feat(util): add cell util

This is a small util that provides a single tracked property
for easily creating tracked state within the function-based
resources.

ember-resources/package.json

@@ -13,6 +13,7 @@
     "./core/class-based": "./dist/core/class-based/index.js",
     "./core/function-based": "./dist/core/function-based/index.js",
     "./util": "./dist/util/index.js",
+    "./util/cell": "./dist/util/cell.js",
     "./util/map": "./dist/util/map.js",
     "./util/helper": "./dist/util/helper.js",
     "./util/remote-data": "./dist/util/remote-data.js",
@@ -33,6 +34,9 @@
       "util": [
         "dist/util/index.d.ts"
       ],
+      "util/cell": [
+        "dist/util/cell.d.ts"
+      ],
       "util/function": [
         "dist/util/function.d.ts"
       ],

ember-resources/src/index.ts

@@ -3,6 +3,9 @@ export { Resource } from './core/class-based';
 export { resource, resourceFactory } from './core/function-based';
 export { use } from './core/use';
 
+// Public API -- Utilities
+export { cell } from 'util/cell';
+
 // Public Type Utilities
 export type { ExpandArgs } from './core/class-based/types';
 export type { ArgsWrapper, Thunk } from './core/types';

ember-resources/src/util/cell.ts

@@ -0,0 +1,57 @@
+import { tracked } from '@glimmer/tracking';
+import { assert } from '@ember/debug';
+
+class Cell<T> {
+  @tracked current: T;
+
+  constructor(initialValue: T) {
+    this.current = initialValue;
+  }
+
+  toggle = () => {
+    assert(
+      `toggle can only be used when 'current' is a boolean type`,
+      typeof this.current === 'boolean' || this.current === undefined
+    );
+
+    (this.current as boolean) = !this.current;
+  };
+}
+
+/**
+ * Small state utility for helping reduce the number of imports
+ * when working with resources in isolation.
+ *
+ * The return value is an instance of a class with a single
+ * `@tracked` property, `current`. If `current` is a boolean,
+ * there is a `toggle` method available as well.
+ *
+ * For example, a Clock:
+ *
+ * ```js
+ * import { resource, cell } from 'ember-resources';
+ *
+ * const Clock = resource(({ on }) => {
+ *   let time = cell(new Date());
+ *   let interval = setInterval(() => time.current = new Date(), 1000);
+ *
+ *   on.cleanup(() => clearInterval(interval));
+ *
+ *   let formatter = new Intl.DateTimeFormat('en-US', {
+ *     hour: 'numeric',
+ *     minute: 'numeric',
+ *     second: 'numeric',
+ *     hour12: true,
+ *   });
+ *
+ *   return () => formatter.format(time.current);
+ * });
+ *
+ * <template>
+ *   It is: <time>{{Clock}}</time>
+ * </template>
+ * ```
+ */
+export function cell<T>(initialValue: T): Cell<T> {
+  return new Cell(initialValue);
+}

testing/ember-app/tests/utils/cell/rendering-test.ts

@@ -0,0 +1,26 @@
+import { render, settled } from '@ember/test-helpers';
+import { hbs } from 'ember-cli-htmlbars';
+import { module, test } from 'qunit';
+import { setupRenderingTest } from 'ember-qunit';
+
+import { cell } from 'ember-resources';
+
+module('Utils | cell | rendering', function (hooks) {
+  setupRenderingTest(hooks);
+
+  test('it works', async function (assert) {
+    let state = cell();
+
+    this.setProperties({ state });
+
+    await render(hbs`{{this.state.current}}`);
+
+    assert.dom().doesNotContainText('hello');
+
+    state.current = 'hello';
+
+    await settled();
+
+    assert.dom().hasText('hello');
+  });
+});