Ben Nadel
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Ben Nadel at Scotch On The Rock (SOTR) 2010 (Brussels) with: Aaron Longnion
Ben Nadel at Scotch On The Rock (SOTR) 2010 (Brussels) with: Aaron Longnion@aqlong )

Attaching Dynamic Event Handlers To Proxied Child Elements In ReactJS

By Ben Nadel on

A month ago, I took a look at creating an AngularJS-inspired "directive" component in ReactJS. The goal of this component was to create a "higher order component" that had a more natural feel and informational flow to it. I liked it as an approach; but, I didn't like that the directive component had to manually enable all of its event bindings. By doing that, it lost out on the event-delegation and state queuing implemented by ReactJS. And, it also means that if the proxied element every gets swapped out, the directive component has to handle that explicitly. To improve this approach, I wanted to look at using ReactJS to attach dynamic event handlers to the proxied child element.


 
 
 

 
 
 
 
 

Run this demo in my JavaScript Demos project on GitHub.

It's easy to think about state in terms of the data being rendered. But, event handler bindings can also be driven by the component state. You probably have a bunch of onClick={} bindings hard-coded in your JSX markup, which makes a lot of sense; but, those don't have to be hard-coded. They can be injected using the spread-operator or through non-JSX element creation, just like any other property. And, just like any other property, they can be driven by the state.

The reason that ReactJS event bindings are so flexible is because they aren't bound directly to the rendered element. Instead, ReactJS implements event delegation, binding the centralized event handlers at the root element and then distributing events, to the target elements, as they happen. The use of event delegation means that the target element can be created, destroyed, and re-created without ever having to worry about re-binding the event handlers.

This is some sweet stuff. And, it means that I can refactor my previous approach to use state driven event-bindings. In the following demo, I have an absolutely positioned draggable element. The root component is really just a state machine that handles the rendering of the container and the draggable dongle. It defers all of the complex user-mouse-interaction to a "directive component" that proxies the content and mutates the state through passed-in property hooks.

As the "directive component" proxies the single child element, it defines event handlers as additional props on that single child. The event handlers are driven by the state of the directive component and change based on the phase of the interaction. In order to not mutate the child props directly, however, I'm using React.cloneElement() which allows us to create a copy of the proxied element with the event handlers "mixed in" to the props collection.

As you look at the following code, take note that I never explicitly unbind an event handler - I only ever define new combinations of event handlers that pertain to a given phase of the interaction. And, to really drive home the extremely dynamic nature of event bindings in ReactJS, I'm swapping out the top element every second. This is demonstrate that, as the "ref" value changes, all of the event bindings remain in tact.

  • <!doctype html>
  • <html>
  • <head>
  • <meta charset="utf-8" />
  •  
  • <title>
  • Attaching Dynamic Event Handlers To Proxied Child Elements In ReactJS
  • </title>
  •  
  • <link rel="stylesheet" type="text/css" href="./demo.css"></link>
  • </head>
  • <body>
  •  
  • <h1>
  • Attaching Dynamic Event Handlers To Proxied Child Elements In ReactJS
  • </h1>
  •  
  • <div id="content">
  • <!-- App will be rendered here. -->
  • </div>
  •  
  •  
  • <!-- Load scripts. -->
  • <script type="text/javascript" src="../../vendor/reactjs/react-0.13.3.min.js"></script>
  • <script type="text/javascript" src="../../vendor/reactjs/JSXTransformer-0.13.3.js"></script>
  • <script type="text/javascript" src="../../vendor/jquery/jquery-2.1.0.min.js"></script>
  • <script type="text/javascript" src="../../vendor/lodash/lodash-3.9.3.min.js"></script>
  • <script type="text/jsx">
  •  
  • // I manage the root component.
  • var Demo = React.createClass({
  •  
  • // I return the initial state of the component.
  • getInitialState: function() {
  •  
  • return({
  • x: 300,
  • y: 300,
  • isActivated: false,
  • containerType: "article"
  • });
  •  
  • },
  •  
  •  
  • // ---
  • // PUBLIC METHODS.
  • // ---
  •  
  •  
  • // I put the dongle in an activate state.
  • activate: function() {
  •  
  • this.setState({
  • isActivated: true
  • });
  •  
  • },
  •  
  •  
  • // When the component mounts, we want to start an interval that will
  • // continuously change the TYPE of the root level container element. This
  • // will help us explore the power of the event-delegation system that
  • // ReactJS uses, and what that means for dynamically-bound event handlers.
  • componentDidMount: function() {
  •  
  • setInterval( this.toggleContainerType, 1000 );
  •  
  • },
  •  
  •  
  • // I put the dongle in a deactivated state.
  • deactivate: function() {
  •  
  • this.setState({
  • isActivated: false
  • });
  •  
  • },
  •  
  •  
  • // I return the virtual DOM represented by the current component state.
  • render: function() {
  •  
  • // As the setInterval() kicks in, we will be toggling back and forth
  • // between an "Article" element and a "Section" element. This will force
  • // ReactJS to physically destroy and create DOM elements, rather than
  • // trying to reconcile attribute-based differences.
  • var Container = this.state.containerType;
  •  
  • var dongleClass = this.state.isActivated
  • ? "dongle activated"
  • : "dongle"
  • ;
  •  
  • var dongleStyle = {
  • left: this.state.x,
  • top: this.state.y
  • };
  •  
  • // When we render our component, we are wrapping the "content" in an
  • // AngularJS-inspired "directive component." This DemoDirective will take
  • // care of manging the complex user interactions (ie, mouse events), while
  • // the current component will worry about managing state and layout.
  • return(
  • <DemoDirective
  • activate={ this.activate }
  • deactivate={ this.deactivate }
  • setPosition={ this.setPosition }>
  •  
  • <Container className="container">
  •  
  • <div className={ dongleClass } style={ dongleStyle }>
  • <span className="label">
  • { this.state.x } / { this.state.y }
  • </span>
  • </div>
  •  
  • </Container>
  •  
  • </DemoDirective>
  • );
  •  
  • },
  •  
  •  
  • // I set the x/y coordinates of the dongle.
  • setPosition: function( x, y ) {
  •  
  • this.setState({
  • x: x,
  • y: y
  • });
  •  
  • },
  •  
  •  
  • // ---
  • // PRIVATE METHODS.
  • // ---
  •  
  •  
  • // I cycle to the next container type.
  • // --
  • // NOTE: We are doing this in order to force ReactJS to physically destroy
  • // and create a new top-level DOM element during virtual DOM reconciliation.
  • toggleContainerType: function() {
  •  
  • var newType = ( this.state.containerType === "article" )
  • ? "section"
  • : "article"
  • ;
  •  
  • this.setState({
  • containerType: newType
  • });
  •  
  • }
  •  
  • });
  •  
  •  
  • // --------------------------------------------------------------------------- //
  • // --------------------------------------------------------------------------- //
  •  
  •  
  • var DemoDirective = React.createClass({
  •  
  • // NOTE: We are requiring that "children" be a single element. This way, when
  • // we render, we can use the single child-element as the virtual DOM.
  • propTypes: {
  • activate: React.PropTypes.func.isRequired,
  • children: React.PropTypes.element.isRequired,
  • deactive: React.PropTypes.func.isRequired,
  • setPosition: React.PropTypes.func.isRequired
  • },
  •  
  •  
  • // I return the initial state of the component.
  • getInitialState: function() {
  •  
  • this.initialMouseDown = null;
  • this.initialMouseOffset = null;
  • this.dragThreshold = 15;
  •  
  • // Since this component is an AngularJS-inspired "directive component",
  • // we're going to have it handle complex UI interactions. In this case,
  • // we'll be using the state to keep track of which event-handlers are
  • // being bound to the virtual DOM (through ReactJS' event delegation).
  • return({
  • handlers: {}
  • });
  •  
  • },
  •  
  •  
  • // ---
  • // PUBLIC METHODS.
  • // ---
  •  
  •  
  • // I get called once, on the client, when the component is rendered on the DOM.
  • componentDidMount: function() {
  •  
  • console.info( "Directive component did mount." );
  •  
  • this.setState({
  • handlers: {
  • onMouseDown: this.phasePreDragHandleMouseDown
  • }
  • });
  •  
  • },
  •  
  •  
  • // I return the virtual DOM represented by the current component state.
  • render: function() {
  •  
  • // When we render the "directive component", we're really just rendering
  • // the child element as there is no additional markup to render. However,
  • // since we are adding dynamic behavior, we need to clone the the child
  • // in order to mix-in the props for mouse-interaction.
  • // --
  • // NOTE: Since we are using "props" to bind mouse-event handlers, we know
  • // that ReactJS is going to bind them using event delegation. This means
  • // that we do NOT HAVE TO WORRY about the element unmounting or event
  • // changing. Heck, it means we don't even have to unbind event handlers
  • // when we want to change them - we simply define a new set of handlers.
  • var props = _.extend(
  • {
  • ref: this.handleRefChange
  • },
  • this.state.handlers
  • );
  •  
  • return( React.cloneElement( this.props.children, props ) );
  •  
  • },
  •  
  •  
  •  
  • // ---
  • // PRIVATE METHODS.
  • // ---
  •  
  •  
  • // I get called when ever the targeted ref is created or destroyed.
  • handleRefChange: function( ref ) {
  •  
  • if ( ref ) {
  •  
  • console.warn( "Ref changed:", ref.getDOMNode().tagName );
  •  
  • }
  •  
  • },
  •  
  •  
  • // INTERACTION PHASE: Drag - I handle the mouse-move event.
  • phaseDragHandleMouseMove: function( event ) {
  •  
  • this.props.setPosition(
  • Math.floor( event.pageX - this.initialMouseOffset.x ),
  • Math.floor( event.pageY - this.initialMouseOffset.y )
  • );
  •  
  • },
  •  
  •  
  • // INTERACTION PHASE: Drag - I handle the mouse-up event.
  • phaseDragHandleMouseUp: function( event ) {
  •  
  • this.initialMouseDown = null;
  • this.initialMouseOffset = null;
  •  
  • this.setState({
  • handlers: {
  • onMouseDown: this.phasePreDragHandleMouseDown
  • }
  • });
  •  
  • this.props.deactivate();
  •  
  • },
  •  
  •  
  • // INTERACTION PHASE: Pre-Drag - I handle the mouse-down event.
  • phasePreDragHandleMouseDown: function( event ) {
  •  
  • var dongle = $( event.target ).closest( ".dongle" );
  •  
  • // If the user moused-down in the dongle, then we want to start tracking
  • // their mouse movements to see if they exceed the drag threshold.
  • if ( dongle.length ) {
  •  
  • var offset = dongle.offset();
  •  
  • this.initialMouseDown = {
  • x: event.pageX,
  • y: event.pageY
  • };
  •  
  • this.initialMouseOffset = {
  • x: ( event.pageX - offset.left ),
  • y: ( event.pageY - offset.top )
  • };
  •  
  • this.setState({
  • handlers: {
  • onMouseMove: this.phasePreDragHandleMouseMove,
  • onMouseUp: this.phasePreDragHandleMouseUp
  • }
  • });
  •  
  • }
  •  
  • },
  •  
  •  
  • // INTERACTION PHASE: Pre-Drag - I handle the mouse-move event.
  • phasePreDragHandleMouseMove: function( event ) {
  •  
  • var maxDelta = Math.max(
  • Math.abs( event.pageX - this.initialMouseDown.x ),
  • Math.abs( event.pageY - this.initialMouseDown.y )
  • );
  •  
  • // If the user has exceeded the pre-drag threshold, push the component
  • // into the drag state. This uses a different set of event handlers.
  • if ( maxDelta >= this.dragThreshold ) {
  •  
  • this.setState({
  • handlers: {
  • onMouseMove: this.phaseDragHandleMouseMove,
  • onMouseUp: this.phaseDragHandleMouseUp
  • }
  • });
  •  
  • this.props.activate();
  •  
  • this.phaseDragHandleMouseMove( event );
  •  
  • }
  •  
  • },
  •  
  •  
  • // INTERACTION PHASE: Pre-Drag - I handle the mouse-up event.
  • phasePreDragHandleMouseUp: function( event ) {
  •  
  • this.initialMouseDown = null;
  • this.initialMouseOffset = null;
  •  
  • this.setState({
  • handlers: {
  • onMouseDown: this.phasePreDragHandleMouseDown
  • }
  • });
  •  
  • }
  •  
  • });
  •  
  •  
  • // --------------------------------------------------------------------------- //
  • // --------------------------------------------------------------------------- //
  •  
  •  
  • // Render the root Demo and mount it inside the given element.
  • React.render( <Demo />, document.getElementById( "content" ) );
  •  
  • </script>
  •  
  • </body>
  • </html>

Whether or not you like this kind of approach, I think there's still some really interesting ReactJS stuff going on here. Coming from a "physical DOM" (Document Object Model) background, it's very different, very challenging, and ultimately very freeing to think about event bindings in a virtual DOM context. But, it certainly takes some getting used to. If nothing else, I hope this post helped you think about ReactJS in a different way.




Reader Comments

Woah -- it's almost magic!

I'm curious how this is working, though. Event handlers in React are usually done via props on the component, right? But here we're not setting those. Instead we're setting this.state.onEvent. Is that what React does behind the scenes when we're setting props on a component? Is this behind the scenes usage of state properties documented anywhere? How did you find out about this?

Also, I'm curious how this might work on an input, or something that can maintain :focus (or something else that relies on the element not being removed). This particular example only has to use state to decide how to render itself. Do you think we'd see the input become unfocused?

Reply to this Comment

@Josh,

It does feel magical. But, we are using props to set the events. I just happen to be storing the event handlers in the state. A less dynamic version of this might look something like:

var props = { onClick: this.handleClick };

<div {...props}>Click me!</div>

This is using the spread-operator (...) to dump a collection of props into the element. We just don't see this very much since most event handlers are hard-coded in the JSX markup.

In my example, I'm using the React.cloneElement(), which has the signature:

React.cloneElement( element, props, children )

... so, I'm essentially creating a new Props collection, cloning the element, and then mixing in the new props (which contain the event handlers). It just so happens that I am storing referencing to the appropriate event handlers in the State of the wrapper element. And that's because they need to change depending on the phase of the interaction.

No doubt, this took me a lot of trial and error to get working as I was learning this as I went :D

Reply to this Comment

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