</button>

</div>

</a>

</div>

Share on Twitter

</button>

Share on Google

</button>

Share on Facebook

</button>

Share on Weibo

</button>

Share on Instapaper

</button>

</div>

<h1>

</h1>

</div>

<html><head></head><body><a id="edit-link" href="https://github.com/mobxjs/mobx/tree/gh-pages/docs/refguide/observer-component.md" class="btn fa fa-edit pull-left">  Edit This Page</a><h1 id="-observer"><a name="-observer" class="plugin-anchor" href="#-observer"></a>@observer</h1>

<a style="color: white; background:green;padding:5px;margin:5px;border-radius:2px" href="https://egghead.io/courses/manage-complex-state-in-react-apps-with-mobx" target="_blank">egghead.io lesson 1: observable & observer</a>

The <code>observer</code> function / decorator can be used to turn ReactJS components into reactive components.

It wraps the component's render function in <code>mobx.autorun</code> to make sure that any data that is used during the rendering of a component forces a re-rendering upon change.

It is available through the separate <code>mobx-react</code> package.

<pre><code class="lang-javascript">import {observer} from "mobx-react";

var timerData = observable({

secondsPassed: 0

});

setInterval(() => {

timerData.secondsPassed++;

}, 1000);

@observer class Timer extends React.Component {

render() {

return (<span>Seconds passed: { this.props.timerData.secondsPassed } </span> )

}

};

ReactDOM.render(<Timer timerData={timerData} />, document.body);

</code></pre>

Tip: when <code>observer</code> needs to be combined with other decorators or higher-order-components, make sure that <code>observer</code> is the innermost (first applied) decorator;

otherwise it might do nothing at all.

Note that using <code>@observer</code> as decorator is optional, <code>observer(class Timer ... { })</code> achieves exactly the same.

<h2 id="gotcha-dereference-values-inside-your-components"><a name="gotcha-dereference-values-inside-your-components" class="plugin-anchor" href="#gotcha-dereference-values-inside-your-components"></a>Gotcha: dereference values inside your components</h2>

MobX can do a lot, but it cannot make primitive values observable (although it can wrap them in an object see <a href="boxed.html">boxed observables</a>).

So not the values that are observable, but the properties of an object. This means that <code>@observer</code> actually reacts to the fact that you dereference a value.

So in our above example, the <code>Timer</code> component would not react if it was initialized as follows:

<pre><code class="lang-javascript">React.render(<Timer timerData={timerData.secondsPassed} />, document.body)

</code></pre>

In this snippet just the current value of <code>secondsPassed</code> is passed to the <code>Timer</code>, which is the immutable value <code>0</code> (all primitives are immutable in JS).

That number won't change anymore in the future, so <code>Timer</code> will never update. It is the property <code>secondsPassed</code> that will change in the future,

so we need to access it in the component. Or in other words: values need to be passed by reference and not by value.

<h2 id="es5-support"><a name="es5-support" class="plugin-anchor" href="#es5-support"></a>ES5 support</h2>

In ES5 environments, observer components can be simple declared using <code>observer(React.createClass({ ...</code>. See also the <a href="../best/syntax.md">syntax guide</a>

<h2 id="stateless-function-components"><a name="stateless-function-components" class="plugin-anchor" href="#stateless-function-components"></a>Stateless function components</h2>

The above timer widget could also be written using stateless function components that are passed through <code>observer</code>:

<pre><code class="lang-javascript">import {observer} from "mobx-react";

const Timer = observer(({ timerData }) =>

<span>Seconds passed: { timerData.secondsPassed } </span>

);

</code></pre>

<h2 id="observable-local-component-state"><a name="observable-local-component-state" class="plugin-anchor" href="#observable-local-component-state"></a>Observable local component state</h2>

Just like normal classes, you can introduce observable properties on a component by using the <code>@observable</code> decorator.

This means that you can have local state in components that doesn't need to be manged by React's verbose and imperative <code>setState</code> mechanism, but is as powerful.

The reactive state will be picked up by <code>render</code> but will not explicitly invoke other React lifecycle methods except <code>componentWillUpdate</code> and <code>componentDidUpdate</code>.

If you need other React lifecycle methods, just use the normal React <code>state</code> based APIs.

The example above could also have been written as:

<pre><code class="lang-javascript">import {observer} from "mobx-react"

import {observable} from "mobx"

@observer class Timer extends React.Component {

@observable secondsPassed = 0

componentWillMount() {

setInterval(() => {

this.secondsPassed++

}, 1000)

}

render() {

return (Seconds passed: { this.secondsPassed } )

}

ReactDOM.render(<Timer />, document.body)

</code></pre>

For more advantages of using observable local component state, see <a href="https://medium.com/@mweststrate/3-reasons-why-i-stopped-using-react-setstate-ab73fc67a42e" target="_blank">3 reasons why I stopped using <code>setState</code></a>.

<h2 id="connect-components-to-provided-stores-using-inject"><a name="connect-components-to-provided-stores-using-inject" class="plugin-anchor" href="#connect-components-to-provided-stores-using-inject"></a>Connect components to provided stores using <code>inject</code></h2>

<a style="color: white; background:green;padding:5px;margin:5px;border-radius:2px" href="https://egghead.io/lessons/react-connect-mobx-observer-components-to-the-store-with-the-react-provider" target="_blank">egghead.io lesson 8: inject stores with Provider</a>

The <code>mobx-react</code> package also provides the <code>Provider</code> component that can be used to pass down stores using React's context mechanism.

To connect to those stores, pass a list of store names to <code>inject</code>, which will make the stores available as props.

N.B. the syntax to inject stores has changed since mobx-react 4, always use <code>inject(stores)(component)</code> or <code>@inject(stores) class Component...</code>.

Passing store names directly to <code>observer</code> is deprecated.

Example:

<pre><code class="lang-javascript">const colors = observable({

foreground: '#000',

background: '#fff'

});

const App = () =>

</Provider>;

const Button = inject("colors")(observer(({ colors, label, onClick }) =>

<button style={{

color: colors.foreground,

backgroundColor: colors.background

}}

onClick={onClick}

>{label}<button>

));

// later..

colors.foreground = 'blue';

// all buttons updated

</code></pre>

See for more information the <a href="https://github.com/mobxjs/mobx-react#provider-and-inject" target="_blank"><code>mobx-react</code> docs</a>.

<h2 id="when-to-apply-observer"><a name="when-to-apply-observer" class="plugin-anchor" href="#when-to-apply-observer"></a>When to apply <code>observer</code>?</h2>

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.

With <code>@observer</code> there is no need to distinguish 'smart' components from 'dumb' components for the purpose of rendering.

It is still a good separation of concerns for where to handle events, make requests etc.

All components become responsible for updating when their own dependencies change.

Its overhead is neglectable and it makes sure that whenever you start using observable data the component will respond to it.

See this <a href="https://www.reddit.com/r/reactjs/comments/4vnxg5/free_eggheadio_course_learn_mobx_react_in_30/d61oh0l" target="_blank">thread</a> for more details.

<h2 id="-observer-and-purecomponent"><a name="-observer-and-purecomponent" class="plugin-anchor" href="#-observer-and-purecomponent"></a><code>observer</code> and <code>PureComponent</code></h2>

<code>observer</code> also prevents re-renderings when the props of the component have only shallowly changed, which makes a lot of sense if the data passed into the component is reactive.

This behavior is similar to <a href="https://reactjs.org/docs/react-api.html#reactpurecomponent" target="_blank">React PureComponent</a>, except that state changes are still always processed.

If a component provides its own <code>shouldComponentUpdate</code>, that one takes precedence.

See for an explanation this <a href="https://github.com/mobxjs/mobx/issues/101" target="_blank">github issue</a>

<h2 id="-componentwillreact-lifecycle-hook"><a name="-componentwillreact-lifecycle-hook" class="plugin-anchor" href="#-componentwillreact-lifecycle-hook"></a><code>componentWillReact</code> (lifecycle hook)</h2>

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 <code>mobx-react</code> you can define a new life cycle hook, <code>componentWillReact</code> (pun intended) that will be triggered when a component will be scheduled to re-render because

data it observes has changed. This makes it easy to trace renders back to the action that caused the rendering.

<pre><code class="lang-javascript">import {observer} from "mobx-react";

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