breaking(breakpoint): add breakpointObserver store (#1092)

* breaking: re-name event "on:match" to "on:change" in `Breakpoint.svelte`

* feat: add `breakpointObserver` read-only store

* refactor: export breakpoint constants from `breakpoints.js`

src/Breakpoint/breakpointObserver.js

@@ -0,0 +1,80 @@
+import { onMount } from "svelte";
+import { derived, writable } from "svelte/store";
+import { breakpoints } from "./breakpoints";
+
+/**
+ * Creates a readable store that returns the current breakpoint size.
+ * It also provides functions for creating derived stores used to do comparisons.
+ */
+export function breakpointObserver() {
+  const store = writable(undefined);
+
+  onMount(() => {
+    /** @type {Record<import("./breakpoints").BreakpointSize, MediaQueryList>} */
+    const match = {
+      sm: window.matchMedia(`(max-width: ${breakpoints.md}px)`),
+      md: window.matchMedia(
+        `(min-width: ${breakpoints.md}px) and (max-width: ${breakpoints.lg}px)`
+      ),
+      lg: window.matchMedia(
+        `(min-width: ${breakpoints.lg}px) and (max-width: ${breakpoints.xlg}px)`
+      ),
+      xlg: window.matchMedia(
+        `(min-width: ${breakpoints.xlg}px) and (max-width: ${breakpoints.max}px)`
+      ),
+      max: window.matchMedia(`(min-width: ${breakpoints.max}px)`),
+    };
+    const matchers = Object.entries(match);
+    const sizeByMedia = Object.fromEntries(
+      matchers.map(([size, queryList]) => [queryList.media, size])
+    );
+
+    const size = matchers.find(([size, queryList]) => queryList.matches)[0];
+    store.set(size);
+
+    /** @type {(e: MediaQueryListEvent) => void} */
+    function handleChange({ matches, media }) {
+      const size = sizeByMedia[media];
+      if (matches) store.set(size);
+    }
+
+    matchers.forEach(([size, queryList]) =>
+      queryList.addEventListener("change", handleChange)
+    );
+
+    return () => {
+      matchers.forEach(([size, queryList]) =>
+        queryList.removeEventListener("change", handleChange)
+      );
+    };
+  });
+
+  return {
+    subscribe: store.subscribe,
+
+    /**
+     * Returns a store readable store that returns whether the current
+     * breakpoint is smaller than {@link size}.
+     * @param {import("./breakpoints").BreakpointSize} size Size to compare against.
+     */
+    smallerThan: (size) => {
+      checkSizeValid(size);
+      return derived(store, ($size) => breakpoints[$size] < breakpoints[size]);
+    },
+
+    /**
+     * Returns a store readable store that returns whether the current
+     * breakpoint is larger than {@link size}.
+     * @param {import("./breakpoints").BreakpointSize} size Size to compare against.
+     */
+    largerThan: (size) => {
+      checkSizeValid(size);
+      return derived(store, ($size) => breakpoints[$size] > breakpoints[size]);
+    },
+  };
+}
+
+function checkSizeValid(size) {
+  if (size in breakpoints == false)
+    throw new Error(`"${size}" is not a valid breakpoint size.`);
+}