Dragging SVGs with React

I’ve recently started a new version of a mapping tool at work that lets people create choropleths from CSV files. The tool combines React and some D3 libraries to create SVG maps of the United States, including data labels.

These labels are, by default, placed at the centroid of the state’s path shape, with a few offsets manually specified for some of the weirder state boundaries. But even with manual offsets, these generated labels can still be positioned poorly. I wanted the ability to click and drag these labels into a better position.

SVGs are not always accommodating. They do not implement the drag and drop API, so no ondrag events for us. And while I do use D3 libraries in this project, it’s mainly just d3-geofor parsing topojson and creating the US state paths rather than creating SVG elements or managing data. React handles the programming state and component creation/modification. That means d3-drag, which seems tightly coupled to D3’s paradigm for creating/modifying SVG elements in the DOM, would probably be a pain to shoehorn into this.

So, let’s implement drag and drop manually!

First, here’s the basic Label component. It’s just two text elements inside a g element that will be added to the SVG. The component also has the style user-select: none to prevent selecting the text rather than dragging it.

const Label = ({ center, adjustment, name, value }) => {
	// Use the centroid coordinates and manual adjustments
	// from props to set X and Y of label in the SVG
	const labelX = center[0] + adjustment[0];
	const labelY = center[1] + adjustment[1];

	return (
		<g
			style={{ userSelect: 'none' }}
			transform={`translate(${labelX}, ${labelY})`}>
			<text>{name}</text>
			<text>{value}</text>
		</g>
	);
};

While SVG may not implement the drag and drop API, we can still use mouse events! We’ll use the mousedown event to know that we’re trying to drag, the mousemove event to decide how far we’ve dragged and update position accordingly, and the mouseup event to know that we’re done dragging.

const Label = ({ center, adjustment }) => {
	const [dragging, setDragging] = useState(false);

	const labelX = center[0] + adjustment[0];
	const labelY = center[1] + adjustment[1];

	return (
		<g
			style={{ userSelect: 'none' }}
			transform={`translate(${labelX}, ${labelY})`}
			onMouseDown={e => {
				// We have clicked the label, starting the drag.
				setDragging(true);
			}}
			onMouseMove={e => {
				// As long as we haven't let go of the mouse button,
				// we are still dragging.
				if (dragging) {
					// Drag behavior will go here.
				}
			}}
			onMouseUp={() => {
				// We let go of the mouse, ending our drag.
				setDragging(false);
			}}>
			...
		</g>
	);
};

Our Label component now knows whether or not it is being dragged. To actually reposition the component, we need to track the coordinates we started at and how far we’ve dragged away from them.

const Label = ({ center, adjustment }) => {
	const [dragging, setDragging] = useState(false);
	const [coordinates, setCoordinates] = useState({ x: 0, y: 0 });
	const [origin, setOrigin] = useState({ x: 0, y: 0 });

	// Add our new coordinates to the X and Y position values.
	const labelX = center[0] + adjustment[0] + coordinates.x;
	const labelY = center[1] + adjustment[1] + coordinates.y;

	return (
		<g
			style={{ userSelect: 'none' }}
			transform={`translate(${labelX}, ${labelY})`}
			onMouseDown={e => {
				// Record our starting point.
				setOrigin({ x: e.clientX, y: e.clientY });
				setDragging(true);
			}}
			onMouseMove={e => {
				if (dragging) {
					// Set state for the change in coordinates.
					setCoordinates({
						x: e.clientX - origin.x,
						y: e.clientY - origin.y,
					});
				}
			}}
			onMouseUp={() => {
				setDragging(false);
			}}>
			...
		</g>
	);
};

And that’s it! We’ve now got a drag-able label inside our SVG, and all without needing any libraries to do it.