This week we made a circle bounce back and forth.

Trivial, right? Yes, as trivial as it gets. The bouncing circle wasn’t the point, the point was how it’s made. This: react-transitionable-component. My first not-just-a-cool-experiment open-source contribution to the React ecosystem.

It’s an abstract component that makes building transitions easy.

Over the past few weeks, I’ve noticed that making a transition always follows the same pattern: move props to state, render from state instead of props, use d3.transition to transition prop values on every componentWillReceiveProps. What if we can abstract all that work away? 

We started with a jerky, jumping circle.

And ended with a smoothly transitioning circle.

Making a circle that transitions like that isn’t hard. It takes about twenty minutes to show and explain to a novice in both React and D3. At least that’s how much it takes when I do workshops.

So who cares about making an abstraction, right? I care. Doing it from scratch every time gets old, fast. I wanted a way to say, “make this component use transitions for everything,” without worrying about implementation every time.

As you can expect from a project that took less than two hours from start to finish, there’s little code to show. 40 lines that look like this:

class TransitionableComponent extends Component {
    constructor(props) {
        // copy all props to state
        // call _defineEasing and _defineDuration
    }

    _defineEasing(easing) {
        // if this.easing undefined, get easing function from d3
    }

    _defineDuration(duration) {
        // if this.duration undefined, set it
    }

    componentWillReceiveProps(newProps) {
        this._defineEasing(newProps.easing);
        this._defineDuration(newProps.duration);

        const node = d3.select(this.refs.node);

        let transition = node.transition()
                             .ease(this.easing);

        if (this.duration !== undefined) {
            transition.duration(this.duration);
        }

        Object.keys(newProps)
              .forEach((k) => {
                  transition.attr(k, newProps[k]);
              });

        transition.on('end', () => this.setState(newProps));
    }
}

I cut out the boring bits. You can see them on GitHub, here.

The fun bit is in componentWillReceiveProps. We update our easing function and our duration, which should’ve been dynamic getters instead. Then we get the node and start a transition()with an easing function, and a duration, if it’s there. After that, we walk through every prop and add it to the pile of transitioning attributes with .attr. When the transition is over, we use the 'end' callback to update the component state and ensure React understands what’s going on.

You’ll notice TransitionableComponent doesn’t have a render()function. It’s an abstract component meant to be extended by a real component. Something like this:

class Circle extends TransitionableComponent {
    render() {
        return <circle cx={this.state.cx}
                       cy={this.state.cy}
                       r={this.state.r} />
    }
}

That’s a transitionable circle now. Any props passed into it are transitioned. You have to be careful to use this.state instead of this.props when using them though.

Using the Circle component looks like this:

    <Circle cx="100" cy="100" r="5" easing="cubicInOut" duration="1500" />

See, easy.

Join me next time when we clean this up to use dynamic getters, avoid importing the entire d3 library and build some examples.