Managing SVG Interaction With The Pointer Events Property

Managing SVG Interaction With The Pointer Events Property

Managing SVG Interaction With The Pointer Events Property

Tiffany Brown

2018-05-16T14:15:25+02:00
2018-05-16T12:57:14+00:00

Try clicking or tapping the SVG image below. If you put your pointer in the right place (the shaded path) then you should have Smashing Magazine’s homepage open in a new browser tab. If you tried to click on some white space, you might be really confused instead.

See the Pen Amethyst by Tiffany Brown (@webinista) on CodePen.

This is the dilemma I faced during a recent project that included links within SVG images. Sometimes when I clicked the image, the link worked. Other times it didn’t. Confusing, right?

I turned to the SVG specification to learn more about what might be happening and whether SVG offers a fix. The answer: pointer-events.

Not to be confused with DOM (Document Object Model) pointer events, pointer-events is both a CSS property and an SVG element attribute. With it, we can manage which parts of an SVG document or element can receive events from a pointing device such as a mouse, trackpad, or finger.

A note about terminology: “pointer events” is also the name of a device-agnostic, web platform feature for user input. However, in this article — and for the purposes of the pointer-events property — the phrase “pointer events” also includes mouse and touch events.

Outside Of The Box: SVG’s “Shape Model”

Using CSS with HTML imposes a box layout model on HTML. In the box layout model, every element generates a rectangle around its contents. That rectangle may be inline, inline-level, atomic inline-level, or block, but it’s still a rectangle with four right angles and four edges. When we add a link or an event listener to an element, the interactive area matches the dimensions of the rectangle.

Note: Adding a clip-path to an interactive element alters its interactive bounds. In other words, if you add a hexagonal clip-path path to an a element, only the points within the clipping path will be clickable. Similarly, adding a skew transformation will turn rectangles into rhomboids.

SVG does not have a box layout model. You see, when an SVG document is contained by an HTML document, within a CSS layout, the root SVG element adheres to the box layout model. Its child elements do not. As a result, most CSS layout-related properties don’t apply to SVG.

So instead, SVG has what I’ll call a ‘shape model’. When we add a link or an event listener to an SVG document or element, the interactive area will not necessarily be a rectangle. SVG elements do have a bounding box. The bounding box is defined as: the tightest fitting rectangle aligned with the axes of that element’s user coordinate system that entirely encloses it and its descendants. But initially, which parts of an SVG document are interactive depends on which parts are visible and/or painted.

Painted vs. Visible Elements

SVG elements can be “filled” but they can also be “stroked”. Fill refers to the interior of a shape. Stroke refers to its outline.

Together, “fill” and “stroke” are painting operations that render elements to the screen or page (also known as the canvas). When we talk about painted elements, we mean that the element has a fill and/or a stroke. Usually, this means the element is also visible.

However, an SVG element can be painted without being visible. This can happen if the visible attribute value or CSS property is hidden or when display is none. The element is there and occupies theoretical space. We just can’t see it (and assistive technology may not detect it).

Perhaps more confusingly, an element can also be visible — that is, have a computed visibility value of visible — without being painted. This happens when elements lack both a stroke and a fill.

Note: Color values with alpha transparency (e.g. rgba(0,0,0,0)) do not affect whether an element is painted or visible. In other words, if an element has an alpha transparent fill or stroke, it’s painted even if it can’t be seen.

Knowing when an element is painted, visible, or neither is crucial to understanding the impact of each pointer-events value.


Source: Smashing Magazine