[comp] add CardDeckFeed implement role:feed pattern

app/src/components/CardDeckFeed.vue

@@ -0,0 +1,80 @@
+<script>
+// Implementation of the feed pattern
+// https://www.w3.org/WAI/ARIA/apg/patterns/feed/
+
+export default {
+  name: 'CardDeckFeed',
+
+  props: {
+    stacks: { type: Number, default: 21 }
+  },
+
+  data () {
+    return {
+      busy: false,
+      range: this.stacks,
+      childrenCount: this.$slots.default.length
+    }
+  },
+
+  methods: {
+    getTopParent (prev) {
+      return prev.parentElement === this.$refs.feed ? prev : this.getTopParent(prev.parentElement)
+    },
+
+    onScroll (e) {
+      const elem = this.$refs.feed
+      if (window.innerHeight > elem.clientHeight + elem.getBoundingClientRect().top - 200) {
+        this.busy = true
+        this.range = Math.min(this.range + this.stacks, this.childrenCount)
+        this.$nextTick().then(() => {
+          this.busy = false
+        })
+      }
+    },
+
+    onKeydown (e) {
+      if (['PageUp', 'PageDown'].includes(e.code)) {
+        e.preventDefault()
+        const key = e.code === 'PageUp' ? 'previous' : 'next'
+        const sibling = this.getTopParent(e.target)[`${key}ElementSibling`]
+        if (sibling) {
+          sibling.focus()
+          sibling.scrollIntoView({ block: 'center' })
+        }
+      }
+      // FIXME Add `Home` and `End` shorcuts
+    }
+  },
+
+  mounted () {
+    window.addEventListener('scroll', this.onScroll)
+    this.$refs.feed.addEventListener('keydown', this.onKeydown)
+  },
+
+  beforeUpdate () {
+    const slots = this.$slots.default
+    if (this.childrenCount !== slots.length) {
+      this.range = this.stacks
+      this.childrenCount = slots.length
+    }
+  },
+
+  render (h) {
+    return h(
+      'b-card-group',
+      {
+        attrs: { role: 'feed', 'aria-busy': this.busy.toString() },
+        props: { deck: true },
+        ref: 'feed'
+      },
+      this.$slots.default.slice(0, this.range)
+    )
+  },
+
+  beforeDestroy () {
+    window.removeEventListener('scroll', this.onScroll)
+    this.$refs.feed.removeEventListener('keydown', this.onKeydown)
+  }
+}
+</script>