utility: nextSubscriber (#23)

* next: nextSubscriber

* package: export nextSubscriber, bump version

* package: update scripts

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@okcontract/cells",
-  "version": "0.2.2",
+  "version": "0.2.3",
   "description": "Simplified reactive functional programming for the web",
   "private": false,
   "main": "dist/cells.umd.cjs",
@@ -39,8 +39,8 @@
     "coverage": "vitest run --coverage",
     "definitions": "tsc --project tsconfig.build.json",
     "prepublishOnly": "npm test && npm run build && npm run check && npm run definitions",
-    "check": "npx @biomejs/biome check src --apply",
-    "format": "npx @biomejs/biome format src --write",
+    "check": "npx @biomejs/biome check src",
+    "format": "npx @biomejs/biome format src --write && npx @biomejs/biome check src --apply",
     "formatReadme": "prettier README.md --prose-wrap always --print-width 78 -w"
   },
   "repository": {
@@ -49,4 +49,4 @@
   },
   "author": "Henri Binsztok",
   "license": "MIT"
-}
+}

src/index.ts

@@ -41,7 +41,6 @@ export {
   reduce,
   sort
 } from "./array";
-
 export { Debugger } from "./debug";
-
 export { jsonStringify } from "./json";
+export { nextSubscriber } from "./next";

src/next.test.ts

@@ -0,0 +1,27 @@
+import { expect, test } from "vitest";
+
+import { nextSubscriber } from "./next";
+import { sleep } from "./promise";
+import { SheetProxy } from "./proxy";
+import { Sheet } from "./sheet";
+
+test("nextSubscriber", async () => {
+  const sheet = new Sheet();
+  const proxy = new SheetProxy(sheet);
+  const cell = proxy.new(1, "init");
+
+  let f = 0;
+  nextSubscriber(cell, (v) => {
+    f = v;
+  });
+
+  // we wait
+  await sleep(10);
+  expect(f).toBe(0);
+
+  cell.set(2);
+  expect(f).toBe(2);
+
+  cell.set(3);
+  expect(f).toBe(2);
+});

src/next.ts

@@ -0,0 +1,24 @@
+import { AnyCell } from "./cell";
+import { Unsubscriber } from "./types";
+
+/**
+ * nextSubscriber subscribes to get the next value of a cell. This is useful
+ * when a cell is already defined, but we know it will be updated (e.g. for a
+ * ValueCell) and we want that next value.
+ */
+export const nextSubscriber = <V>(
+  cell: AnyCell<V>,
+  cb: (v: V) => void,
+  _expectedCount = 2
+) => {
+  // biome-ignore lint/style/useConst: need reference
+  let uns: Unsubscriber;
+  let count = 0;
+  uns = cell.subscribe((arg) => {
+    count++;
+    if (count !== _expectedCount) return;
+    if (arg instanceof Error) throw arg;
+    cb(arg);
+    queueMicrotask(uns);
+  });
+};