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# Intro to the React Framework

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In today's world of Javascript Application frameworks, design philosophy is the key differentiating factor. If you compare the popular JS frameworks, such as EmberJS, AngularJS, Backbone, Knockout, etc. you are sure to find differences in their abstractions, thinking models, and of course the terminology. This is a direct consequence of the underlying design philosophy. But, in principle, they all do one thing, which is to abstract out the DOM in such a way that you don't deal directly with HTML Elements.

I personally think that a framework becomes interesting when it provides a set of abstractions that enable a different mode of thinking. In this aspect, react, the new JS framework from the folks at Facebook, will force you to rethink (to some extent) how you decompose the UI and interactions of your application. Having reached version 0.4.1 (as of this writing), React provides a surprisingly simple, yet effective model for building JS apps that mixes a delightful cocktail of a different kind.

In this article, we'll explore the building blocks of React and embrace a style of thinking that may seem counter-intuitive on the first go. But, as the React docs say: "Give it Five Minutes" and then you will see how this approach will become more natural.

## Motivations

The story of React started within the confines of Facebook, where it brewed for a while. Having reached a stable-enough state, the developers decided to open-source it a few months back. Interestingly the Instagram website is also powered by the React Framework.

React approaches the DOM-abstraction problem with a slightly different take. To understand how this is different, lets quickly gloss over the techniques adopted by the frameworks I mentioned earlier.

### A High Level Overview of JS Application Frameworks

The MVC (Model-View-Controller) design pattern is fundamental to UI development, not just in web apps, but in front-end applications on any platform. In case of web apps, the DOM is the physical representation of a View. The DOM itself is generated from a textual html-template that is pulled from a different file, script-block or a precompiled template function. The View is an entity that brings the textual template to life as a DOM fragment. It also sets up event-handlers and takes care of manipulating the DOM tree as part of its lifecycle.

For the View to be useful, it needs to show some data, and possibly allow user interaction. The data is the Model, which comes from some data-source (a database, web-service, local-storage, etc.). Frameworks provide a way of "binding" the data to the view, such that changes in data are automatically reflected with changes on the view. This automatic process is called data-binding and there are APIs/techniques to make this as seamless as possible.

The MVC triad is completed by the Controller, which engages the View and the Model and orchestrates the flow of data (Model) into the View and user-events out from the View, possibly leading to changes in the Model.

Frameworks that automatically handle the flow of data back and forth between the View and Model maintain an internal event-loop. This event-loop is needed to listen to certain user events, data-change events, external triggers, etc and then determine if there is any change from the previous run of the loop. If there are changes, at either end (View or Model), the framework ensures that both are brought back in sync.

### What Makes React Different?

With React, the View-part of the MVC triad takes prominence and is rolled into an entity called the Component. The Component maintains an immutable property bag called props, and a state that represents the user-driven state of the UI. The view-generation part of the Component is rather interesting and possibly the reason that makes React stand out compared to other frameworks. Instead of constructing a physical DOM directly from a template file/script/function, the Component generates an intermediate DOM that is a stand-in for the real HTML DOM. An additional step is then taken to translate this intermediate DOM into the real HTML DOM.

As part of the intermediate DOM generation, the Component also attaches event-handlers and binds the data contained in props and state.

If the idea of an intermediate-DOM sounds a little alien, don't be too alarmed. You have already seen this strategy adopted by language runtimes (aka Virtual Machines) for interpreted languages. Our very own JavaScript runtime, first generates an intermediate representation before spitting out the native code. This is also true for other VM-based languages such as Java, C#, Ruby, Python, etc.

React cleverly adopts this strategy to create an intermediate DOM before generating the final HTML DOM. The intermediate-DOM is just a JavaScript object graph and is not rendered directly. There is a translation step that creates the real DOM. This is the underlying technique that makes React do fast DOM manipulations.

## React In Depth

To get a better picture of how React makes it all work, let's dive a little deeper; starting with the Component. The Component is the primary building block in React. You can compose the UI of your application by assembling a tree of Components. Each Component provides an implementation for the render() method, where it creates the intermediate-DOM. Calling React.renderComponent() on the root Component results in recursively going down the Component-tree and building up the intermediate-DOM. The intermediate-DOM is then converted into the real HTML DOM.

Since the intermediate-DOM creation is an integral part of the Component, React provides a convenient XML-based extension to JavaScript, called JSX, to build the component tree as a set of XML nodes. This makes it easier to visualize and reason about the DOM. JSX also simplifies the association of event-handlers and properties as xml attributes. Since JSX is an extension language, there is a tool (command-line and in-browser) to generate the final JavaScript. A JSX XML node maps directly to a Component. It is worth pointing out that React works independent of JSX and the JSX language only makes it easy to create the intermediate DOM.

### Tooling

The core React framework can be downloaded from their website. Additionally, for the JSX → JS transformation, you can either use the in-browser JSXTransformer or use the command line tool, called react-tools (installed via NPM). You will need an installation of Node.js to download it. The command-line tool allows you to precompile the JSX files and avoid the translation within the browser. This is definitely recommended if your JSX files are large or many in number.

### A Simple Component

Alright, we have seen a lot of theory so far, and I am sure you are itching to see some real code. Let's dive into our first example:

Although simple, the code above does cover a good amount of the React surface area:

• We create the Simple component by using React.createClass and passing in an object that implements some core functions. The most important one is the render(), which generates the intermediate-DOM.
• Here we are using JSX to define the DOM and also attach the mousedown event-handler. The {} syntax is useful for incorporating JavaScript expressions for attributes (onMouseDown={this.handleClick}) and child-nodes (<span class="count">{this.state.count}</span>). Event handlers associated using the {} syntax are automatically bound to the instance of the component. Thus this inside the event-handler function refers to the component instance. The comment on the first line /** @jsx React.DOM */ is a cue for the JSX transformer to do the translation to JS. Without this comment line, no translation will take place.

We can run the command-line tool (jsx) in watch mode and auto-compile changes from JSX → JS. The source files are in /src folder and the output is generated in /build.

Here is the generated JS file:

Notice how the <div/> and <span/> tags map to instances of React.DOM.div and React.DOM.span.

• Now let's get back to our code example. Inside handleMouseDown, we make use of this.props to read the message property that was passed in. We set the message on the last line of the snippet, in the call to React.renderComponent() where we create the <Simple/> component. The purpose of this.props is to store the data that was passed in to the component. It is considered immutable and only a higher-level component is allowed to make changes and pass it down the component tree.
• Inside handleMouseDown we also set some user state with this.setState() to track the number of times the message was displayed. You will notice that we use this.state in the render() method. Anytime you call setState(), React also triggers the render() method to keep the DOM in sync. Besides React.renderComponent(), setState() is another way to force a visual refresh.

### Synthetic Events

The events exposed on the intermediate-DOM, such as the onMouseDown, also act as a layer of indirection before they get set on the real-DOM. These events are thus refered to as Synthetic Events. React adopts event-delegation, which is a well-known technique, and attaches events only at the root-level of the real-DOM. Thus there is only one true event-handler on the real-DOM. Additionally these synthetic events also provide a level of consistency by hiding browser and element differences.

The combination of the intermediate-DOM and synthetic events gives you a standard and consistent way of defining UIs across different browsers and even devices.

### Component Lifecycle

Components in the React framework have a specific lifecycle and embody a state-machine that has three distinct states.

The Component comes to life after being Mounted. Mounting results in going through a render-pass that generates the component-tree (intermediate-DOM). This tree is converted and placed into a container-node of the real DOM. This is a direct outcome of the call to React.renderComponent().

Once mounted, the component stays in the Update state. A component gets updated when you change state using setState() or change props using setProps(). This in turn results in calling render(), which brings the DOM in sync with the data (props + state). Between subsequent updates, React will calculate the delta between the previous component-tree and the newly generated tree. This is a highly optimized step (and a flagship feature) that minimizes the manipulation on the real DOM.

The final state is Unmounted. This happens when you explicitly call React.unmountAndReleaseReactRootNode() or automatically if a component was a child that was no longer generated in a render() call. Most often you don't have to deal with this and just let React do the proper thing.

Now it would have been a big remiss, if React didn't tell you when it moved between the Mounted-Update-Unmounted states. Thankfully that is not the case and there are hooks you can override to get notified of lifecycle changes. The names speak for themselves:

• getInitialState(): prepare initial state of the Component
• componentWillMount()
• componentDidMount()
• componentWillReceiveProps()
• shouldComponentUpdate(): useful if you want to control when a render should be skipped.
• componentWillUpdate()
• render()
• componentDidUpdate()
• componentWillUnmount()

The componentWill* methods are called before the state change and the componentDid* methods are called after.

Some of the method names do seem to have taken a cue from the Cocoa frameworks in Mac and iOS

### Miscellaneous Features

Within a component-tree, data should always flow down. A parent-component should set the props of a child-component to pass any data from the parent to the child. This is termed as the Owner-Owned pair. On the other hand user-events (mouse, keyboard, touches) will always bubble up from the child all the way to the root component, unless handled in between.

When you create the intermediate-DOM in render(), you can also assign a ref property to a child component. You can then refer to it from the parent using the refs property. This is depicted in the snippet below.

As part of the component metadata, you can set the initial-state (getInitialState()), which we saw earlier within the lifecycle methods. You can also set the default values of the props with getDefaultProps() and also establish some validation rules on these props using propTypes. The docs give a nice overview of the different kinds of validations (type checks, required, etc.) you can perform.

React also supports the concept of a Mixin to extract reusable pieces of behavior that can be injected into disparate Components. You can pass the mixins using the mixins property of a Component.

Now, lets get real and build a more comprehensive Component that uses these features.

## A Shape Editor Built Using React

In this example, we will build an editor that accepts a simple DSL (Domain Specific Language) for creating shapes. As you type in, you will see the corresponding output on the side, giving you live feedback.

The DSL allows you to create three kinds of shapes: Ellipse, Rectangle and Text. Each shape is specified on a separate line along with a bunch of styling properties. The syntax is straightforward and borrows a bit from CSS. To parse a line, we use a Regex that looks like:

As an example, the following set of lines describe two rectangles and a text label...

...generating the output shown below:

### Setting Up

Alright, lets go ahead and build this editor. We will start out with the HTML file (index.html), where we put in the top-level markup and include the libraries and application scripts. I am only showing the relevant parts here:

In the above snippet, the container div holds our React generated DOM. Our application scripts are included from the /build directory. We are using JSX within our components and the command line watcher (jsx), puts the converted JS files into /build. Note that this watcher command is part of the react-tools NPM module.

The editor is broken down into a set of components, which are listed below:

• ShapeEditor: the root Component in the component tree
• ShapeCanvas: responsible for generating the shape-Components (Ellipse, Rectangle, Text). It is contained within the ShapeEditor.
• ShapeParser: responsible for parsing text and extracting the list of shape definitions. It parses line by line with the Regex we saw earlier. Invalid lines are ignored. This is not really a component, but a helper JS object, used by the ShapeEditor.
• Ellipse, Rectangle, Text: the shape Components. These become children of the ShapeCanvas.
• ShapePropertyMixin: provides helper functions for extracting styles found in the shape definitions. This is mixed-into the three shape-Components using the mixins property.
• app: the entry-point for the editor. It generates the root component (ShapeEditor) and allows you to pick a shape sample from the drop-down.

The relationship of these entities is shown in the annotated component-tree:

### The ShapeEditor Component

Lets look at the implementation of some of these components, starting with the ShapeEditor.

As the name suggests, the ShapeEditor provides the editing experience by generating the <textarea/> and the live feedback on the <ShapeCanvas/<. It listens to the onChange event (events in React are always named with camel case) on the <textarea/> and on every change, sets the text property of the component's state. As mentioned earlier, whenever you set the state using setState(), render is called automatically. In this case, the render() of the ShapeEditor gets called where we parse the text from the state and rebuild the shapes. Note that we are starting with an initial state of empty text, which is set in the getInitialState() hook.

For parsing the text into a set of shapes, We use an instance of the ShapeParser. I've left out the details of the parser to keep the discussion focused on React. The parser instance is created in the componentWillMount() hook. This is called just before the component mounts and is a good place to do any initializations before the first render happens.

It is generally recommended that you funnel all your complex processing through the render() method. Event handlers just set the state while render() is the hub for all your core logic.

The ShapeEditor uses this idea to do the parsing inside of its render() and forwards the detected shapes by setting the shapes property of the ShapeCanvas. This is how data flows down into the component tree, from the owner (ShapeEditor) to the owned (ShapeCanvas).

One last thing to note in here is that we have the first line comment to indicate JSX → JS translation.

### ShapeCanvas to Generate the Shapes

Next, we will move on to the ShapeCanvas and the Ellipse, Rectangle and Text components.

p>The ShapeCanvas is rather straightforward with its core responsibility of generating the respective <Ellipse/>, <Rectangle/> and <Text/> components from the passed in shape definitions (this.props.shapes). For each shape, we pass in the parsed properties with the attribute expression: properties={shape.properties}.

One thing different here is that our component tree is not static, like we have in ShapeEditor. Instead it's dynamically generated by looping over the passed in shapes. We also show the "No Shapes Found" message if there is nothing to show.

### The Shapes: Ellipse, Rectangle, Text

All of the shapes have a similar structure and differ only in the styling. They also make use of the ShapePropertyMixin to handle the style generation.

Here's Ellipse:

The implementation for extractStyle() is provided by the ShapePropertyMixin.

The Rectangle component follows suit, of course without the border-radius style. The Text component has an extra property called value which sets the inner text for the <div/>.

Here's Text, to make this clear:

### Tying It All Together With App.js

app.js is where we bring it all together. Here we render the root component, the ShapeEditor and also provide support to switch between a few sample shapes. When you pick a different sample from the drop down, we load some predefined text into the ShapeEditor and cause the ShapeCanvas to update. This happens in the readShapes() method.

To exercise the creative side, here is a robot built using the Shape Editor:

## And That's React for you!

Phew! This has been a rather long article and having reached this point, you should have a sense of achievement!

We have explored a lot of concepts here: the integral role of Components in the framework, use of JSX to easily describe a component tree (aka intermediate-DOM), various hooks to plug into the component lifecyle, use of state and props to drive the render process, use of Mixins to factor out reusable behavior and finally pulling all of this together with the Shape Editor example.

I hope this article gives you enough boost to build a few React apps for yourself. To continue your exploration, here are few handy links:

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