feat(core): add a utility service for easy TrackByFunction implementation

npm/ng-packs/packages/core/src/lib/services/track-by.service.ts

@@ -0,0 +1,18 @@
+import { Injectable, TrackByFunction } from '@angular/core';
+
+@Injectable({
+  providedIn: 'root',
+})
+export class TrackByService<ItemType = any> {
+  by<T = ItemType>(key: keyof T): TrackByFunction<T> {
+    return ({}, item) => item[key];
+  }
+
+  byDeep<T = ItemType>(...keys: (string | number)[]): TrackByFunction<T> {
+    return ({}, item) => keys.reduce((acc, key) => acc[key], item);
+  }
+
+  bySelf<T = ItemType>(): TrackByFunction<T> {
+    return ({}, item) => item;
+  }
+}

npm/ng-packs/packages/core/src/lib/tests/track-by.service.spec.ts

@@ -0,0 +1,33 @@
+import { TrackByService } from '../services/track-by.service';
+
+describe('TrackByService', () => {
+  const service = new TrackByService();
+
+  describe('#by', () => {
+    it('should return a function which tracks a property', () => {
+      expect(service.by('x')(284, { x: 1036 })).toBe(1036);
+    });
+  });
+
+  describe('#byDeep', () => {
+    it('should return a function which tracks a deeply-nested property', () => {
+      expect(
+        service.byDeep(
+          'a',
+          'b',
+          'c',
+          1,
+          'x',
+        )(284, {
+          a: { b: { c: [{ x: 1035 }, { x: 1036 }, { x: 1037 }] } },
+        }),
+      ).toBe(1036);
+    });
+  });
+
+  describe('#bySelf', () => {
+    it('should return a function which tracks the item', () => {
+      expect(service.bySelf()(284, 'X')).toBe('X');
+    });
+  });
+});