feat: added computedPrevious helper fn (#181)

docs/src/content/docs/utilities/Signals/computed-previous.md

@@ -0,0 +1,36 @@
+---
+title: computedPrevious
+description: ngxtension/computed-previous
+badge: stable
+contributor: enea-jahollari
+---
+
+`computedPrevious` is a helper function that returns a `signal` that emits the previous value of the passed `signal`. It's useful when you want to keep track of the previous value of a signal.
+It will emit `null` as the previous value when the signal is first created.
+
+```ts
+import { computedPrevious } from 'ngxtension/computed-previous';
+```
+
+## Usage
+
+`computedPrevious` accepts a `Signal` and returns a `Signal` that emits the previous value of the passed `Signal`.
+
+```ts
+const a = signal(1);
+const b = computedPrevious(a);
+
+console.log(b()); // null
+
+a.set(2);
+console.log(b()); // 1
+
+a.set(3);
+console.log(b()); // 2
+```
+
+## API
+
+```ts
+computedPrevious<T>(signal: Signal<T>): Signal<T | null>
+```

libs/ngxtension/computed-previous/README.md

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+# ngxtension/computed-previous
+
+Secondary entry point of `ngxtension`. It can be used by importing from `ngxtension/computed-previous`.

libs/ngxtension/computed-previous/ng-package.json

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+{
+	"lib": {
+		"entryFile": "src/index.ts"
+	}
+}

libs/ngxtension/computed-previous/project.json

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+{
+	"name": "ngxtension/computed-previous",
+	"$schema": "../../../node_modules/nx/schemas/project-schema.json",
+	"projectType": "library",
+	"sourceRoot": "libs/ngxtension/computed-previous/src",
+	"targets": {
+		"test": {
+			"executor": "@nx/jest:jest",
+			"outputs": ["{workspaceRoot}/coverage/{projectRoot}"],
+			"options": {
+				"jestConfig": "libs/ngxtension/jest.config.ts",
+				"testPathPattern": ["computed-previous"],
+				"passWithNoTests": true
+			},
+			"configurations": {
+				"ci": {
+					"ci": true,
+					"codeCoverage": true
+				}
+			}
+		},
+		"lint": {
+			"executor": "@nx/eslint:lint",
+			"outputs": ["{options.outputFile}"],
+			"options": {
+				"lintFilePatterns": [
+					"libs/ngxtension/computed-previous/**/*.ts",
+					"libs/ngxtension/computed-previous/**/*.html"
+				]
+			}
+		}
+	}
+}

libs/ngxtension/computed-previous/src/computed-previous.spec.ts

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+import { Component, signal } from '@angular/core';
+import { TestBed } from '@angular/core/testing';
+import { computedPrevious } from './computed-previous';
+
+describe(computedPrevious.name, () => {
+	@Component({ standalone: true, template: '{{previous()}}' })
+	class Test {
+		value = signal(0);
+		previous = computedPrevious(this.value);
+	}
+
+	function setup() {
+		const fixture = TestBed.createComponent(Test);
+		fixture.detectChanges();
+		return fixture.componentInstance;
+	}
+
+	it('should work properly', () => {
+		const cmp = setup();
+
+		expect(cmp.value()).toEqual(0);
+		expect(cmp.previous()).toEqual(null);
+
+		cmp.value.set(1);
+
+		expect(cmp.value()).toEqual(1);
+		expect(cmp.previous()).toEqual(0);
+
+		cmp.value.set(2);
+
+		expect(cmp.value()).toEqual(2);
+		expect(cmp.previous()).toEqual(1);
+
+		cmp.value.set(2);
+
+		expect(cmp.value()).toEqual(2);
+		expect(cmp.previous()).toEqual(1);
+
+		cmp.value.set(3);
+
+		expect(cmp.value()).toEqual(3);
+		expect(cmp.previous()).toEqual(2);
+	});
+});

libs/ngxtension/computed-previous/src/computed-previous.ts

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+import { computed, type Signal } from '@angular/core';
+
+/**
+ * Returns a signal that emits the previous value of the given signal.
+ * The first time the signal is emitted, the previous value will be `null`.
+ *
+ * @example
+ * ```ts
+ * const value = signal(0);
+ * const previous = computedPrevious(value);
+ *
+ * effect(() => {
+ *  console.log('Current value:', value());
+ *  console.log('Previous value:', previous());
+ * });
+ *
+ * Logs:
+ * // Current value: 0
+ * // Previous value: null
+ *
+ * value.set(1);
+ *
+ * Logs:
+ * // Current value: 1
+ * // Previous value: 0
+ *```
+
+ * @param s Signal to compute previous value for
+ * @returns Signal that emits previous value of `s`
+ */
+export function computedPrevious<T>(s: Signal<T>): Signal<T | null> {
+	let current = null as T;
+	let previous = null as T;
+
+	return computed(() => {
+		const value = s();
+		if (value !== current) {
+			previous = current;
+			current = value;
+		}
+		return previous;
+	});
+}

libs/ngxtension/computed-previous/src/index.ts

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+export * from './computed-previous';

tsconfig.base.json

@@ -33,6 +33,9 @@
 			"ngxtension/computed-from": [
 				"libs/ngxtension/computed-from/src/index.ts"
 			],
+			"ngxtension/computed-previous": [
+				"libs/ngxtension/computed-previous/src/index.ts"
+			],
 			"ngxtension/connect": ["libs/ngxtension/connect/src/index.ts"],
 			"ngxtension/create-effect": [
 				"libs/ngxtension/create-effect/src/index.ts"