From a1bf9980d0a1e32dcf64a8f52c51b974c9f4d657 Mon Sep 17 00:00:00 2001
From: Arman Ozak <arman@armanozak.com>
Date: Thu, 19 Mar 2020 19:14:49 +0300
Subject: [PATCH] feat(core): add a utility service for easy TrackByFunction
 implementation

---
 .../core/src/lib/services/track-by.service.ts | 18 ++++++++++
 .../src/lib/tests/track-by.service.spec.ts    | 33 +++++++++++++++++++
 2 files changed, 51 insertions(+)
 create mode 100644 npm/ng-packs/packages/core/src/lib/services/track-by.service.ts
 create mode 100644 npm/ng-packs/packages/core/src/lib/tests/track-by.service.spec.ts

diff --git a/npm/ng-packs/packages/core/src/lib/services/track-by.service.ts b/npm/ng-packs/packages/core/src/lib/services/track-by.service.ts
new file mode 100644
index 0000000000..ca2b92db5f
--- /dev/null
+++ b/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;
+  }
+}
diff --git a/npm/ng-packs/packages/core/src/lib/tests/track-by.service.spec.ts b/npm/ng-packs/packages/core/src/lib/tests/track-by.service.spec.ts
new file mode 100644
index 0000000000..19868e8ad5
--- /dev/null
+++ b/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');
+    });
+  });
+});