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.
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?
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
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
All Or None Or Something In Between: The Values
pointer-events is both a CSS property and an SVG element attribute. Its initial value is
auto, which means that only the painted and visible portions will receive pointer events. Most other values can be split into two groups:
- Values that require an element to be visible; and
- Values that do not.
all fall into the latter category. Their visibility-dependent counterparts —
visible — fall into the former.
The SVG 2.0 specification also defines a
bounding-box value. When the value of
bounding-box, the rectangular area around the element can also receive pointer events. As of this writing, only Chrome 65+ supports the
none is also a valid value. It prevents the element and its children from receiving any pointer events. The
pointer-events CSS property can be used with HTML elements too. But when used with HTML, only
none are valid values.
pointer-events values are better demonstrated than explained, let’s look at some demos.
Here we have a circle with a fill and a stroke applied. It’s both painted and visible. The entire circle can receive pointer events, but the area outside of the circle cannot.
Disable the fill, so that its value is
none. Now if you try to hover, click, or tap the interior of the circle, nothing happens. But if you do the same for the stroke area, pointer events are still dispatched. Changing the
fill value to
none means that this area visible, but not painted.
Let’s make a small change to our markup. We’ll add
pointer-events="visible" to our
circle element, while keeping
See the Pen How adding pointer-events: all affects interactivity by Tiffany Brown (@webinista) on CodePen.
Now the unpainted area encircled by the stroke can receive pointer events.
Augmenting The Clickable Area Of An SVG Image
Let’s return to the image from the beginning of this article. Our “amethyst” is a
path element, as opposed to a group of polygons each with a
fill. That means we can’t just add
pointer-events="all" and call it a day.
Instead, we need to augment the click area. Let’s use what we know about painted and visible elements. In the example below, I’ve added a rectangle to our image markup.
Even though this rectangle is unseen, it’s still technically visible (i.e.
visibility: visible). Its lack of a fill, however, means that it is not painted. Our image looks the same. Indeed it still works the same — clicking white space still doesn’t trigger a navigation operation. We still need to add a
pointer-events attribute to our
a element. Using the
all values will work here.
Now the entire image can receive pointer events.
bounding-box would eliminate the need for a phantom element. All points within the bounding box would receive pointer events, including the white space enclosed by the path. But again:
pointer-events="bounding-box" isn’t widely supported. Until it is, we can use unpainted elements.
pointer-events When Mixing SVG And HTML
Another case where
pointer-events may be helpful: using SVG inside of an HTML button.
In most browsers — Firefox and Internet Explorer 11 are exceptions here — the value of
event.target will be an SVG element instead of our HTML button. Let’s add
pointer-events="none" to our opening SVG tag.
See the Pen How pointer-events: none can be used with SVG and HTML by Tiffany Brown (@webinista) on CodePen.
Now when users click or tap our button, the
event.target will refer to our
function keyword instead of an arrow function and
this instead of
event.target fixes this problem. Using
SVG supports the same kind of interactivity we’re used to with HTML. We can use it to create charts that respond to clicks or taps. We can create linked areas that don’t adhere to the CSS and HTML box model. And with the addition of
pointer-events, we can improve the way our SVG documents behave in response to user interaction.
Browser support for SVG
pointer-events is robust. Every browser that supports SVG supports the property for SVG documents and elements. When used with HTML elements, support is slightly less robust. It isn’t available in Internet Explorer 10 or its predecessors, or any version of Opera Mini.
We’ve just scratched the surface of
pointer-events in this piece. For a more in-depth, technical treatment, read through the SVG Specification. MDN (Mozilla Developer Network) Web Docs offers more web developer-friendly documentation for
pointer-events, complete with examples.
Source link https://www.smashingmagazine.com/2018/05/svg-interaction-pointer-events-property/