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Using the Grid Shepherd Technique to Order Data with CSS

May 28th, 2019

Shepherds are good attending to their sheep, bringing order and structure to their herds. Even if there are hundreds of those wooly animals, a shepherd still herds them back to the farm at the end of the day.

When dealing with data, programmers often don’t know if it is correctly filtered or sorted. This is especially painful when iterating over an array then displaying the data on a site without knowing the locations of each element receiving it. The Grid Shepherd is a technique that helps position and sort items where you want them to be, using CSS Grid instead of JavaScript.

That’s what we’re going to look at in this post. The Grid Shepherd technique can bring order and structure to the data we work with while giving us greater visibility to where and how it’s being used than we would be able to otherwise.

Let’s dig in.

Sorting with JavaScript

We’re going to start by iterating over an unordered array of farm animals. Imagine that cows and sheep are happily grazing the fields. They can be grouped together programmatically with the Array.prototype.sort method and listed on a page:

let animals = [
  { name: 'Edna', animal: 'cow'   },
  { name: 'Liam', animal: 'sheep' },
  { name: 'Fink', animal: 'sheep' },
  { name: 'Olga', animal: 'cow'   },
]
let sortedAnimals = animals.sort((a, b) => {
  if (a.animal < b.animal) return -1
  if (a.animal > b.animal) return 1
  return 0
})
console.log(sortedAnimals)
/* Returns:
  [ { name: 'Elga', animal: 'cow'   },
    { name: 'Olga', animal: 'cow'   },
    { name: 'Liam', animal: 'sheep' },
    { name: 'Fink', animal: 'sheep' } ]
*/

Meet the Grid Shepherd

The Grid Shepherd method makes sorting data possible without the use of JavaScript. Instead, we rely on CSS Grid to do the lifting for us.

The structure is exactly the same as the JavaScript array of objects above, only represented in DOM nodes instead.

<main>
  <div class="cow">Edna</div>
  <div class="sheep">Liam</div>
  <div class="sheep">Jenn</div>
  <div class="cow">Fink</div>
</main>

See the Pen
1. Start
by David Bernegger (@Achilles_2)
on CodePen.

To herd the animals, we have to fence them into a common area, which is what we’re using the <main> element to do. By setting that fence with display: grid, we’re creating a grid formatting context where we can define the column (or row) each animal should occupy.

.sheep { grid-column: 1; }
.cow { grid-column: 2; }

And with grid-auto-flow: dense, each animal orders itself into the first available spot of each defined area. This can also be used with as many different sort options as you want — simply define another column and the data will be shepherded magically into it.

main
  display: grid;
  grid-auto-flow: dense;
}

.sheep { grid-column: 1; }
.cow { grid-column: 2; }

See the Pen
2. Final
by David Bernegger (@Achilles_2)
on CodePen.

Pro Shepherding

We can take our herding example further with CSS Counters. That way, we can count how many animals we have in each column and — applying Heydon Pickering’s quantity queries from Lea Verou’s Talk in 2011 — conditionally style the them depending on how many there are.

Quantity queries rely on some sort of selector for counting the classes — which would be great with the :nth-child(An+B [of S]?) pseudo-class notation, but it’s currently only available in Safari). That means we have to use the :nth-of-type() selector as a workaround.

We need some new element types for this to work. This could be realized through Web Components or renaming any HTML element to a custom name. This works even if these elements are not in the HTML spec, as browsers use HTMLUnknownElement for undefined tags which results in them behaving much like a div. The document looks now like this:

<fence>
  <sheep>Lisa</sheep>
  <sheep>Bonnie</sheep>
  <cow>Olaf</cow>
  <sheep>Jenn</sheep>
</fence>

Now we can access our custom element types. Let’s apply a red background when the number of sheep or the cows is equal to or less than 10.

sheep:nth-last-of-type(n+10),
sheep:nth-last-of-type(n+10) ~ sheep,
cow:nth-last-of-type(n+10),
cow:nth-last-of-type(n+10) ~ cow, {
  background-color: red;
}

Additionally the the counters can be simply realized by using counter-reset: countsheep countcow; on the parent element and using the before selector to target each element and count up.

sheep::before {
  counter-increment: countsheep;  
  content: counter(countsheep); 
}

Here, we’re going to reach for Vue to dynamically add and remove animals using Vue transitions with two different sort options. Watch as the animals naturally occupy the correct columns, even as more are added and some are removed:

See the Pen
3. Final with Vue Transitions
by David Bernegger (@Achilles_2)
on CodePen.

Grid Shepherd can also be used with any non-ordered data to:

  • separate and count voters in a poll (maybe as two sections with their corresponding profile picture) with live insertion;
  • group people / co-workers according to their position, age, height; and
  • create any hierarchical structure

Shepherding and accessibility

grid-auto-flow: dense does not change the DOM structure of the grid — it merely reorders the contained elements visually. A side effect can be seen in the last example when ordering alphabetically as the counter numbers get mixed up. Changing the DOM structure not only affects people who use screen readers, but also effects tab traversal.

Also note that a flat document structure might not be good for screen readers. Instead, I would treat these presentational grids as graphs and provide the information with a longer textual alternative.

Round ‘em up!

It’s pretty neat to see how a powerful CSS layout tool like grid can be leveraged for use cases that fall outside of traditional layouts needs and into things where we may have reached for other languages in the past. In this case, we can see how the layout benefits of CSS Grid and the dynamic data-handling capabilities of JavaScript overlap and how that gives us more choices — and power — to bend rendered data to our will.